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| <div class="chapter"> |
| <div class="titlepage"><div><div><h1 class="title"> |
| <a name="mc-manual"></a>4. Memcheck: a memory error detector</h1></div></div></div> |
| <div class="toc"> |
| <p><b>Table of Contents</b></p> |
| <dl class="toc"> |
| <dt><span class="sect1"><a href="mc-manual.html#mc-manual.overview">4.1. Overview</a></span></dt> |
| <dt><span class="sect1"><a href="mc-manual.html#mc-manual.errormsgs">4.2. Explanation of error messages from Memcheck</a></span></dt> |
| <dd><dl> |
| <dt><span class="sect2"><a href="mc-manual.html#mc-manual.badrw">4.2.1. Illegal read / Illegal write errors</a></span></dt> |
| <dt><span class="sect2"><a href="mc-manual.html#mc-manual.uninitvals">4.2.2. Use of uninitialised values</a></span></dt> |
| <dt><span class="sect2"><a href="mc-manual.html#mc-manual.bad-syscall-args">4.2.3. Use of uninitialised or unaddressable values in system |
| calls</a></span></dt> |
| <dt><span class="sect2"><a href="mc-manual.html#mc-manual.badfrees">4.2.4. Illegal frees</a></span></dt> |
| <dt><span class="sect2"><a href="mc-manual.html#mc-manual.rudefn">4.2.5. When a heap block is freed with an inappropriate deallocation |
| function</a></span></dt> |
| <dt><span class="sect2"><a href="mc-manual.html#mc-manual.overlap">4.2.6. Overlapping source and destination blocks</a></span></dt> |
| <dt><span class="sect2"><a href="mc-manual.html#mc-manual.fishyvalue">4.2.7. Fishy argument values</a></span></dt> |
| <dt><span class="sect2"><a href="mc-manual.html#mc-manual.leaks">4.2.8. Memory leak detection</a></span></dt> |
| </dl></dd> |
| <dt><span class="sect1"><a href="mc-manual.html#mc-manual.options">4.3. Memcheck Command-Line Options</a></span></dt> |
| <dt><span class="sect1"><a href="mc-manual.html#mc-manual.suppfiles">4.4. Writing suppression files</a></span></dt> |
| <dt><span class="sect1"><a href="mc-manual.html#mc-manual.machine">4.5. Details of Memcheck's checking machinery</a></span></dt> |
| <dd><dl> |
| <dt><span class="sect2"><a href="mc-manual.html#mc-manual.value">4.5.1. Valid-value (V) bits</a></span></dt> |
| <dt><span class="sect2"><a href="mc-manual.html#mc-manual.vaddress">4.5.2. Valid-address (A) bits</a></span></dt> |
| <dt><span class="sect2"><a href="mc-manual.html#mc-manual.together">4.5.3. Putting it all together</a></span></dt> |
| </dl></dd> |
| <dt><span class="sect1"><a href="mc-manual.html#mc-manual.monitor-commands">4.6. Memcheck Monitor Commands</a></span></dt> |
| <dt><span class="sect1"><a href="mc-manual.html#mc-manual.clientreqs">4.7. Client Requests</a></span></dt> |
| <dt><span class="sect1"><a href="mc-manual.html#mc-manual.mempools">4.8. Memory Pools: describing and working with custom allocators</a></span></dt> |
| <dt><span class="sect1"><a href="mc-manual.html#mc-manual.mpiwrap">4.9. Debugging MPI Parallel Programs with Valgrind</a></span></dt> |
| <dd><dl> |
| <dt><span class="sect2"><a href="mc-manual.html#mc-manual.mpiwrap.build">4.9.1. Building and installing the wrappers</a></span></dt> |
| <dt><span class="sect2"><a href="mc-manual.html#mc-manual.mpiwrap.gettingstarted">4.9.2. Getting started</a></span></dt> |
| <dt><span class="sect2"><a href="mc-manual.html#mc-manual.mpiwrap.controlling">4.9.3. Controlling the wrapper library</a></span></dt> |
| <dt><span class="sect2"><a href="mc-manual.html#mc-manual.mpiwrap.limitations.functions">4.9.4. Functions</a></span></dt> |
| <dt><span class="sect2"><a href="mc-manual.html#mc-manual.mpiwrap.limitations.types">4.9.5. Types</a></span></dt> |
| <dt><span class="sect2"><a href="mc-manual.html#mc-manual.mpiwrap.writingwrappers">4.9.6. Writing new wrappers</a></span></dt> |
| <dt><span class="sect2"><a href="mc-manual.html#mc-manual.mpiwrap.whattoexpect">4.9.7. What to expect when using the wrappers</a></span></dt> |
| </dl></dd> |
| </dl> |
| </div> |
| <p>To use this tool, you may specify <code class="option">--tool=memcheck</code> |
| on the Valgrind command line. You don't have to, though, since Memcheck |
| is the default tool.</p> |
| <div class="sect1"> |
| <div class="titlepage"><div><div><h2 class="title" style="clear: both"> |
| <a name="mc-manual.overview"></a>4.1. Overview</h2></div></div></div> |
| <p>Memcheck is a memory error detector. It can detect the following |
| problems that are common in C and C++ programs.</p> |
| <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "> |
| <li class="listitem"><p>Accessing memory you shouldn't, e.g. overrunning and underrunning |
| heap blocks, overrunning the top of the stack, and accessing memory after |
| it has been freed.</p></li> |
| <li class="listitem"><p>Using undefined values, i.e. values that have not been initialised, |
| or that have been derived from other undefined values.</p></li> |
| <li class="listitem"><p>Incorrect freeing of heap memory, such as double-freeing heap |
| blocks, or mismatched use of |
| <code class="function">malloc</code>/<code class="computeroutput">new</code>/<code class="computeroutput">new[]</code> |
| versus |
| <code class="function">free</code>/<code class="computeroutput">delete</code>/<code class="computeroutput">delete[]</code></p></li> |
| <li class="listitem"><p>Overlapping <code class="computeroutput">src</code> and |
| <code class="computeroutput">dst</code> pointers in |
| <code class="computeroutput">memcpy</code> and related |
| functions.</p></li> |
| <li class="listitem"><p>Passing a fishy (presumably negative) value to the |
| <code class="computeroutput">size</code> parameter of a memory |
| allocation function.</p></li> |
| <li class="listitem"><p>Memory leaks.</p></li> |
| </ul></div> |
| <p>Problems like these can be difficult to find by other means, |
| often remaining undetected for long periods, then causing occasional, |
| difficult-to-diagnose crashes.</p> |
| <p>Memcheck also provides <a class="xref" href="manual-core.html#manual-core.xtree" title="2.9. Execution Trees">Execution Trees</a> memory |
| profiling using the command line |
| option <code class="computeroutput">--xtree-memory</code> and the monitor command |
| <code class="computeroutput">xtmemory</code>.</p> |
| </div> |
| <div class="sect1"> |
| <div class="titlepage"><div><div><h2 class="title" style="clear: both"> |
| <a name="mc-manual.errormsgs"></a>4.2. Explanation of error messages from Memcheck</h2></div></div></div> |
| <p>Memcheck issues a range of error messages. This section presents a |
| quick summary of what error messages mean. The precise behaviour of the |
| error-checking machinery is described in <a class="xref" href="mc-manual.html#mc-manual.machine" title="4.5. Details of Memcheck's checking machinery">Details of Memcheck's checking machinery</a>.</p> |
| <div class="sect2"> |
| <div class="titlepage"><div><div><h3 class="title"> |
| <a name="mc-manual.badrw"></a>4.2.1. Illegal read / Illegal write errors</h3></div></div></div> |
| <p>For example:</p> |
| <pre class="programlisting"> |
| Invalid read of size 4 |
| at 0x40F6BBCC: (within /usr/lib/libpng.so.2.1.0.9) |
| by 0x40F6B804: (within /usr/lib/libpng.so.2.1.0.9) |
| by 0x40B07FF4: read_png_image(QImageIO *) (kernel/qpngio.cpp:326) |
| by 0x40AC751B: QImageIO::read() (kernel/qimage.cpp:3621) |
| Address 0xBFFFF0E0 is not stack'd, malloc'd or free'd |
| </pre> |
| <p>This happens when your program reads or writes memory at a place |
| which Memcheck reckons it shouldn't. In this example, the program did a |
| 4-byte read at address 0xBFFFF0E0, somewhere within the system-supplied |
| library libpng.so.2.1.0.9, which was called from somewhere else in the |
| same library, called from line 326 of <code class="filename">qpngio.cpp</code>, |
| and so on.</p> |
| <p>Memcheck tries to establish what the illegal address might relate |
| to, since that's often useful. So, if it points into a block of memory |
| which has already been freed, you'll be informed of this, and also where |
| the block was freed. Likewise, if it should turn out to be just off |
| the end of a heap block, a common result of off-by-one-errors in |
| array subscripting, you'll be informed of this fact, and also where the |
| block was allocated. If you use the <code class="option"><a class="xref" href="manual-core.html#opt.read-var-info">--read-var-info</a></code> option Memcheck will run more slowly |
| but may give a more detailed description of any illegal address.</p> |
| <p>In this example, Memcheck can't identify the address. Actually |
| the address is on the stack, but, for some reason, this is not a valid |
| stack address -- it is below the stack pointer and that isn't allowed. |
| In this particular case it's probably caused by GCC generating invalid |
| code, a known bug in some ancient versions of GCC.</p> |
| <p>Note that Memcheck only tells you that your program is about to |
| access memory at an illegal address. It can't stop the access from |
| happening. So, if your program makes an access which normally would |
| result in a segmentation fault, you program will still suffer the same |
| fate -- but you will get a message from Memcheck immediately prior to |
| this. In this particular example, reading junk on the stack is |
| non-fatal, and the program stays alive.</p> |
| </div> |
| <div class="sect2"> |
| <div class="titlepage"><div><div><h3 class="title"> |
| <a name="mc-manual.uninitvals"></a>4.2.2. Use of uninitialised values</h3></div></div></div> |
| <p>For example:</p> |
| <pre class="programlisting"> |
| Conditional jump or move depends on uninitialised value(s) |
| at 0x402DFA94: _IO_vfprintf (_itoa.h:49) |
| by 0x402E8476: _IO_printf (printf.c:36) |
| by 0x8048472: main (tests/manuel1.c:8) |
| </pre> |
| <p>An uninitialised-value use error is reported when your program |
| uses a value which hasn't been initialised -- in other words, is |
| undefined. Here, the undefined value is used somewhere inside the |
| <code class="function">printf</code> machinery of the C library. This error was |
| reported when running the following small program:</p> |
| <pre class="programlisting"> |
| int main() |
| { |
| int x; |
| printf ("x = %d\n", x); |
| }</pre> |
| <p>It is important to understand that your program can copy around |
| junk (uninitialised) data as much as it likes. Memcheck observes this |
| and keeps track of the data, but does not complain. A complaint is |
| issued only when your program attempts to make use of uninitialised |
| data in a way that might affect your program's externally-visible behaviour. |
| In this example, <code class="varname">x</code> is uninitialised. Memcheck observes |
| the value being passed to <code class="function">_IO_printf</code> and thence to |
| <code class="function">_IO_vfprintf</code>, but makes no comment. However, |
| <code class="function">_IO_vfprintf</code> has to examine the value of |
| <code class="varname">x</code> so it can turn it into the corresponding ASCII string, |
| and it is at this point that Memcheck complains.</p> |
| <p>Sources of uninitialised data tend to be:</p> |
| <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "> |
| <li class="listitem"><p>Local variables in procedures which have not been initialised, |
| as in the example above.</p></li> |
| <li class="listitem"><p>The contents of heap blocks (allocated with |
| <code class="function">malloc</code>, <code class="function">new</code>, or a similar |
| function) before you (or a constructor) write something there. |
| </p></li> |
| </ul></div> |
| <p>To see information on the sources of uninitialised data in your |
| program, use the <code class="option">--track-origins=yes</code> option. This |
| makes Memcheck run more slowly, but can make it much easier to track down |
| the root causes of uninitialised value errors.</p> |
| </div> |
| <div class="sect2"> |
| <div class="titlepage"><div><div><h3 class="title"> |
| <a name="mc-manual.bad-syscall-args"></a>4.2.3. Use of uninitialised or unaddressable values in system |
| calls</h3></div></div></div> |
| <p>Memcheck checks all parameters to system calls: |
| </p> |
| <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "> |
| <li class="listitem"><p>It checks all the direct parameters themselves, whether they are |
| initialised.</p></li> |
| <li class="listitem"><p>Also, if a system call needs to read from a buffer provided by |
| your program, Memcheck checks that the entire buffer is addressable |
| and its contents are initialised.</p></li> |
| <li class="listitem"><p>Also, if the system call needs to write to a user-supplied |
| buffer, Memcheck checks that the buffer is addressable.</p></li> |
| </ul></div> |
| <p> |
| </p> |
| <p>After the system call, Memcheck updates its tracked information to |
| precisely reflect any changes in memory state caused by the system |
| call.</p> |
| <p>Here's an example of two system calls with invalid parameters:</p> |
| <pre class="programlisting"> |
| #include <stdlib.h> |
| #include <unistd.h> |
| int main( void ) |
| { |
| char* arr = malloc(10); |
| int* arr2 = malloc(sizeof(int)); |
| write( 1 /* stdout */, arr, 10 ); |
| exit(arr2[0]); |
| } |
| </pre> |
| <p>You get these complaints ...</p> |
| <pre class="programlisting"> |
| Syscall param write(buf) points to uninitialised byte(s) |
| at 0x25A48723: __write_nocancel (in /lib/tls/libc-2.3.3.so) |
| by 0x259AFAD3: __libc_start_main (in /lib/tls/libc-2.3.3.so) |
| by 0x8048348: (within /auto/homes/njn25/grind/head4/a.out) |
| Address 0x25AB8028 is 0 bytes inside a block of size 10 alloc'd |
| at 0x259852B0: malloc (vg_replace_malloc.c:130) |
| by 0x80483F1: main (a.c:5) |
| |
| Syscall param exit(error_code) contains uninitialised byte(s) |
| at 0x25A21B44: __GI__exit (in /lib/tls/libc-2.3.3.so) |
| by 0x8048426: main (a.c:8) |
| </pre> |
| <p>... because the program has (a) written uninitialised junk |
| from the heap block to the standard output, and (b) passed an |
| uninitialised value to <code class="function">exit</code>. Note that the first |
| error refers to the memory pointed to by |
| <code class="computeroutput">buf</code> (not |
| <code class="computeroutput">buf</code> itself), but the second error |
| refers directly to <code class="computeroutput">exit</code>'s argument |
| <code class="computeroutput">arr2[0]</code>.</p> |
| </div> |
| <div class="sect2"> |
| <div class="titlepage"><div><div><h3 class="title"> |
| <a name="mc-manual.badfrees"></a>4.2.4. Illegal frees</h3></div></div></div> |
| <p>For example:</p> |
| <pre class="programlisting"> |
| Invalid free() |
| at 0x4004FFDF: free (vg_clientmalloc.c:577) |
| by 0x80484C7: main (tests/doublefree.c:10) |
| Address 0x3807F7B4 is 0 bytes inside a block of size 177 free'd |
| at 0x4004FFDF: free (vg_clientmalloc.c:577) |
| by 0x80484C7: main (tests/doublefree.c:10) |
| </pre> |
| <p>Memcheck keeps track of the blocks allocated by your program |
| with <code class="function">malloc</code>/<code class="computeroutput">new</code>, |
| so it can know exactly whether or not the argument to |
| <code class="function">free</code>/<code class="computeroutput">delete</code> is |
| legitimate or not. Here, this test program has freed the same block |
| twice. As with the illegal read/write errors, Memcheck attempts to |
| make sense of the address freed. If, as here, the address is one |
| which has previously been freed, you wil be told that -- making |
| duplicate frees of the same block easy to spot. You will also get this |
| message if you try to free a pointer that doesn't point to the start of a |
| heap block.</p> |
| </div> |
| <div class="sect2"> |
| <div class="titlepage"><div><div><h3 class="title"> |
| <a name="mc-manual.rudefn"></a>4.2.5. When a heap block is freed with an inappropriate deallocation |
| function</h3></div></div></div> |
| <p>In the following example, a block allocated with |
| <code class="function">new[]</code> has wrongly been deallocated with |
| <code class="function">free</code>:</p> |
| <pre class="programlisting"> |
| Mismatched free() / delete / delete [] |
| at 0x40043249: free (vg_clientfuncs.c:171) |
| by 0x4102BB4E: QGArray::~QGArray(void) (tools/qgarray.cpp:149) |
| by 0x4C261C41: PptDoc::~PptDoc(void) (include/qmemarray.h:60) |
| by 0x4C261F0E: PptXml::~PptXml(void) (pptxml.cc:44) |
| Address 0x4BB292A8 is 0 bytes inside a block of size 64 alloc'd |
| at 0x4004318C: operator new[](unsigned int) (vg_clientfuncs.c:152) |
| by 0x4C21BC15: KLaola::readSBStream(int) const (klaola.cc:314) |
| by 0x4C21C155: KLaola::stream(KLaola::OLENode const *) (klaola.cc:416) |
| by 0x4C21788F: OLEFilter::convert(QCString const &) (olefilter.cc:272) |
| </pre> |
| <p>In <code class="literal">C++</code> it's important to deallocate memory in a |
| way compatible with how it was allocated. The deal is:</p> |
| <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "> |
| <li class="listitem"><p>If allocated with |
| <code class="function">malloc</code>, |
| <code class="function">calloc</code>, |
| <code class="function">realloc</code>, |
| <code class="function">valloc</code> or |
| <code class="function">memalign</code>, you must |
| deallocate with <code class="function">free</code>.</p></li> |
| <li class="listitem"><p>If allocated with <code class="function">new</code>, you must deallocate |
| with <code class="function">delete</code>.</p></li> |
| <li class="listitem"><p>If allocated with <code class="function">new[]</code>, you must |
| deallocate with <code class="function">delete[]</code>.</p></li> |
| </ul></div> |
| <p>The worst thing is that on Linux apparently it doesn't matter if |
| you do mix these up, but the same program may then crash on a |
| different platform, Solaris for example. So it's best to fix it |
| properly. According to the KDE folks "it's amazing how many C++ |
| programmers don't know this".</p> |
| <p>The reason behind the requirement is as follows. In some C++ |
| implementations, <code class="function">delete[]</code> must be used for |
| objects allocated by <code class="function">new[]</code> because the compiler |
| stores the size of the array and the pointer-to-member to the |
| destructor of the array's content just before the pointer actually |
| returned. <code class="function">delete</code> doesn't account for this and will get |
| confused, possibly corrupting the heap.</p> |
| </div> |
| <div class="sect2"> |
| <div class="titlepage"><div><div><h3 class="title"> |
| <a name="mc-manual.overlap"></a>4.2.6. Overlapping source and destination blocks</h3></div></div></div> |
| <p>The following C library functions copy some data from one |
| memory block to another (or something similar): |
| <code class="function">memcpy</code>, |
| <code class="function">strcpy</code>, |
| <code class="function">strncpy</code>, |
| <code class="function">strcat</code>, |
| <code class="function">strncat</code>. |
| The blocks pointed to by their <code class="computeroutput">src</code> and |
| <code class="computeroutput">dst</code> pointers aren't allowed to overlap. |
| The POSIX standards have wording along the lines "If copying takes place |
| between objects that overlap, the behavior is undefined." Therefore, |
| Memcheck checks for this. |
| </p> |
| <p>For example:</p> |
| <pre class="programlisting"> |
| ==27492== Source and destination overlap in memcpy(0xbffff294, 0xbffff280, 21) |
| ==27492== at 0x40026CDC: memcpy (mc_replace_strmem.c:71) |
| ==27492== by 0x804865A: main (overlap.c:40) |
| </pre> |
| <p>You don't want the two blocks to overlap because one of them could |
| get partially overwritten by the copying.</p> |
| <p>You might think that Memcheck is being overly pedantic reporting |
| this in the case where <code class="computeroutput">dst</code> is less than |
| <code class="computeroutput">src</code>. For example, the obvious way to |
| implement <code class="function">memcpy</code> is by copying from the first |
| byte to the last. However, the optimisation guides of some |
| architectures recommend copying from the last byte down to the first. |
| Also, some implementations of <code class="function">memcpy</code> zero |
| <code class="computeroutput">dst</code> before copying, because zeroing the |
| destination's cache line(s) can improve performance.</p> |
| <p>The moral of the story is: if you want to write truly portable |
| code, don't make any assumptions about the language |
| implementation.</p> |
| </div> |
| <div class="sect2"> |
| <div class="titlepage"><div><div><h3 class="title"> |
| <a name="mc-manual.fishyvalue"></a>4.2.7. Fishy argument values</h3></div></div></div> |
| <p>All memory allocation functions take an argument specifying the |
| size of the memory block that should be allocated. Clearly, the requested |
| size should be a non-negative value and is typically not excessively large. |
| For instance, it is extremely unlikly that the size of an allocation |
| request exceeds 2**63 bytes on a 64-bit machine. It is much more likely that |
| such a value is the result of an erroneous size calculation and is in effect |
| a negative value (that just happens to appear excessively large because |
| the bit pattern is interpreted as an unsigned integer). |
| Such a value is called a "fishy value". |
| |
| The <code class="varname">size</code> argument of the following allocation functions |
| is checked for being fishy: |
| <code class="function">malloc</code>, |
| <code class="function">calloc</code>, |
| <code class="function">realloc</code>, |
| <code class="function">memalign</code>, |
| <code class="function">new</code>, |
| <code class="function">new []</code>. |
| <code class="function">__builtin_new</code>, |
| <code class="function">__builtin_vec_new</code>, |
| For <code class="function">calloc</code> both arguments are being checked. |
| </p> |
| <p>For example:</p> |
| <pre class="programlisting"> |
| ==32233== Argument 'size' of function malloc has a fishy (possibly negative) value: -3 |
| ==32233== at 0x4C2CFA7: malloc (vg_replace_malloc.c:298) |
| ==32233== by 0x400555: foo (fishy.c:15) |
| ==32233== by 0x400583: main (fishy.c:23) |
| </pre> |
| <p>In earlier Valgrind versions those values were being referred to |
| as "silly arguments" and no back-trace was included. |
| </p> |
| </div> |
| <div class="sect2"> |
| <div class="titlepage"><div><div><h3 class="title"> |
| <a name="mc-manual.leaks"></a>4.2.8. Memory leak detection</h3></div></div></div> |
| <p>Memcheck keeps track of all heap blocks issued in response to |
| calls to |
| <code class="function">malloc</code>/<code class="function">new</code> et al. |
| So when the program exits, it knows which blocks have not been freed. |
| </p> |
| <p>If <code class="option">--leak-check</code> is set appropriately, for each |
| remaining block, Memcheck determines if the block is reachable from pointers |
| within the root-set. The root-set consists of (a) general purpose registers |
| of all threads, and (b) initialised, aligned, pointer-sized data words in |
| accessible client memory, including stacks.</p> |
| <p>There are two ways a block can be reached. The first is with a |
| "start-pointer", i.e. a pointer to the start of the block. The second is with |
| an "interior-pointer", i.e. a pointer to the middle of the block. There are |
| several ways we know of that an interior-pointer can occur:</p> |
| <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "> |
| <li class="listitem"><p>The pointer might have originally been a start-pointer and have been |
| moved along deliberately (or not deliberately) by the program. In |
| particular, this can happen if your program uses tagged pointers, i.e. |
| if it uses the bottom one, two or three bits of a pointer, which are |
| normally always zero due to alignment, in order to store extra |
| information.</p></li> |
| <li class="listitem"><p>It might be a random junk value in memory, entirely unrelated, just |
| a coincidence.</p></li> |
| <li class="listitem"><p>It might be a pointer to the inner char array of a C++ |
| <code class="computeroutput">std::string</code>. For example, some |
| compilers add 3 words at the beginning of the std::string to |
| store the length, the capacity and a reference count before the |
| memory containing the array of characters. They return a pointer |
| just after these 3 words, pointing at the char array.</p></li> |
| <li class="listitem"><p>Some code might allocate a block of memory, and use the first 8 |
| bytes to store (block size - 8) as a 64bit number. |
| <code class="computeroutput">sqlite3MemMalloc</code> does this.</p></li> |
| <li class="listitem"><p>It might be a pointer to an array of C++ objects (which possess |
| destructors) allocated with <code class="computeroutput">new[]</code>. In |
| this case, some compilers store a "magic cookie" containing the array |
| length at the start of the allocated block, and return a pointer to just |
| past that magic cookie, i.e. an interior-pointer. |
| See <a class="ulink" href="http://theory.uwinnipeg.ca/gnu/gcc/gxxint_14.html" target="_top">this |
| page</a> for more information.</p></li> |
| <li class="listitem"><p>It might be a pointer to an inner part of a C++ object using |
| multiple inheritance. </p></li> |
| </ul></div> |
| <p>You can optionally activate heuristics to use during the leak |
| search to detect the interior pointers corresponding to |
| the <code class="computeroutput">stdstring</code>, |
| <code class="computeroutput">length64</code>, |
| <code class="computeroutput">newarray</code> |
| and <code class="computeroutput">multipleinheritance</code> cases. If the |
| heuristic detects that an interior pointer corresponds to such a case, |
| the block will be considered as reachable by the interior |
| pointer. In other words, the interior pointer will be treated |
| as if it were a start pointer.</p> |
| <p>With that in mind, consider the nine possible cases described by the |
| following figure.</p> |
| <pre class="programlisting"> |
| Pointer chain AAA Leak Case BBB Leak Case |
| ------------- ------------- ------------- |
| (1) RRR ------------> BBB DR |
| (2) RRR ---> AAA ---> BBB DR IR |
| (3) RRR BBB DL |
| (4) RRR AAA ---> BBB DL IL |
| (5) RRR ------?-----> BBB (y)DR, (n)DL |
| (6) RRR ---> AAA -?-> BBB DR (y)IR, (n)DL |
| (7) RRR -?-> AAA ---> BBB (y)DR, (n)DL (y)IR, (n)IL |
| (8) RRR -?-> AAA -?-> BBB (y)DR, (n)DL (y,y)IR, (n,y)IL, (_,n)DL |
| (9) RRR AAA -?-> BBB DL (y)IL, (n)DL |
| |
| Pointer chain legend: |
| - RRR: a root set node or DR block |
| - AAA, BBB: heap blocks |
| - --->: a start-pointer |
| - -?->: an interior-pointer |
| |
| Leak Case legend: |
| - DR: Directly reachable |
| - IR: Indirectly reachable |
| - DL: Directly lost |
| - IL: Indirectly lost |
| - (y)XY: it's XY if the interior-pointer is a real pointer |
| - (n)XY: it's XY if the interior-pointer is not a real pointer |
| - (_)XY: it's XY in either case |
| </pre> |
| <p>Every possible case can be reduced to one of the above nine. Memcheck |
| merges some of these cases in its output, resulting in the following four |
| leak kinds.</p> |
| <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "> |
| <li class="listitem"><p>"Still reachable". This covers cases 1 and 2 (for the BBB blocks) |
| above. A start-pointer or chain of start-pointers to the block is |
| found. Since the block is still pointed at, the programmer could, at |
| least in principle, have freed it before program exit. "Still reachable" |
| blocks are very common and arguably not a problem. So, by default, |
| Memcheck won't report such blocks individually.</p></li> |
| <li class="listitem"><p>"Definitely lost". This covers case 3 (for the BBB blocks) above. |
| This means that no pointer to the block can be found. The block is |
| classified as "lost", because the programmer could not possibly have |
| freed it at program exit, since no pointer to it exists. This is likely |
| a symptom of having lost the pointer at some earlier point in the |
| program. Such cases should be fixed by the programmer.</p></li> |
| <li class="listitem"><p>"Indirectly lost". This covers cases 4 and 9 (for the BBB blocks) |
| above. This means that the block is lost, not because there are no |
| pointers to it, but rather because all the blocks that point to it are |
| themselves lost. For example, if you have a binary tree and the root |
| node is lost, all its children nodes will be indirectly lost. Because |
| the problem will disappear if the definitely lost block that caused the |
| indirect leak is fixed, Memcheck won't report such blocks individually |
| by default.</p></li> |
| <li class="listitem"><p>"Possibly lost". This covers cases 5--8 (for the BBB blocks) |
| above. This means that a chain of one or more pointers to the block has |
| been found, but at least one of the pointers is an interior-pointer. |
| This could just be a random value in memory that happens to point into a |
| block, and so you shouldn't consider this ok unless you know you have |
| interior-pointers.</p></li> |
| </ul></div> |
| <p>(Note: This mapping of the nine possible cases onto four leak kinds is |
| not necessarily the best way that leaks could be reported; in particular, |
| interior-pointers are treated inconsistently. It is possible the |
| categorisation may be improved in the future.)</p> |
| <p>Furthermore, if suppressions exists for a block, it will be reported |
| as "suppressed" no matter what which of the above four kinds it belongs |
| to.</p> |
| <p>The following is an example leak summary.</p> |
| <pre class="programlisting"> |
| LEAK SUMMARY: |
| definitely lost: 48 bytes in 3 blocks. |
| indirectly lost: 32 bytes in 2 blocks. |
| possibly lost: 96 bytes in 6 blocks. |
| still reachable: 64 bytes in 4 blocks. |
| suppressed: 0 bytes in 0 blocks. |
| </pre> |
| <p>If heuristics have been used to consider some blocks as |
| reachable, the leak summary details the heuristically reachable subset |
| of 'still reachable:' per heuristic. In the below example, of the 95 |
| bytes still reachable, 87 bytes (56+7+8+16) have been considered |
| heuristically reachable. |
| </p> |
| <pre class="programlisting"> |
| LEAK SUMMARY: |
| definitely lost: 4 bytes in 1 blocks |
| indirectly lost: 0 bytes in 0 blocks |
| possibly lost: 0 bytes in 0 blocks |
| still reachable: 95 bytes in 6 blocks |
| of which reachable via heuristic: |
| stdstring : 56 bytes in 2 blocks |
| length64 : 16 bytes in 1 blocks |
| newarray : 7 bytes in 1 blocks |
| multipleinheritance: 8 bytes in 1 blocks |
| suppressed: 0 bytes in 0 blocks |
| </pre> |
| <p>If <code class="option">--leak-check=full</code> is specified, |
| Memcheck will give details for each definitely lost or possibly lost block, |
| including where it was allocated. (Actually, it merges results for all |
| blocks that have the same leak kind and sufficiently similar stack traces |
| into a single "loss record". The |
| <code class="option">--leak-resolution</code> lets you control the |
| meaning of "sufficiently similar".) It cannot tell you when or how or why |
| the pointer to a leaked block was lost; you have to work that out for |
| yourself. In general, you should attempt to ensure your programs do not |
| have any definitely lost or possibly lost blocks at exit.</p> |
| <p>For example:</p> |
| <pre class="programlisting"> |
| 8 bytes in 1 blocks are definitely lost in loss record 1 of 14 |
| at 0x........: malloc (vg_replace_malloc.c:...) |
| by 0x........: mk (leak-tree.c:11) |
| by 0x........: main (leak-tree.c:39) |
| |
| 88 (8 direct, 80 indirect) bytes in 1 blocks are definitely lost in loss record 13 of 14 |
| at 0x........: malloc (vg_replace_malloc.c:...) |
| by 0x........: mk (leak-tree.c:11) |
| by 0x........: main (leak-tree.c:25) |
| </pre> |
| <p>The first message describes a simple case of a single 8 byte block |
| that has been definitely lost. The second case mentions another 8 byte |
| block that has been definitely lost; the difference is that a further 80 |
| bytes in other blocks are indirectly lost because of this lost block. |
| The loss records are not presented in any notable order, so the loss record |
| numbers aren't particularly meaningful. The loss record numbers can be used |
| in the Valgrind gdbserver to list the addresses of the leaked blocks and/or give |
| more details about how a block is still reachable.</p> |
| <p>The option <code class="option">--show-leak-kinds=<set></code> |
| controls the set of leak kinds to show |
| when <code class="option">--leak-check=full</code> is specified. </p> |
| <p>The <code class="option"><set></code> of leak kinds is specified |
| in one of the following ways: |
| |
| </p> |
| <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "> |
| <li class="listitem"><p>a comma separated list of one or more of |
| <code class="option">definite indirect possible reachable</code>.</p></li> |
| <li class="listitem"><p><code class="option">all</code> to specify the complete set (all leak kinds).</p></li> |
| <li class="listitem"><p><code class="option">none</code> for the empty set.</p></li> |
| </ul></div> |
| <p> |
| |
| </p> |
| <p> The default value for the leak kinds to show is |
| <code class="option">--show-leak-kinds=definite,possible</code>. |
| </p> |
| <p>To also show the reachable and indirectly lost blocks in |
| addition to the definitely and possibly lost blocks, you can |
| use <code class="option">--show-leak-kinds=all</code>. To only show the |
| reachable and indirectly lost blocks, use |
| <code class="option">--show-leak-kinds=indirect,reachable</code>. The reachable |
| and indirectly lost blocks will then be presented as shown in |
| the following two examples.</p> |
| <pre class="programlisting"> |
| 64 bytes in 4 blocks are still reachable in loss record 2 of 4 |
| at 0x........: malloc (vg_replace_malloc.c:177) |
| by 0x........: mk (leak-cases.c:52) |
| by 0x........: main (leak-cases.c:74) |
| |
| 32 bytes in 2 blocks are indirectly lost in loss record 1 of 4 |
| at 0x........: malloc (vg_replace_malloc.c:177) |
| by 0x........: mk (leak-cases.c:52) |
| by 0x........: main (leak-cases.c:80) |
| </pre> |
| <p>Because there are different kinds of leaks with different |
| severities, an interesting question is: which leaks should be |
| counted as true "errors" and which should not? |
| </p> |
| <p> The answer to this question affects the numbers printed in |
| the <code class="computeroutput">ERROR SUMMARY</code> line, and also the |
| effect of the <code class="option">--error-exitcode</code> option. First, a leak |
| is only counted as a true "error" |
| if <code class="option">--leak-check=full</code> is specified. Then, the |
| option <code class="option">--errors-for-leak-kinds=<set></code> controls |
| the set of leak kinds to consider as errors. The default value |
| is <code class="option">--errors-for-leak-kinds=definite,possible</code> |
| </p> |
| </div> |
| </div> |
| <div class="sect1"> |
| <div class="titlepage"><div><div><h2 class="title" style="clear: both"> |
| <a name="mc-manual.options"></a>4.3. Memcheck Command-Line Options</h2></div></div></div> |
| <div class="variablelist"> |
| <a name="mc.opts.list"></a><dl class="variablelist"> |
| <dt> |
| <a name="opt.leak-check"></a><span class="term"> |
| <code class="option">--leak-check=<no|summary|yes|full> [default: summary] </code> |
| </span> |
| </dt> |
| <dd><p>When enabled, search for memory leaks when the client |
| program finishes. If set to <code class="varname">summary</code>, it says how |
| many leaks occurred. If set to <code class="varname">full</code> or |
| <code class="varname">yes</code>, each individual leak will be shown |
| in detail and/or counted as an error, as specified by the options |
| <code class="option">--show-leak-kinds</code> and |
| <code class="option">--errors-for-leak-kinds</code>. </p></dd> |
| <dt> |
| <a name="opt.leak-resolution"></a><span class="term"> |
| <code class="option">--leak-resolution=<low|med|high> [default: high] </code> |
| </span> |
| </dt> |
| <dd> |
| <p>When doing leak checking, determines how willing |
| Memcheck is to consider different backtraces to |
| be the same for the purposes of merging multiple leaks into a single |
| leak report. When set to <code class="varname">low</code>, only the first |
| two entries need match. When <code class="varname">med</code>, four entries |
| have to match. When <code class="varname">high</code>, all entries need to |
| match.</p> |
| <p>For hardcore leak debugging, you probably want to use |
| <code class="option">--leak-resolution=high</code> together with |
| <code class="option">--num-callers=40</code> or some such large number. |
| </p> |
| <p>Note that the <code class="option">--leak-resolution</code> setting |
| does not affect Memcheck's ability to find |
| leaks. It only changes how the results are presented.</p> |
| </dd> |
| <dt> |
| <a name="opt.show-leak-kinds"></a><span class="term"> |
| <code class="option">--show-leak-kinds=<set> [default: definite,possible] </code> |
| </span> |
| </dt> |
| <dd> |
| <p>Specifies the leak kinds to show in a <code class="varname">full</code> |
| leak search, in one of the following ways: </p> |
| <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "> |
| <li class="listitem"><p>a comma separated list of one or more of |
| <code class="option">definite indirect possible reachable</code>.</p></li> |
| <li class="listitem"><p><code class="option">all</code> to specify the complete set (all leak kinds). |
| It is equivalent to |
| <code class="option">--show-leak-kinds=definite,indirect,possible,reachable</code>.</p></li> |
| <li class="listitem"><p><code class="option">none</code> for the empty set.</p></li> |
| </ul></div> |
| </dd> |
| <dt> |
| <a name="opt.errors-for-leak-kinds"></a><span class="term"> |
| <code class="option">--errors-for-leak-kinds=<set> [default: definite,possible] </code> |
| </span> |
| </dt> |
| <dd><p>Specifies the leak kinds to count as errors in a |
| <code class="varname">full</code> leak search. The |
| <code class="option"><set></code> is specified similarly to |
| <code class="option">--show-leak-kinds</code> |
| </p></dd> |
| <dt> |
| <a name="opt.leak-check-heuristics"></a><span class="term"> |
| <code class="option">--leak-check-heuristics=<set> [default: all] </code> |
| </span> |
| </dt> |
| <dd> |
| <p>Specifies the set of leak check heuristics to be used |
| during leak searches. The heuristics control which interior pointers |
| to a block cause it to be considered as reachable. |
| The heuristic set is specified in one of the following ways:</p> |
| <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "> |
| <li class="listitem"><p>a comma separated list of one or more of |
| <code class="option">stdstring length64 newarray multipleinheritance</code>.</p></li> |
| <li class="listitem"><p><code class="option">all</code> to activate the complete set of |
| heuristics. |
| It is equivalent to |
| <code class="option">--leak-check-heuristics=stdstring,length64,newarray,multipleinheritance</code>.</p></li> |
| <li class="listitem"><p><code class="option">none</code> for the empty set.</p></li> |
| </ul></div> |
| </dd> |
| <dt> |
| <a name="opt.show-reachable"></a><span class="term"> |
| <code class="option">--show-reachable=<yes|no> </code> |
| , </span><span class="term"> |
| <code class="option">--show-possibly-lost=<yes|no> </code> |
| </span> |
| </dt> |
| <dd> |
| <p>These options provide an alternative way to specify the leak kinds to show: |
| </p> |
| <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "> |
| <li class="listitem"><p> |
| <code class="option">--show-reachable=no --show-possibly-lost=yes</code> is equivalent to |
| <code class="option">--show-leak-kinds=definite,possible</code>. |
| </p></li> |
| <li class="listitem"><p> |
| <code class="option">--show-reachable=no --show-possibly-lost=no</code> is equivalent to |
| <code class="option">--show-leak-kinds=definite</code>. |
| </p></li> |
| <li class="listitem"><p> |
| <code class="option">--show-reachable=yes</code> is equivalent to |
| <code class="option">--show-leak-kinds=all</code>. |
| </p></li> |
| </ul></div> |
| </dd> |
| <dt> |
| <a name="opt.xtree-leak"></a><span class="term"> |
| <code class="option">--xtree-leak=<no|yes> [no] </code> |
| </span> |
| </dt> |
| <dd> |
| <p>If set to yes, the results for the leak search done at exit will be |
| output in a 'Callgrind Format' execution tree file. Note that this |
| automatically sets the option <code class="option">--leak-check=full</code>. |
| The produced file |
| will contain the following events:</p> |
| <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "> |
| <li class="listitem"><p><code class="option">RB</code> : Reachable Bytes</p></li> |
| <li class="listitem"><p><code class="option">PB</code> : Possibly lost Bytes</p></li> |
| <li class="listitem"><p><code class="option">IB</code> : Indirectly lost Bytes</p></li> |
| <li class="listitem"><p><code class="option">DB</code> : Definitely lost Bytes (direct plus indirect)</p></li> |
| <li class="listitem"><p><code class="option">DIB</code> : Definitely Indirectly lost Bytes (subset of DB)</p></li> |
| <li class="listitem"><p><code class="option">RBk</code> : reachable Blocks</p></li> |
| <li class="listitem"><p><code class="option">PBk</code> : Possibly lost Blocks</p></li> |
| <li class="listitem"><p><code class="option">IBk</code> : Indirectly lost Blocks</p></li> |
| <li class="listitem"><p><code class="option">DBk</code> : Definitely lost Blocks</p></li> |
| </ul></div> |
| <p>The increase or decrease for all events above will also be output in |
| the file to provide the delta (increase or decreaseà between 2 |
| successive leak searches. For example, <code class="option">iRB</code> is the |
| increase of the <code class="option">RB</code> event, <code class="option">dPBk</code> is the |
| decrease of <code class="option">PBk</code> event. The values for the increase and |
| decrease events will be zero for the first leak search done.</p> |
| <p>See <a class="xref" href="manual-core.html#manual-core.xtree" title="2.9. Execution Trees">Execution Trees</a> for a detailed explanation |
| about execution trees.</p> |
| </dd> |
| <dt> |
| <a name="opt.xtree-leak-file"></a><span class="term"> |
| <code class="option">--xtree-leak-file=<filename> [default: |
| xtleak.kcg.%p] </code> |
| </span> |
| </dt> |
| <dd> |
| <p>Specifies that Valgrind should produce the xtree leak |
| report in the specified file. Any <code class="option">%p</code>, |
| <code class="option">%q</code> or <code class="option">%n</code> sequences appearing in |
| the filename are expanded |
| in exactly the same way as they are for <code class="option">--log-file</code>. |
| See the description of <a class="xref" href="manual-core.html#opt.log-file">--log-file</a> |
| for details. </p> |
| <p>See <a class="xref" href="manual-core.html#manual-core.xtree" title="2.9. Execution Trees">Execution Trees</a> |
| for a detailed explanation about execution trees formats. </p> |
| </dd> |
| <dt> |
| <a name="opt.undef-value-errors"></a><span class="term"> |
| <code class="option">--undef-value-errors=<yes|no> [default: yes] </code> |
| </span> |
| </dt> |
| <dd><p>Controls whether Memcheck reports |
| uses of undefined value errors. Set this to |
| <code class="varname">no</code> if you don't want to see undefined value |
| errors. It also has the side effect of speeding up |
| Memcheck somewhat. |
| </p></dd> |
| <dt> |
| <a name="opt.track-origins"></a><span class="term"> |
| <code class="option">--track-origins=<yes|no> [default: no] </code> |
| </span> |
| </dt> |
| <dd> |
| <p>Controls whether Memcheck tracks |
| the origin of uninitialised values. By default, it does not, |
| which means that although it can tell you that an |
| uninitialised value is being used in a dangerous way, it |
| cannot tell you where the uninitialised value came from. This |
| often makes it difficult to track down the root problem. |
| </p> |
| <p>When set |
| to <code class="varname">yes</code>, Memcheck keeps |
| track of the origins of all uninitialised values. Then, when |
| an uninitialised value error is |
| reported, Memcheck will try to show the |
| origin of the value. An origin can be one of the following |
| four places: a heap block, a stack allocation, a client |
| request, or miscellaneous other sources (eg, a call |
| to <code class="varname">brk</code>). |
| </p> |
| <p>For uninitialised values originating from a heap |
| block, Memcheck shows where the block was |
| allocated. For uninitialised values originating from a stack |
| allocation, Memcheck can tell you which |
| function allocated the value, but no more than that -- typically |
| it shows you the source location of the opening brace of the |
| function. So you should carefully check that all of the |
| function's local variables are initialised properly. |
| </p> |
| <p>Performance overhead: origin tracking is expensive. It |
| halves Memcheck's speed and increases |
| memory use by a minimum of 100MB, and possibly more. |
| Nevertheless it can drastically reduce the effort required to |
| identify the root cause of uninitialised value errors, and so |
| is often a programmer productivity win, despite running |
| more slowly. |
| </p> |
| <p>Accuracy: Memcheck tracks origins |
| quite accurately. To avoid very large space and time |
| overheads, some approximations are made. It is possible, |
| although unlikely, that Memcheck will report an incorrect origin, or |
| not be able to identify any origin. |
| </p> |
| <p>Note that the combination |
| <code class="option">--track-origins=yes</code> |
| and <code class="option">--undef-value-errors=no</code> is |
| nonsensical. Memcheck checks for and |
| rejects this combination at startup. |
| </p> |
| </dd> |
| <dt> |
| <a name="opt.partial-loads-ok"></a><span class="term"> |
| <code class="option">--partial-loads-ok=<yes|no> [default: yes] </code> |
| </span> |
| </dt> |
| <dd> |
| <p>Controls how Memcheck handles 32-, 64-, 128- and 256-bit |
| naturally aligned loads from addresses for which some bytes are |
| addressable and others are not. When <code class="varname">yes</code>, such |
| loads do not produce an address error. Instead, loaded bytes |
| originating from illegal addresses are marked as uninitialised, and |
| those corresponding to legal addresses are handled in the normal |
| way.</p> |
| <p>When <code class="varname">no</code>, loads from partially invalid |
| addresses are treated the same as loads from completely invalid |
| addresses: an illegal-address error is issued, and the resulting |
| bytes are marked as initialised.</p> |
| <p>Note that code that behaves in this way is in violation of |
| the ISO C/C++ standards, and should be considered broken. If |
| at all possible, such code should be fixed.</p> |
| </dd> |
| <dt> |
| <a name="opt.expensive-definedness-checks"></a><span class="term"> |
| <code class="option">--expensive-definedness-checks=<yes|no> [default: no] </code> |
| </span> |
| </dt> |
| <dd><p>Controls whether Memcheck should employ more precise but also more |
| expensive (time consuming) algorithms when checking the definedness of a |
| value. The default setting is not to do that and it is usually |
| sufficient. However, for highly optimised code valgrind may sometimes |
| incorrectly complain. |
| Invoking valgrind with <code class="option">--expensive-definedness-checks=yes</code> |
| helps but comes at a performance cost. Runtime degradation of |
| 25% have been observed but the extra cost depends a lot on the |
| application at hand. |
| </p></dd> |
| <dt> |
| <a name="opt.keep-stacktraces"></a><span class="term"> |
| <code class="option">--keep-stacktraces=alloc|free|alloc-and-free|alloc-then-free|none [default: alloc-and-free] </code> |
| </span> |
| </dt> |
| <dd> |
| <p>Controls which stack trace(s) to keep for malloc'd and/or |
| free'd blocks. |
| </p> |
| <p>With <code class="varname">alloc-then-free</code>, a stack trace is |
| recorded at allocation time, and is associated with the block. |
| When the block is freed, a second stack trace is recorded, and |
| this replaces the allocation stack trace. As a result, any "use |
| after free" errors relating to this block can only show a stack |
| trace for where the block was freed. |
| </p> |
| <p>With <code class="varname">alloc-and-free</code>, both allocation |
| and the deallocation stack traces for the block are stored. |
| Hence a "use after free" error will |
| show both, which may make the error easier to diagnose. |
| Compared to <code class="varname">alloc-then-free</code>, this setting |
| slightly increases Valgrind's memory use as the block contains two |
| references instead of one. |
| </p> |
| <p>With <code class="varname">alloc</code>, only the allocation stack |
| trace is recorded (and reported). With <code class="varname">free</code>, |
| only the deallocation stack trace is recorded (and reported). |
| These values somewhat decrease Valgrind's memory and cpu usage. |
| They can be useful depending on the error types you are |
| searching for and the level of detail you need to analyse |
| them. For example, if you are only interested in memory leak |
| errors, it is sufficient to record the allocation stack traces. |
| </p> |
| <p>With <code class="varname">none</code>, no stack traces are recorded |
| for malloc and free operations. If your program allocates a lot |
| of blocks and/or allocates/frees from many different stack |
| traces, this can significantly decrease cpu and/or memory |
| required. Of course, few details will be reported for errors |
| related to heap blocks. |
| </p> |
| <p>Note that once a stack trace is recorded, Valgrind keeps |
| the stack trace in memory even if it is not referenced by any |
| block. Some programs (for example, recursive algorithms) can |
| generate a huge number of stack traces. If Valgrind uses too |
| much memory in such circumstances, you can reduce the memory |
| required with the options <code class="varname">--keep-stacktraces</code> |
| and/or by using a smaller value for the |
| option <code class="varname">--num-callers</code>. |
| </p> |
| <p>If you want to use |
| <code class="computeroutput">--xtree-memory=full</code> memory profiling |
| (see <a class="xref" href="manual-core.html#manual-core.xtree" title="2.9. Execution Trees">Execution Trees</a> ), then you cannot |
| specify <code class="varname">--keep-stacktraces=free</code> |
| or <code class="varname">--keep-stacktraces=none</code>.</p> |
| </dd> |
| <dt> |
| <a name="opt.freelist-vol"></a><span class="term"> |
| <code class="option">--freelist-vol=<number> [default: 20000000] </code> |
| </span> |
| </dt> |
| <dd> |
| <p>When the client program releases memory using |
| <code class="function">free</code> (in <code class="literal">C</code>) or |
| <code class="computeroutput">delete</code> |
| (<code class="literal">C++</code>), that memory is not immediately made |
| available for re-allocation. Instead, it is marked inaccessible |
| and placed in a queue of freed blocks. The purpose is to defer as |
| long as possible the point at which freed-up memory comes back |
| into circulation. This increases the chance that |
| Memcheck will be able to detect invalid |
| accesses to blocks for some significant period of time after they |
| have been freed.</p> |
| <p>This option specifies the maximum total size, in bytes, of the |
| blocks in the queue. The default value is twenty million bytes. |
| Increasing this increases the total amount of memory used by |
| Memcheck but may detect invalid uses of freed |
| blocks which would otherwise go undetected.</p> |
| </dd> |
| <dt> |
| <a name="opt.freelist-big-blocks"></a><span class="term"> |
| <code class="option">--freelist-big-blocks=<number> [default: 1000000] </code> |
| </span> |
| </dt> |
| <dd> |
| <p>When making blocks from the queue of freed blocks available |
| for re-allocation, Memcheck will in priority re-circulate the blocks |
| with a size greater or equal to <code class="option">--freelist-big-blocks</code>. |
| This ensures that freeing big blocks (in particular freeing blocks bigger than |
| <code class="option">--freelist-vol</code>) does not immediately lead to a re-circulation |
| of all (or a lot of) the small blocks in the free list. In other words, |
| this option increases the likelihood to discover dangling pointers |
| for the "small" blocks, even when big blocks are freed.</p> |
| <p>Setting a value of 0 means that all the blocks are re-circulated |
| in a FIFO order. </p> |
| </dd> |
| <dt> |
| <a name="opt.workaround-gcc296-bugs"></a><span class="term"> |
| <code class="option">--workaround-gcc296-bugs=<yes|no> [default: no] </code> |
| </span> |
| </dt> |
| <dd> |
| <p>When enabled, assume that reads and writes some small |
| distance below the stack pointer are due to bugs in GCC 2.96, and |
| does not report them. The "small distance" is 256 bytes by |
| default. Note that GCC 2.96 is the default compiler on some ancient |
| Linux distributions (RedHat 7.X) and so you may need to use this |
| option. Do not use it if you do not have to, as it can cause real |
| errors to be overlooked. A better alternative is to use a more |
| recent GCC in which this bug is fixed.</p> |
| <p>You may also need to use this option when working with |
| GCC 3.X or 4.X on 32-bit PowerPC Linux. This is because |
| GCC generates code which occasionally accesses below the |
| stack pointer, particularly for floating-point to/from integer |
| conversions. This is in violation of the 32-bit PowerPC ELF |
| specification, which makes no provision for locations below the |
| stack pointer to be accessible.</p> |
| <p>This option is deprecated as of version 3.12 and may be |
| removed from future versions. You should instead use |
| <code class="option">--ignore-range-below-sp</code> to specify the exact |
| range of offsets below the stack pointer that should be ignored. |
| A suitable equivalent |
| is <code class="option">--ignore-range-below-sp=1024-1</code>. |
| </p> |
| </dd> |
| <dt> |
| <a name="opt.ignore-range-below-sp"></a><span class="term"> |
| <code class="option">--ignore-range-below-sp=<number>-<number> </code> |
| </span> |
| </dt> |
| <dd><p>This is a more general replacement for the deprecated |
| <code class="option">--workaround-gcc296-bugs</code> option. When |
| specified, it causes Memcheck not to report errors for accesses |
| at the specified offsets below the stack pointer. The two |
| offsets must be positive decimal numbers and -- somewhat |
| counterintuitively -- the first one must be larger, in order to |
| imply a non-wraparound address range to ignore. For example, |
| to ignore 4 byte accesses at 8192 bytes below the stack |
| pointer, |
| use <code class="option">--ignore-range-below-sp=8192-8189</code>. Only |
| one range may be specified. |
| </p></dd> |
| <dt> |
| <a name="opt.show-mismatched-frees"></a><span class="term"> |
| <code class="option">--show-mismatched-frees=<yes|no> [default: yes] </code> |
| </span> |
| </dt> |
| <dd> |
| <p>When enabled, Memcheck checks that heap blocks are |
| deallocated using a function that matches the allocating |
| function. That is, it expects <code class="varname">free</code> to be |
| used to deallocate blocks allocated |
| by <code class="varname">malloc</code>, <code class="varname">delete</code> for |
| blocks allocated by <code class="varname">new</code>, |
| and <code class="varname">delete[]</code> for blocks allocated |
| by <code class="varname">new[]</code>. If a mismatch is detected, an |
| error is reported. This is in general important because in some |
| environments, freeing with a non-matching function can cause |
| crashes.</p> |
| <p>There is however a scenario where such mismatches cannot |
| be avoided. That is when the user provides implementations of |
| <code class="varname">new</code>/<code class="varname">new[]</code> that |
| call <code class="varname">malloc</code> and |
| of <code class="varname">delete</code>/<code class="varname">delete[]</code> that |
| call <code class="varname">free</code>, and these functions are |
| asymmetrically inlined. For example, imagine |
| that <code class="varname">delete[]</code> is inlined |
| but <code class="varname">new[]</code> is not. The result is that |
| Memcheck "sees" all <code class="varname">delete[]</code> calls as direct |
| calls to <code class="varname">free</code>, even when the program source |
| contains no mismatched calls.</p> |
| <p>This causes a lot of confusing and irrelevant error |
| reports. <code class="varname">--show-mismatched-frees=no</code> disables |
| these checks. It is not generally advisable to disable them, |
| though, because you may miss real errors as a result.</p> |
| </dd> |
| <dt> |
| <a name="opt.ignore-ranges"></a><span class="term"> |
| <code class="option">--ignore-ranges=0xPP-0xQQ[,0xRR-0xSS] </code> |
| </span> |
| </dt> |
| <dd><p>Any ranges listed in this option (and multiple ranges can be |
| specified, separated by commas) will be ignored by Memcheck's |
| addressability checking.</p></dd> |
| <dt> |
| <a name="opt.malloc-fill"></a><span class="term"> |
| <code class="option">--malloc-fill=<hexnumber> </code> |
| </span> |
| </dt> |
| <dd><p>Fills blocks allocated |
| by <code class="computeroutput">malloc</code>, |
| <code class="computeroutput">new</code>, etc, but not |
| by <code class="computeroutput">calloc</code>, with the specified |
| byte. This can be useful when trying to shake out obscure |
| memory corruption problems. The allocated area is still |
| regarded by Memcheck as undefined -- this option only affects its |
| contents. Note that <code class="option">--malloc-fill</code> does not |
| affect a block of memory when it is used as argument |
| to client requests VALGRIND_MEMPOOL_ALLOC or |
| VALGRIND_MALLOCLIKE_BLOCK. |
| </p></dd> |
| <dt> |
| <a name="opt.free-fill"></a><span class="term"> |
| <code class="option">--free-fill=<hexnumber> </code> |
| </span> |
| </dt> |
| <dd><p>Fills blocks freed |
| by <code class="computeroutput">free</code>, |
| <code class="computeroutput">delete</code>, etc, with the |
| specified byte value. This can be useful when trying to shake out |
| obscure memory corruption problems. The freed area is still |
| regarded by Memcheck as not valid for access -- this option only |
| affects its contents. Note that <code class="option">--free-fill</code> does not |
| affect a block of memory when it is used as argument to |
| client requests VALGRIND_MEMPOOL_FREE or VALGRIND_FREELIKE_BLOCK. |
| </p></dd> |
| </dl> |
| </div> |
| </div> |
| <div class="sect1"> |
| <div class="titlepage"><div><div><h2 class="title" style="clear: both"> |
| <a name="mc-manual.suppfiles"></a>4.4. Writing suppression files</h2></div></div></div> |
| <p>The basic suppression format is described in |
| <a class="xref" href="manual-core.html#manual-core.suppress" title="2.5. Suppressing errors">Suppressing errors</a>.</p> |
| <p>The suppression-type (second) line should have the form:</p> |
| <pre class="programlisting"> |
| Memcheck:suppression_type</pre> |
| <p>The Memcheck suppression types are as follows:</p> |
| <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "> |
| <li class="listitem"><p><code class="varname">Value1</code>, |
| <code class="varname">Value2</code>, |
| <code class="varname">Value4</code>, |
| <code class="varname">Value8</code>, |
| <code class="varname">Value16</code>, |
| meaning an uninitialised-value error when |
| using a value of 1, 2, 4, 8 or 16 bytes.</p></li> |
| <li class="listitem"><p><code class="varname">Cond</code> (or its old |
| name, <code class="varname">Value0</code>), meaning use |
| of an uninitialised CPU condition code.</p></li> |
| <li class="listitem"><p><code class="varname">Addr1</code>, |
| <code class="varname">Addr2</code>, |
| <code class="varname">Addr4</code>, |
| <code class="varname">Addr8</code>, |
| <code class="varname">Addr16</code>, |
| meaning an invalid address during a |
| memory access of 1, 2, 4, 8 or 16 bytes respectively.</p></li> |
| <li class="listitem"><p><code class="varname">Jump</code>, meaning an |
| jump to an unaddressable location error.</p></li> |
| <li class="listitem"><p><code class="varname">Param</code>, meaning an |
| invalid system call parameter error.</p></li> |
| <li class="listitem"><p><code class="varname">Free</code>, meaning an |
| invalid or mismatching free.</p></li> |
| <li class="listitem"><p><code class="varname">Overlap</code>, meaning a |
| <code class="computeroutput">src</code> / |
| <code class="computeroutput">dst</code> overlap in |
| <code class="function">memcpy</code> or a similar function.</p></li> |
| <li class="listitem"><p><code class="varname">Leak</code>, meaning |
| a memory leak.</p></li> |
| </ul></div> |
| <p><code class="computeroutput">Param</code> errors have a mandatory extra |
| information line at this point, which is the name of the offending |
| system call parameter. </p> |
| <p><code class="computeroutput">Leak</code> errors have an optional |
| extra information line, with the following format:</p> |
| <pre class="programlisting"> |
| match-leak-kinds:<set></pre> |
| <p>where <code class="computeroutput"><set></code> specifies which |
| leak kinds are matched by this suppression entry. |
| <code class="computeroutput"><set></code> is specified in the |
| same way as with the option <code class="option">--show-leak-kinds</code>, that is, |
| one of the following:</p> |
| <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "> |
| <li class="listitem">a comma separated list of one or more of |
| <code class="option">definite indirect possible reachable</code>. |
| </li> |
| <li class="listitem"> |
| <code class="option">all</code> to specify the complete set (all leak kinds). |
| </li> |
| <li class="listitem"> |
| <code class="option">none</code> for the empty set. |
| </li> |
| </ul></div> |
| <p>If this optional extra line is not present, the suppression |
| entry will match all leak kinds.</p> |
| <p>Be aware that leak suppressions that are created using |
| <code class="option">--gen-suppressions</code> will contain this optional extra |
| line, and therefore may match fewer leaks than you expect. You may |
| want to remove the line before using the generated |
| suppressions.</p> |
| <p>The other Memcheck error kinds do not have extra lines.</p> |
| <p> |
| If you give the <code class="option">-v</code> option, Valgrind will print |
| the list of used suppressions at the end of execution. |
| For a leak suppression, this output gives the number of different |
| loss records that match the suppression, and the number of bytes |
| and blocks suppressed by the suppression. |
| If the run contains multiple leak checks, the number of bytes and blocks |
| are reset to zero before each new leak check. Note that the number of different |
| loss records is not reset to zero.</p> |
| <p>In the example below, in the last leak search, 7 blocks and 96 bytes have |
| been suppressed by a suppression with the name |
| <code class="option">some_leak_suppression</code>:</p> |
| <pre class="programlisting"> |
| --21041-- used_suppression: 10 some_other_leak_suppression s.supp:14 suppressed: 12,400 bytes in 1 blocks |
| --21041-- used_suppression: 39 some_leak_suppression s.supp:2 suppressed: 96 bytes in 7 blocks |
| </pre> |
| <p>For <code class="varname">ValueN</code> and <code class="varname">AddrN</code> |
| errors, the first line of the calling context is either the name of |
| the function in which the error occurred, or, failing that, the full |
| path of the <code class="filename">.so</code> file or executable containing the |
| error location. For <code class="varname">Free</code> errors, the first line is |
| the name of the function doing the freeing (eg, |
| <code class="function">free</code>, <code class="function">__builtin_vec_delete</code>, |
| etc). For <code class="varname">Overlap</code> errors, the first line is the name of the |
| function with the overlapping arguments (eg. |
| <code class="function">memcpy</code>, <code class="function">strcpy</code>, etc).</p> |
| <p>The last part of any suppression specifies the rest of the |
| calling context that needs to be matched.</p> |
| </div> |
| <div class="sect1"> |
| <div class="titlepage"><div><div><h2 class="title" style="clear: both"> |
| <a name="mc-manual.machine"></a>4.5. Details of Memcheck's checking machinery</h2></div></div></div> |
| <p>Read this section if you want to know, in detail, exactly |
| what and how Memcheck is checking.</p> |
| <div class="sect2"> |
| <div class="titlepage"><div><div><h3 class="title"> |
| <a name="mc-manual.value"></a>4.5.1. Valid-value (V) bits</h3></div></div></div> |
| <p>It is simplest to think of Memcheck implementing a synthetic CPU |
| which is identical to a real CPU, except for one crucial detail. Every |
| bit (literally) of data processed, stored and handled by the real CPU |
| has, in the synthetic CPU, an associated "valid-value" bit, which says |
| whether or not the accompanying bit has a legitimate value. In the |
| discussions which follow, this bit is referred to as the V (valid-value) |
| bit.</p> |
| <p>Each byte in the system therefore has a 8 V bits which follow it |
| wherever it goes. For example, when the CPU loads a word-size item (4 |
| bytes) from memory, it also loads the corresponding 32 V bits from a |
| bitmap which stores the V bits for the process' entire address space. |
| If the CPU should later write the whole or some part of that value to |
| memory at a different address, the relevant V bits will be stored back |
| in the V-bit bitmap.</p> |
| <p>In short, each bit in the system has (conceptually) an associated V |
| bit, which follows it around everywhere, even inside the CPU. Yes, all the |
| CPU's registers (integer, floating point, vector and condition registers) |
| have their own V bit vectors. For this to work, Memcheck uses a great deal |
| of compression to represent the V bits compactly.</p> |
| <p>Copying values around does not cause Memcheck to check for, or |
| report on, errors. However, when a value is used in a way which might |
| conceivably affect your program's externally-visible behaviour, |
| the associated V bits are immediately checked. If any of these indicate |
| that the value is undefined (even partially), an error is reported.</p> |
| <p>Here's an (admittedly nonsensical) example:</p> |
| <pre class="programlisting"> |
| int i, j; |
| int a[10], b[10]; |
| for ( i = 0; i < 10; i++ ) { |
| j = a[i]; |
| b[i] = j; |
| }</pre> |
| <p>Memcheck emits no complaints about this, since it merely copies |
| uninitialised values from <code class="varname">a[]</code> into |
| <code class="varname">b[]</code>, and doesn't use them in a way which could |
| affect the behaviour of the program. However, if |
| the loop is changed to:</p> |
| <pre class="programlisting"> |
| for ( i = 0; i < 10; i++ ) { |
| j += a[i]; |
| } |
| if ( j == 77 ) |
| printf("hello there\n"); |
| </pre> |
| <p>then Memcheck will complain, at the |
| <code class="computeroutput">if</code>, that the condition depends on |
| uninitialised values. Note that it <span class="command"><strong>doesn't</strong></span> complain |
| at the <code class="varname">j += a[i];</code>, since at that point the |
| undefinedness is not "observable". It's only when a decision has to be |
| made as to whether or not to do the <code class="function">printf</code> -- an |
| observable action of your program -- that Memcheck complains.</p> |
| <p>Most low level operations, such as adds, cause Memcheck to use the |
| V bits for the operands to calculate the V bits for the result. Even if |
| the result is partially or wholly undefined, it does not |
| complain.</p> |
| <p>Checks on definedness only occur in three places: when a value is |
| used to generate a memory address, when control flow decision needs to |
| be made, and when a system call is detected, Memcheck checks definedness |
| of parameters as required.</p> |
| <p>If a check should detect undefinedness, an error message is |
| issued. The resulting value is subsequently regarded as well-defined. |
| To do otherwise would give long chains of error messages. In other |
| words, once Memcheck reports an undefined value error, it tries to |
| avoid reporting further errors derived from that same undefined |
| value.</p> |
| <p>This sounds overcomplicated. Why not just check all reads from |
| memory, and complain if an undefined value is loaded into a CPU |
| register? Well, that doesn't work well, because perfectly legitimate C |
| programs routinely copy uninitialised values around in memory, and we |
| don't want endless complaints about that. Here's the canonical example. |
| Consider a struct like this:</p> |
| <pre class="programlisting"> |
| struct S { int x; char c; }; |
| struct S s1, s2; |
| s1.x = 42; |
| s1.c = 'z'; |
| s2 = s1; |
| </pre> |
| <p>The question to ask is: how large is <code class="varname">struct S</code>, |
| in bytes? An <code class="varname">int</code> is 4 bytes and a |
| <code class="varname">char</code> one byte, so perhaps a <code class="varname">struct |
| S</code> occupies 5 bytes? Wrong. All non-toy compilers we know |
| of will round the size of <code class="varname">struct S</code> up to a whole |
| number of words, in this case 8 bytes. Not doing this forces compilers |
| to generate truly appalling code for accessing arrays of |
| <code class="varname">struct S</code>'s on some architectures.</p> |
| <p>So <code class="varname">s1</code> occupies 8 bytes, yet only 5 of them will |
| be initialised. For the assignment <code class="varname">s2 = s1</code>, GCC |
| generates code to copy all 8 bytes wholesale into <code class="varname">s2</code> |
| without regard for their meaning. If Memcheck simply checked values as |
| they came out of memory, it would yelp every time a structure assignment |
| like this happened. So the more complicated behaviour described above |
| is necessary. This allows GCC to copy |
| <code class="varname">s1</code> into <code class="varname">s2</code> any way it likes, and a |
| warning will only be emitted if the uninitialised values are later |
| used.</p> |
| </div> |
| <div class="sect2"> |
| <div class="titlepage"><div><div><h3 class="title"> |
| <a name="mc-manual.vaddress"></a>4.5.2. Valid-address (A) bits</h3></div></div></div> |
| <p>Notice that the previous subsection describes how the validity of |
| values is established and maintained without having to say whether the |
| program does or does not have the right to access any particular memory |
| location. We now consider the latter question.</p> |
| <p>As described above, every bit in memory or in the CPU has an |
| associated valid-value (V) bit. In addition, all bytes in memory, but |
| not in the CPU, have an associated valid-address (A) bit. This |
| indicates whether or not the program can legitimately read or write that |
| location. It does not give any indication of the validity of the data |
| at that location -- that's the job of the V bits -- only whether or not |
| the location may be accessed.</p> |
| <p>Every time your program reads or writes memory, Memcheck checks |
| the A bits associated with the address. If any of them indicate an |
| invalid address, an error is emitted. Note that the reads and writes |
| themselves do not change the A bits, only consult them.</p> |
| <p>So how do the A bits get set/cleared? Like this:</p> |
| <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "> |
| <li class="listitem"><p>When the program starts, all the global data areas are |
| marked as accessible.</p></li> |
| <li class="listitem"><p>When the program does |
| <code class="function">malloc</code>/<code class="computeroutput">new</code>, |
| the A bits for exactly the area allocated, and not a byte more, |
| are marked as accessible. Upon freeing the area the A bits are |
| changed to indicate inaccessibility.</p></li> |
| <li class="listitem"><p>When the stack pointer register (<code class="literal">SP</code>) moves |
| up or down, A bits are set. The rule is that the area from |
| <code class="literal">SP</code> up to the base of the stack is marked as |
| accessible, and below <code class="literal">SP</code> is inaccessible. (If |
| that sounds illogical, bear in mind that the stack grows down, not |
| up, on almost all Unix systems, including GNU/Linux.) Tracking |
| <code class="literal">SP</code> like this has the useful side-effect that the |
| section of stack used by a function for local variables etc is |
| automatically marked accessible on function entry and inaccessible |
| on exit.</p></li> |
| <li class="listitem"><p>When doing system calls, A bits are changed appropriately. |
| For example, <code class="literal">mmap</code> |
| magically makes files appear in the process' |
| address space, so the A bits must be updated if <code class="literal">mmap</code> |
| succeeds.</p></li> |
| <li class="listitem"><p>Optionally, your program can tell Memcheck about such changes |
| explicitly, using the client request mechanism described |
| above.</p></li> |
| </ul></div> |
| </div> |
| <div class="sect2"> |
| <div class="titlepage"><div><div><h3 class="title"> |
| <a name="mc-manual.together"></a>4.5.3. Putting it all together</h3></div></div></div> |
| <p>Memcheck's checking machinery can be summarised as |
| follows:</p> |
| <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "> |
| <li class="listitem"><p>Each byte in memory has 8 associated V (valid-value) bits, |
| saying whether or not the byte has a defined value, and a single A |
| (valid-address) bit, saying whether or not the program currently has |
| the right to read/write that address. As mentioned above, heavy |
| use of compression means the overhead is typically around 25%.</p></li> |
| <li class="listitem"><p>When memory is read or written, the relevant A bits are |
| consulted. If they indicate an invalid address, Memcheck emits an |
| Invalid read or Invalid write error.</p></li> |
| <li class="listitem"><p>When memory is read into the CPU's registers, the relevant V |
| bits are fetched from memory and stored in the simulated CPU. They |
| are not consulted.</p></li> |
| <li class="listitem"><p>When a register is written out to memory, the V bits for that |
| register are written back to memory too.</p></li> |
| <li class="listitem"><p>When values in CPU registers are used to generate a memory |
| address, or to determine the outcome of a conditional branch, the V |
| bits for those values are checked, and an error emitted if any of |
| them are undefined.</p></li> |
| <li class="listitem"><p>When values in CPU registers are used for any other purpose, |
| Memcheck computes the V bits for the result, but does not check |
| them.</p></li> |
| <li class="listitem"><p>Once the V bits for a value in the CPU have been checked, they |
| are then set to indicate validity. This avoids long chains of |
| errors.</p></li> |
| <li class="listitem"> |
| <p>When values are loaded from memory, Memcheck checks the A bits |
| for that location and issues an illegal-address warning if needed. |
| In that case, the V bits loaded are forced to indicate Valid, |
| despite the location being invalid.</p> |
| <p>This apparently strange choice reduces the amount of confusing |
| information presented to the user. It avoids the unpleasant |
| phenomenon in which memory is read from a place which is both |
| unaddressable and contains invalid values, and, as a result, you get |
| not only an invalid-address (read/write) error, but also a |
| potentially large set of uninitialised-value errors, one for every |
| time the value is used.</p> |
| <p>There is a hazy boundary case to do with multi-byte loads from |
| addresses which are partially valid and partially invalid. See |
| details of the option <code class="option">--partial-loads-ok</code> for details. |
| </p> |
| </li> |
| </ul></div> |
| <p>Memcheck intercepts calls to <code class="function">malloc</code>, |
| <code class="function">calloc</code>, <code class="function">realloc</code>, |
| <code class="function">valloc</code>, <code class="function">memalign</code>, |
| <code class="function">free</code>, <code class="computeroutput">new</code>, |
| <code class="computeroutput">new[]</code>, |
| <code class="computeroutput">delete</code> and |
| <code class="computeroutput">delete[]</code>. The behaviour you get |
| is:</p> |
| <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "> |
| <li class="listitem"><p><code class="function">malloc</code>/<code class="function">new</code>/<code class="computeroutput">new[]</code>: |
| the returned memory is marked as addressable but not having valid |
| values. This means you have to write to it before you can read |
| it.</p></li> |
| <li class="listitem"><p><code class="function">calloc</code>: returned memory is marked both |
| addressable and valid, since <code class="function">calloc</code> clears |
| the area to zero.</p></li> |
| <li class="listitem"><p><code class="function">realloc</code>: if the new size is larger than |
| the old, the new section is addressable but invalid, as with |
| <code class="function">malloc</code>. If the new size is smaller, the |
| dropped-off section is marked as unaddressable. You may only pass to |
| <code class="function">realloc</code> a pointer previously issued to you by |
| <code class="function">malloc</code>/<code class="function">calloc</code>/<code class="function">realloc</code>.</p></li> |
| <li class="listitem"><p><code class="function">free</code>/<code class="computeroutput">delete</code>/<code class="computeroutput">delete[]</code>: |
| you may only pass to these functions a pointer previously issued |
| to you by the corresponding allocation function. Otherwise, |
| Memcheck complains. If the pointer is indeed valid, Memcheck |
| marks the entire area it points at as unaddressable, and places |
| the block in the freed-blocks-queue. The aim is to defer as long |
| as possible reallocation of this block. Until that happens, all |
| attempts to access it will elicit an invalid-address error, as you |
| would hope.</p></li> |
| </ul></div> |
| </div> |
| </div> |
| <div class="sect1"> |
| <div class="titlepage"><div><div><h2 class="title" style="clear: both"> |
| <a name="mc-manual.monitor-commands"></a>4.6. Memcheck Monitor Commands</h2></div></div></div> |
| <p>The Memcheck tool provides monitor commands handled by Valgrind's |
| built-in gdbserver (see <a class="xref" href="manual-core-adv.html#manual-core-adv.gdbserver-commandhandling" title="3.2.5. Monitor command handling by the Valgrind gdbserver">Monitor command handling by the Valgrind gdbserver</a>). |
| </p> |
| <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "> |
| <li class="listitem"> |
| <p><code class="varname">xb <addr> [<len>]</code> |
| shows the definedness (V) bits and values for <len> (default 1) |
| bytes starting at <addr>. |
| For each 8 bytes, two lines are output. |
| </p> |
| <p> |
| The first line shows the validity bits for 8 bytes. |
| The definedness of each byte in the range is given using two hexadecimal |
| digits. These hexadecimal digits encode the validity of each bit of the |
| corresponding byte, |
| using 0 if the bit is defined and 1 if the bit is undefined. |
| If a byte is not addressable, its validity bits are replaced |
| by <code class="varname">__</code> (a double underscore). |
| </p> |
| <p> |
| The second line shows the values of the bytes below the corresponding |
| validity bits. The format used to show the bytes data is similar to the |
| GDB command 'x /<len>xb <addr>'. The value for a non |
| addressable bytes is shown as ?? (two question marks). |
| </p> |
| <p> |
| In the following example, <code class="varname">string10</code> is an array |
| of 10 characters, in which the even numbered bytes are |
| undefined. In the below example, the byte corresponding |
| to <code class="varname">string10[5]</code> is not addressable. |
| </p> |
| <pre class="programlisting"> |
| (gdb) p &string10 |
| $4 = (char (*)[10]) 0x804a2f0 |
| (gdb) mo xb 0x804a2f0 10 |
| ff 00 ff 00 ff __ ff 00 |
| 0x804A2F0: 0x3f 0x6e 0x3f 0x65 0x3f 0x?? 0x3f 0x65 |
| ff 00 |
| 0x804A2F8: 0x3f 0x00 |
| Address 0x804A2F0 len 10 has 1 bytes unaddressable |
| (gdb) |
| </pre> |
| <p> The command xb cannot be used with registers. To get |
| the validity bits of a register, you must start Valgrind with the |
| option <code class="option">--vgdb-shadow-registers=yes</code>. The validity |
| bits of a register can then be obtained by printing the 'shadow 1' |
| corresponding register. In the below x86 example, the register |
| eax has all its bits undefined, while the register ebx is fully |
| defined. |
| </p> |
| <pre class="programlisting"> |
| (gdb) p /x $eaxs1 |
| $9 = 0xffffffff |
| (gdb) p /x $ebxs1 |
| $10 = 0x0 |
| (gdb) |
| </pre> |
| </li> |
| <li class="listitem"> |
| <p><code class="varname">get_vbits <addr> [<len>]</code> |
| shows the definedness (V) bits for <len> (default 1) bytes |
| starting at <addr> using the same convention as the |
| <code class="varname">xb</code> command. <code class="varname">get_vbits</code> only |
| shows the V bits (grouped by 4 bytes). It does not show the values. |
| If you want to associate V bits with the corresponding byte values, the |
| <code class="varname">xb</code> command will be easier to use, in particular |
| on little endian computers when associating undefined parts of an integer |
| with their V bits values. |
| </p> |
| <p> |
| The following example shows the result of <code class="varname">get_vibts</code> |
| on the <code class="varname">string10</code> used in the <code class="varname">xb</code> |
| command explanation. |
| </p> |
| <pre class="programlisting"> |
| (gdb) monitor get_vbits 0x804a2f0 10 |
| ff00ff00 ff__ff00 ff00 |
| Address 0x804A2F0 len 10 has 1 bytes unaddressable |
| (gdb) |
| </pre> |
| </li> |
| <li class="listitem"> |
| <p><code class="varname">make_memory |
| [noaccess|undefined|defined|Definedifaddressable] <addr> |
| [<len>]</code> marks the range of <len> (default 1) |
| bytes at <addr> as having the given status. Parameter |
| <code class="varname">noaccess</code> marks the range as non-accessible, so |
| Memcheck will report an error on any access to it. |
| <code class="varname">undefined</code> or <code class="varname">defined</code> mark |
| the area as accessible, but Memcheck regards the bytes in it |
| respectively as having undefined or defined values. |
| <code class="varname">Definedifaddressable</code> marks as defined, bytes in |
| the range which are already addressible, but makes no change to |
| the status of bytes in the range which are not addressible. Note |
| that the first letter of <code class="varname">Definedifaddressable</code> |
| is an uppercase D to avoid confusion with <code class="varname">defined</code>. |
| </p> |
| <p> |
| In the following example, the first byte of the |
| <code class="varname">string10</code> is marked as defined: |
| </p> |
| <pre class="programlisting"> |
| (gdb) monitor make_memory defined 0x8049e28 1 |
| (gdb) monitor get_vbits 0x8049e28 10 |
| 0000ff00 ff00ff00 ff00 |
| (gdb) |
| </pre> |
| </li> |
| <li class="listitem"> |
| <p><code class="varname">check_memory [addressable|defined] <addr> |
| [<len>]</code> checks that the range of <len> |
| (default 1) bytes at <addr> has the specified accessibility. |
| It then outputs a description of <addr>. In the following |
| example, a detailed description is available because the |
| option <code class="option">--read-var-info=yes</code> was given at Valgrind |
| startup: |
| </p> |
| <pre class="programlisting"> |
| (gdb) monitor check_memory defined 0x8049e28 1 |
| Address 0x8049E28 len 1 defined |
| ==14698== Location 0x8049e28 is 0 bytes inside string10[0], |
| ==14698== declared at prog.c:10, in frame #0 of thread 1 |
| (gdb) |
| </pre> |
| </li> |
| <li class="listitem"> |
| <p><code class="varname">leak_check [full*|summary|xtleak] |
| [kinds <set>|reachable|possibleleak*|definiteleak] |
| [heuristics heur1,heur2,...] |
| [increased*|changed|any] |
| [unlimited*|limited <max_loss_records_output>] |
| </code> |
| performs a leak check. The <code class="varname">*</code> in the arguments |
| indicates the default values. </p> |
| <p> If the <code class="varname">[full*|summary|xtleak]</code> argument is |
| <code class="varname">summary</code>, only a summary of the leak search is given; |
| otherwise a full leak report is produced. A full leak report gives |
| detailed information for each leak: the stack trace where the leaked blocks |
| were allocated, the number of blocks leaked and their total size. When a |
| full report is requested, the next two arguments further specify what |
| kind of leaks to report. A leak's details are shown if they match |
| both the second and third argument. A full leak report might |
| output detailed information for many leaks. The nr of leaks for |
| which information is output can be controlled using |
| the <code class="varname">limited</code> argument followed by the maximum nr |
| of leak records to output. If this maximum is reached, the leak |
| search outputs the records with the biggest number of bytes. |
| </p> |
| <p>The value <code class="varname">xtleak</code> also produces a full leak report, |
| but output it as an xtree in a file xtleak.kcg.%p.%n (see <a class="xref" href="manual-core.html#opt.log-file">--log-file</a>). |
| See <a class="xref" href="manual-core.html#manual-core.xtree" title="2.9. Execution Trees">Execution Trees</a> |
| for a detailed explanation about execution trees formats. |
| See <a class="xref" href="mc-manual.html#opt.xtree-leak">--xtree-leak</a> for the description of the events |
| in a xtree leak file. |
| </p> |
| <p>The <code class="varname">kinds</code> argument controls what kind of blocks |
| are shown for a <code class="varname">full</code> leak search. The set of leak kinds |
| to show can be specified using a <code class="varname"><set></code> similarly |
| to the command line option <code class="option">--show-leak-kinds</code>. |
| Alternatively, the value <code class="varname">definiteleak</code> |
| is equivalent to <code class="varname">kinds definite</code>, the |
| value <code class="varname">possibleleak</code> is equivalent to |
| <code class="varname">kinds definite,possible</code> : it will also show |
| possibly leaked blocks, .i.e those for which only an interior |
| pointer was found. The value <code class="varname">reachable</code> will |
| show all block categories (i.e. is equivalent to <code class="varname">kinds |
| all</code>). |
| </p> |
| <p>The <code class="varname">heuristics</code> argument controls the heuristics |
| used during the leak search. The set of heuristics to use can be specified |
| using a <code class="varname"><set></code> similarly |
| to the command line option <code class="option">--leak-check-heuristics</code>. |
| The default value for the <code class="varname">heuristics</code> argument is |
| <code class="varname">heuristics none</code>. |
| </p> |
| <p>The <code class="varname">[increased*|changed|any]</code> argument controls what |
| kinds of changes are shown for a <code class="varname">full</code> leak search. The |
| value <code class="varname">increased</code> specifies that only block |
| allocation stacks with an increased number of leaked bytes or |
| blocks since the previous leak check should be shown. The |
| value <code class="varname">changed</code> specifies that allocation stacks |
| with any change since the previous leak check should be shown. |
| The value <code class="varname">any</code> specifies that all leak entries |
| should be shown, regardless of any increase or decrease. When |
| If <code class="varname">increased</code> or <code class="varname">changed</code> are |
| specified, the leak report entries will show the delta relative to |
| the previous leak report. |
| </p> |
| <p>The following example shows usage of the |
| <code class="varname">leak_check</code> monitor command on |
| the <code class="varname">memcheck/tests/leak-cases.c</code> regression |
| test. The first command outputs one entry having an increase in |
| the leaked bytes. The second command is the same as the first |
| command, but uses the abbreviated forms accepted by GDB and the |
| Valgrind gdbserver. It only outputs the summary information, as |
| there was no increase since the previous leak search.</p> |
| <pre class="programlisting"> |
| (gdb) monitor leak_check full possibleleak increased |
| ==19520== 16 (+16) bytes in 1 (+1) blocks are possibly lost in loss record 9 of 12 |
| ==19520== at 0x40070B4: malloc (vg_replace_malloc.c:263) |
| ==19520== by 0x80484D5: mk (leak-cases.c:52) |
| ==19520== by 0x804855F: f (leak-cases.c:81) |
| ==19520== by 0x80488E0: main (leak-cases.c:107) |
| ==19520== |
| ==19520== LEAK SUMMARY: |
| ==19520== definitely lost: 32 (+0) bytes in 2 (+0) blocks |
| ==19520== indirectly lost: 16 (+0) bytes in 1 (+0) blocks |
| ==19520== possibly lost: 32 (+16) bytes in 2 (+1) blocks |
| ==19520== still reachable: 96 (+16) bytes in 6 (+1) blocks |
| ==19520== suppressed: 0 (+0) bytes in 0 (+0) blocks |
| ==19520== Reachable blocks (those to which a pointer was found) are not shown. |
| ==19520== To see them, add 'reachable any' args to leak_check |
| ==19520== |
| (gdb) mo l |
| ==19520== LEAK SUMMARY: |
| ==19520== definitely lost: 32 (+0) bytes in 2 (+0) blocks |
| ==19520== indirectly lost: 16 (+0) bytes in 1 (+0) blocks |
| ==19520== possibly lost: 32 (+0) bytes in 2 (+0) blocks |
| ==19520== still reachable: 96 (+0) bytes in 6 (+0) blocks |
| ==19520== suppressed: 0 (+0) bytes in 0 (+0) blocks |
| ==19520== Reachable blocks (those to which a pointer was found) are not shown. |
| ==19520== To see them, add 'reachable any' args to leak_check |
| ==19520== |
| (gdb) |
| </pre> |
| <p>Note that when using Valgrind's gdbserver, it is not |
| necessary to rerun |
| with <code class="option">--leak-check=full</code> |
| <code class="option">--show-reachable=yes</code> to see the reachable |
| blocks. You can obtain the same information without rerunning by |
| using the GDB command <code class="computeroutput">monitor leak_check full |
| reachable any</code> (or, using |
| abbreviation: <code class="computeroutput">mo l f r a</code>). |
| </p> |
| </li> |
| <li class="listitem"> |
| <p><code class="varname">block_list <loss_record_nr>|<loss_record_nr_from>..<loss_record_nr_to> |
| [unlimited*|limited <max_blocks>] |
| [heuristics heur1,heur2,...] |
| </code> |
| shows the list of blocks belonging to |
| <code class="varname"><loss_record_nr></code> (or to the loss records range |
| <code class="varname"><loss_record_nr_from>..<loss_record_nr_to></code>). |
| The nr of blocks to print can be controlled using the |
| <code class="varname">limited</code> argument followed by the maximum nr |
| of blocks to output. |
| If one or more heuristics are given, only prints the loss records |
| and blocks found via one of the given <code class="varname">heur1,heur2,...</code> |
| heuristics. |
| </p> |
| <p> A leak search merges the allocated blocks in loss records : |
| a loss record re-groups all blocks having the same state (for |
| example, Definitely Lost) and the same allocation backtrace. |
| Each loss record is identified in the leak search result |
| by a loss record number. |
| The <code class="varname">block_list</code> command shows the loss record information |
| followed by the addresses and sizes of the blocks which have been |
| merged in the loss record. If a block was found using an heuristic, the block size |
| is followed by the heuristic. |
| </p> |
| <p> If a directly lost block causes some other blocks to be indirectly |
| lost, the block_list command will also show these indirectly lost blocks. |
| The indirectly lost blocks will be indented according to the level of indirection |
| between the directly lost block and the indirectly lost block(s). |
| Each indirectly lost block is followed by the reference of its loss record. |
| </p> |
| <p> The block_list command can be used on the results of a leak search as long |
| as no block has been freed after this leak search: as soon as the program frees |
| a block, a new leak search is needed before block_list can be used again. |
| </p> |
| <p> |
| In the below example, the program leaks a tree structure by losing the pointer to |
| the block A (top of the tree). |
| So, the block A is directly lost, causing an indirect |
| loss of blocks B to G. The first block_list command shows the loss record of A |
| (a definitely lost block with address 0x4028028, size 16). The addresses and sizes |
| of the indirectly lost blocks due to block A are shown below the block A. |
| The second command shows the details of one of the indirect loss records output |
| by the first command. |
| </p> |
| <pre class="programlisting"> |
| A |
| / \ |
| B C |
| / \ / \ |
| D E F G |
| </pre> |
| <pre class="programlisting"> |
| (gdb) bt |
| #0 main () at leak-tree.c:69 |
| (gdb) monitor leak_check full any |
| ==19552== 112 (16 direct, 96 indirect) bytes in 1 blocks are definitely lost in loss record 7 of 7 |
| ==19552== at 0x40070B4: malloc (vg_replace_malloc.c:263) |
| ==19552== by 0x80484D5: mk (leak-tree.c:28) |
| ==19552== by 0x80484FC: f (leak-tree.c:41) |
| ==19552== by 0x8048856: main (leak-tree.c:63) |
| ==19552== |
| ==19552== LEAK SUMMARY: |
| ==19552== definitely lost: 16 bytes in 1 blocks |
| ==19552== indirectly lost: 96 bytes in 6 blocks |
| ==19552== possibly lost: 0 bytes in 0 blocks |
| ==19552== still reachable: 0 bytes in 0 blocks |
| ==19552== suppressed: 0 bytes in 0 blocks |
| ==19552== |
| (gdb) monitor block_list 7 |
| ==19552== 112 (16 direct, 96 indirect) bytes in 1 blocks are definitely lost in loss record 7 of 7 |
| ==19552== at 0x40070B4: malloc (vg_replace_malloc.c:263) |
| ==19552== by 0x80484D5: mk (leak-tree.c:28) |
| ==19552== by 0x80484FC: f (leak-tree.c:41) |
| ==19552== by 0x8048856: main (leak-tree.c:63) |
| ==19552== 0x4028028[16] |
| ==19552== 0x4028068[16] indirect loss record 1 |
| ==19552== 0x40280E8[16] indirect loss record 3 |
| ==19552== 0x4028128[16] indirect loss record 4 |
| ==19552== 0x40280A8[16] indirect loss record 2 |
| ==19552== 0x4028168[16] indirect loss record 5 |
| ==19552== 0x40281A8[16] indirect loss record 6 |
| (gdb) mo b 2 |
| ==19552== 16 bytes in 1 blocks are indirectly lost in loss record 2 of 7 |
| ==19552== at 0x40070B4: malloc (vg_replace_malloc.c:263) |
| ==19552== by 0x80484D5: mk (leak-tree.c:28) |
| ==19552== by 0x8048519: f (leak-tree.c:43) |
| ==19552== by 0x8048856: main (leak-tree.c:63) |
| ==19552== 0x40280A8[16] |
| ==19552== 0x4028168[16] indirect loss record 5 |
| ==19552== 0x40281A8[16] indirect loss record 6 |
| (gdb) |
| |
| </pre> |
| </li> |
| <li class="listitem"> |
| <p><code class="varname">who_points_at <addr> [<len>]</code> |
| shows all the locations where a pointer to addr is found. |
| If len is equal to 1, the command only shows the locations pointing |
| exactly at addr (i.e. the "start pointers" to addr). |
| If len is > 1, "interior pointers" pointing at the len first bytes |
| will also be shown. |
| </p> |
| <p>The locations searched for are the same as the locations |
| used in the leak search. So, <code class="varname">who_points_at</code> can a.o. |
| be used to show why the leak search still can reach a block, or can |
| search for dangling pointers to a freed block. |
| Each location pointing at addr (or pointing inside addr if interior pointers |
| are being searched for) will be described. |
| </p> |
| <p>In the below example, the pointers to the 'tree block A' (see example |
| in command <code class="varname">block_list</code>) is shown before the tree was leaked. |
| The descriptions are detailed as the option <code class="option">--read-var-info=yes</code> |
| was given at Valgrind startup. The second call shows the pointers (start and interior |
| pointers) to block G. The block G (0x40281A8) is reachable via block C (0x40280a8) |
| and register ECX of tid 1 (tid is the Valgrind thread id). |
| It is "interior reachable" via the register EBX. |
| </p> |
| <pre class="programlisting"> |
| (gdb) monitor who_points_at 0x4028028 |
| ==20852== Searching for pointers to 0x4028028 |
| ==20852== *0x8049e20 points at 0x4028028 |
| ==20852== Location 0x8049e20 is 0 bytes inside global var "t" |
| ==20852== declared at leak-tree.c:35 |
| (gdb) monitor who_points_at 0x40281A8 16 |
| ==20852== Searching for pointers pointing in 16 bytes from 0x40281a8 |
| ==20852== *0x40280ac points at 0x40281a8 |
| ==20852== Address 0x40280ac is 4 bytes inside a block of size 16 alloc'd |
| ==20852== at 0x40070B4: malloc (vg_replace_malloc.c:263) |
| ==20852== by 0x80484D5: mk (leak-tree.c:28) |
| ==20852== by 0x8048519: f (leak-tree.c:43) |
| ==20852== by 0x8048856: main (leak-tree.c:63) |
| ==20852== tid 1 register ECX points at 0x40281a8 |
| ==20852== tid 1 register EBX interior points at 2 bytes inside 0x40281a8 |
| (gdb) |
| </pre> |
| <p> When <code class="varname">who_points_at</code> finds an interior pointer, |
| it will report the heuristic(s) with which this interior pointer |
| will be considered as reachable. Note that this is done independently |
| of the value of the option <code class="option">--leak-check-heuristics</code>. |
| In the below example, the loss record 6 indicates a possibly lost |
| block. <code class="varname">who_points_at</code> reports that there is an interior |
| pointer pointing in this block, and that the block can be considered |
| reachable using the heuristic |
| <code class="computeroutput">multipleinheritance</code>. |
| </p> |
| <pre class="programlisting"> |
| (gdb) monitor block_list 6 |
| ==3748== 8 bytes in 1 blocks are possibly lost in loss record 6 of 7 |
| ==3748== at 0x4007D77: operator new(unsigned int) (vg_replace_malloc.c:313) |
| ==3748== by 0x8048954: main (leak_cpp_interior.cpp:43) |
| ==3748== 0x402A0E0[8] |
| (gdb) monitor who_points_at 0x402A0E0 8 |
| ==3748== Searching for pointers pointing in 8 bytes from 0x402a0e0 |
| ==3748== *0xbe8ee078 interior points at 4 bytes inside 0x402a0e0 |
| ==3748== Address 0xbe8ee078 is on thread 1's stack |
| ==3748== block at 0x402a0e0 considered reachable by ptr 0x402a0e4 using multipleinheritance heuristic |
| (gdb) |
| </pre> |
| </li> |
| </ul></div> |
| </div> |
| <div class="sect1"> |
| <div class="titlepage"><div><div><h2 class="title" style="clear: both"> |
| <a name="mc-manual.clientreqs"></a>4.7. Client Requests</h2></div></div></div> |
| <p>The following client requests are defined in |
| <code class="filename">memcheck.h</code>. |
| See <code class="filename">memcheck.h</code> for exact details of their |
| arguments.</p> |
| <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "> |
| <li class="listitem"><p><code class="varname">VALGRIND_MAKE_MEM_NOACCESS</code>, |
| <code class="varname">VALGRIND_MAKE_MEM_UNDEFINED</code> and |
| <code class="varname">VALGRIND_MAKE_MEM_DEFINED</code>. |
| These mark address ranges as completely inaccessible, |
| accessible but containing undefined data, and accessible and |
| containing defined data, respectively. They return -1, when |
| run on Valgrind and 0 otherwise.</p></li> |
| <li class="listitem"><p><code class="varname">VALGRIND_MAKE_MEM_DEFINED_IF_ADDRESSABLE</code>. |
| This is just like <code class="varname">VALGRIND_MAKE_MEM_DEFINED</code> but only |
| affects those bytes that are already addressable.</p></li> |
| <li class="listitem"><p><code class="varname">VALGRIND_CHECK_MEM_IS_ADDRESSABLE</code> and |
| <code class="varname">VALGRIND_CHECK_MEM_IS_DEFINED</code>: check immediately |
| whether or not the given address range has the relevant property, |
| and if not, print an error message. Also, for the convenience of |
| the client, returns zero if the relevant property holds; otherwise, |
| the returned value is the address of the first byte for which the |
| property is not true. Always returns 0 when not run on |
| Valgrind.</p></li> |
| <li class="listitem"><p><code class="varname">VALGRIND_CHECK_VALUE_IS_DEFINED</code>: a quick and easy |
| way to find out whether Valgrind thinks a particular value |
| (lvalue, to be precise) is addressable and defined. Prints an error |
| message if not. It has no return value.</p></li> |
| <li class="listitem"><p><code class="varname">VALGRIND_DO_LEAK_CHECK</code>: does a full memory leak |
| check (like <code class="option">--leak-check=full</code>) right now. |
| This is useful for incrementally checking for leaks between arbitrary |
| places in the program's execution. It has no return value.</p></li> |
| <li class="listitem"><p><code class="varname">VALGRIND_DO_ADDED_LEAK_CHECK</code>: same as |
| <code class="varname"> VALGRIND_DO_LEAK_CHECK</code> but only shows the |
| entries for which there was an increase in leaked bytes or leaked |
| number of blocks since the previous leak search. It has no return |
| value.</p></li> |
| <li class="listitem"><p><code class="varname">VALGRIND_DO_CHANGED_LEAK_CHECK</code>: same as |
| <code class="varname">VALGRIND_DO_LEAK_CHECK</code> but only shows the |
| entries for which there was an increase or decrease in leaked |
| bytes or leaked number of blocks since the previous leak search. It |
| has no return value.</p></li> |
| <li class="listitem"><p><code class="varname">VALGRIND_DO_QUICK_LEAK_CHECK</code>: like |
| <code class="varname">VALGRIND_DO_LEAK_CHECK</code>, except it produces only a leak |
| summary (like <code class="option">--leak-check=summary</code>). |
| It has no return value.</p></li> |
| <li class="listitem"><p><code class="varname">VALGRIND_COUNT_LEAKS</code>: fills in the four |
| arguments with the number of bytes of memory found by the previous |
| leak check to be leaked (i.e. the sum of direct leaks and indirect leaks), |
| dubious, reachable and suppressed. This is useful in test harness code, |
| after calling <code class="varname">VALGRIND_DO_LEAK_CHECK</code> or |
| <code class="varname">VALGRIND_DO_QUICK_LEAK_CHECK</code>.</p></li> |
| <li class="listitem"><p><code class="varname">VALGRIND_COUNT_LEAK_BLOCKS</code>: identical to |
| <code class="varname">VALGRIND_COUNT_LEAKS</code> except that it returns the |
| number of blocks rather than the number of bytes in each |
| category.</p></li> |
| <li class="listitem"><p><code class="varname">VALGRIND_GET_VBITS</code> and |
| <code class="varname">VALGRIND_SET_VBITS</code>: allow you to get and set the |
| V (validity) bits for an address range. You should probably only |
| set V bits that you have got with |
| <code class="varname">VALGRIND_GET_VBITS</code>. Only for those who really |
| know what they are doing.</p></li> |
| <li class="listitem"> |
| <p><code class="varname">VALGRIND_CREATE_BLOCK</code> and |
| <code class="varname">VALGRIND_DISCARD</code>. <code class="varname">VALGRIND_CREATE_BLOCK</code> |
| takes an address, a number of bytes and a character string. The |
| specified address range is then associated with that string. When |
| Memcheck reports an invalid access to an address in the range, it |
| will describe it in terms of this block rather than in terms of |
| any other block it knows about. Note that the use of this macro |
| does not actually change the state of memory in any way -- it |
| merely gives a name for the range. |
| </p> |
| <p>At some point you may want Memcheck to stop reporting errors |
| in terms of the block named |
| by <code class="varname">VALGRIND_CREATE_BLOCK</code>. To make this |
| possible, <code class="varname">VALGRIND_CREATE_BLOCK</code> returns a |
| "block handle", which is a C <code class="varname">int</code> value. You |
| can pass this block handle to <code class="varname">VALGRIND_DISCARD</code>. |
| After doing so, Valgrind will no longer relate addressing errors |
| in the specified range to the block. Passing invalid handles to |
| <code class="varname">VALGRIND_DISCARD</code> is harmless. |
| </p> |
| </li> |
| </ul></div> |
| </div> |
| <div class="sect1"> |
| <div class="titlepage"><div><div><h2 class="title" style="clear: both"> |
| <a name="mc-manual.mempools"></a>4.8. Memory Pools: describing and working with custom allocators</h2></div></div></div> |
| <p>Some programs use custom memory allocators, often for performance |
| reasons. Left to itself, Memcheck is unable to understand the |
| behaviour of custom allocation schemes as well as it understands the |
| standard allocators, and so may miss errors and leaks in your program. What |
| this section describes is a way to give Memcheck enough of a description of |
| your custom allocator that it can make at least some sense of what is |
| happening.</p> |
| <p>There are many different sorts of custom allocator, so Memcheck |
| attempts to reason about them using a loose, abstract model. We |
| use the following terminology when describing custom allocation |
| systems:</p> |
| <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "> |
| <li class="listitem"><p>Custom allocation involves a set of independent "memory pools". |
| </p></li> |
| <li class="listitem"><p>Memcheck's notion of a a memory pool consists of a single "anchor |
| address" and a set of non-overlapping "chunks" associated with the |
| anchor address.</p></li> |
| <li class="listitem"><p>Typically a pool's anchor address is the address of a |
| book-keeping "header" structure.</p></li> |
| <li class="listitem"><p>Typically the pool's chunks are drawn from a contiguous |
| "superblock" acquired through the system |
| <code class="function">malloc</code> or |
| <code class="function">mmap</code>.</p></li> |
| </ul></div> |
| <p>Keep in mind that the last two points above say "typically": the |
| Valgrind mempool client request API is intentionally vague about the |
| exact structure of a mempool. There is no specific mention made of |
| headers or superblocks. Nevertheless, the following picture may help |
| elucidate the intention of the terms in the API:</p> |
| <pre class="programlisting"> |
| "pool" |
| (anchor address) |
| | |
| v |
| +--------+---+ |
| | header | o | |
| +--------+-|-+ |
| | |
| v superblock |
| +------+---+--------------+---+------------------+ |
| | |rzB| allocation |rzB| | |
| +------+---+--------------+---+------------------+ |
| ^ ^ |
| | | |
| "addr" "addr"+"size" |
| </pre> |
| <p> |
| Note that the header and the superblock may be contiguous or |
| discontiguous, and there may be multiple superblocks associated with a |
| single header; such variations are opaque to Memcheck. The API |
| only requires that your allocation scheme can present sensible values |
| of "pool", "addr" and "size".</p> |
| <p> |
| Typically, before making client requests related to mempools, a client |
| program will have allocated such a header and superblock for their |
| mempool, and marked the superblock NOACCESS using the |
| <code class="varname">VALGRIND_MAKE_MEM_NOACCESS</code> client request.</p> |
| <p> |
| When dealing with mempools, the goal is to maintain a particular |
| invariant condition: that Memcheck believes the unallocated portions |
| of the pool's superblock (including redzones) are NOACCESS. To |
| maintain this invariant, the client program must ensure that the |
| superblock starts out in that state; Memcheck cannot make it so, since |
| Memcheck never explicitly learns about the superblock of a pool, only |
| the allocated chunks within the pool.</p> |
| <p> |
| Once the header and superblock for a pool are established and properly |
| marked, there are a number of client requests programs can use to |
| inform Memcheck about changes to the state of a mempool:</p> |
| <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "> |
| <li class="listitem"> |
| <p> |
| <code class="varname">VALGRIND_CREATE_MEMPOOL(pool, rzB, is_zeroed)</code>: |
| This request registers the address <code class="varname">pool</code> as the anchor |
| address for a memory pool. It also provides a size |
| <code class="varname">rzB</code>, specifying how large the redzones placed around |
| chunks allocated from the pool should be. Finally, it provides an |
| <code class="varname">is_zeroed</code> argument that specifies whether the pool's |
| chunks are zeroed (more precisely: defined) when allocated. |
| </p> |
| <p> |
| Upon completion of this request, no chunks are associated with the |
| pool. The request simply tells Memcheck that the pool exists, so that |
| subsequent calls can refer to it as a pool. |
| </p> |
| </li> |
| <li class="listitem"> |
| <p> |
| <code class="varname">VALGRIND_CREATE_MEMPOOL_EXT(pool, rzB, is_zeroed, flags)</code>: |
| Create a memory pool with some flags (that can |
| be OR-ed together) specifying extended behaviour. When flags is |
| zero, the behaviour is identical to |
| <code class="varname">VALGRIND_CREATE_MEMPOOL</code>.</p> |
| <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: circle; "> |
| <li class="listitem"><p> The flag <code class="varname">VALGRIND_MEMPOOL_METAPOOL</code> |
| specifies that the pieces of memory associated with the pool |
| using <code class="varname">VALGRIND_MEMPOOL_ALLOC</code> will be used |
| by the application as superblocks to dole out MALLOC_LIKE |
| blocks using <code class="varname">VALGRIND_MALLOCLIKE_BLOCK</code>. |
| In other words, a meta pool is a "2 levels" pool : first |
| level is the blocks described |
| by <code class="varname">VALGRIND_MEMPOOL_ALLOC</code>. The second |
| level blocks are described |
| using <code class="varname">VALGRIND_MALLOCLIKE_BLOCK</code>. Note |
| that the association between the pool and the second level |
| blocks is implicit : second level blocks will be located |
| inside first level blocks. It is necessary to use |
| the <code class="varname">VALGRIND_MEMPOOL_METAPOOL</code> flag for |
| such 2 levels pools, as otherwise valgrind will detect |
| overlapping memory blocks, and will abort execution |
| (e.g. during leak search). |
| </p></li> |
| <li class="listitem"><p> |
| <code class="varname">VALGRIND_MEMPOOL_AUTO_FREE</code>. Such a meta |
| pool can also be marked as an 'auto free' pool using the |
| flag <code class="varname">VALGRIND_MEMPOOL_AUTO_FREE</code>, which |
| must be OR-ed together with |
| the <code class="varname">VALGRIND_MEMPOOL_METAPOOL</code>. For an |
| 'auto free' pool, <code class="varname">VALGRIND_MEMPOOL_FREE</code> |
| will automatically free the second level blocks that are |
| contained inside the first level block freed |
| with <code class="varname">VALGRIND_MEMPOOL_FREE</code>. In other |
| words, calling <code class="varname">VALGRIND_MEMPOOL_FREE</code> will |
| cause implicit calls |
| to <code class="varname">VALGRIND_FREELIKE_BLOCK</code> for all the |
| second level blocks included in the first level block. |
| Note: it is an error to use |
| the <code class="varname">VALGRIND_MEMPOOL_AUTO_FREE</code> flag |
| without the |
| <code class="varname">VALGRIND_MEMPOOL_METAPOOL</code> flag. |
| </p></li> |
| </ul></div> |
| </li> |
| <li class="listitem"><p><code class="varname">VALGRIND_DESTROY_MEMPOOL(pool)</code>: |
| This request tells Memcheck that a pool is being torn down. Memcheck |
| then removes all records of chunks associated with the pool, as well |
| as its record of the pool's existence. While destroying its records of |
| a mempool, Memcheck resets the redzones of any live chunks in the pool |
| to NOACCESS. |
| </p></li> |
| <li class="listitem"><p><code class="varname">VALGRIND_MEMPOOL_ALLOC(pool, addr, size)</code>: |
| This request informs Memcheck that a <code class="varname">size</code>-byte chunk |
| has been allocated at <code class="varname">addr</code>, and associates the chunk with the |
| specified |
| <code class="varname">pool</code>. If the pool was created with nonzero |
| <code class="varname">rzB</code> redzones, Memcheck will mark the |
| <code class="varname">rzB</code> bytes before and after the chunk as NOACCESS. If |
| the pool was created with the <code class="varname">is_zeroed</code> argument set, |
| Memcheck will mark the chunk as DEFINED, otherwise Memcheck will mark |
| the chunk as UNDEFINED. |
| </p></li> |
| <li class="listitem"><p><code class="varname">VALGRIND_MEMPOOL_FREE(pool, addr)</code>: |
| This request informs Memcheck that the chunk at <code class="varname">addr</code> |
| should no longer be considered allocated. Memcheck will mark the chunk |
| associated with <code class="varname">addr</code> as NOACCESS, and delete its |
| record of the chunk's existence. |
| </p></li> |
| <li class="listitem"> |
| <p><code class="varname">VALGRIND_MEMPOOL_TRIM(pool, addr, size)</code>: |
| This request trims the chunks associated with <code class="varname">pool</code>. |
| The request only operates on chunks associated with |
| <code class="varname">pool</code>. Trimming is formally defined as:</p> |
| <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: circle; "> |
| <li class="listitem"><p> All chunks entirely inside the range |
| <code class="varname">addr..(addr+size-1)</code> are preserved.</p></li> |
| <li class="listitem"><p>All chunks entirely outside the range |
| <code class="varname">addr..(addr+size-1)</code> are discarded, as though |
| <code class="varname">VALGRIND_MEMPOOL_FREE</code> was called on them. </p></li> |
| <li class="listitem"><p>All other chunks must intersect with the range |
| <code class="varname">addr..(addr+size-1)</code>; areas outside the |
| intersection are marked as NOACCESS, as though they had been |
| independently freed with |
| <code class="varname">VALGRIND_MEMPOOL_FREE</code>.</p></li> |
| </ul></div> |
| <p>This is a somewhat rare request, but can be useful in |
| implementing the type of mass-free operations common in custom |
| LIFO allocators.</p> |
| </li> |
| <li class="listitem"> |
| <p><code class="varname">VALGRIND_MOVE_MEMPOOL(poolA, poolB)</code>: This |
| request informs Memcheck that the pool previously anchored at |
| address <code class="varname">poolA</code> has moved to anchor address |
| <code class="varname">poolB</code>. This is a rare request, typically only needed |
| if you <code class="function">realloc</code> the header of a mempool.</p> |
| <p>No memory-status bits are altered by this request.</p> |
| </li> |
| <li class="listitem"> |
| <p> |
| <code class="varname">VALGRIND_MEMPOOL_CHANGE(pool, addrA, addrB, |
| size)</code>: This request informs Memcheck that the chunk |
| previously allocated at address <code class="varname">addrA</code> within |
| <code class="varname">pool</code> has been moved and/or resized, and should be |
| changed to cover the region <code class="varname">addrB..(addrB+size-1)</code>. This |
| is a rare request, typically only needed if you |
| <code class="function">realloc</code> a superblock or wish to extend a chunk |
| without changing its memory-status bits. |
| </p> |
| <p>No memory-status bits are altered by this request. |
| </p> |
| </li> |
| <li class="listitem"><p><code class="varname">VALGRIND_MEMPOOL_EXISTS(pool)</code>: |
| This request informs the caller whether or not Memcheck is currently |
| tracking a mempool at anchor address <code class="varname">pool</code>. It |
| evaluates to 1 when there is a mempool associated with that address, 0 |
| otherwise. This is a rare request, only useful in circumstances when |
| client code might have lost track of the set of active mempools. |
| </p></li> |
| </ul></div> |
| </div> |
| <div class="sect1"> |
| <div class="titlepage"><div><div><h2 class="title" style="clear: both"> |
| <a name="mc-manual.mpiwrap"></a>4.9. Debugging MPI Parallel Programs with Valgrind</h2></div></div></div> |
| <p>Memcheck supports debugging of distributed-memory applications |
| which use the MPI message passing standard. This support consists of a |
| library of wrapper functions for the |
| <code class="computeroutput">PMPI_*</code> interface. When incorporated |
| into the application's address space, either by direct linking or by |
| <code class="computeroutput">LD_PRELOAD</code>, the wrappers intercept |
| calls to <code class="computeroutput">PMPI_Send</code>, |
| <code class="computeroutput">PMPI_Recv</code>, etc. They then |
| use client requests to inform Memcheck of memory state changes caused |
| by the function being wrapped. This reduces the number of false |
| positives that Memcheck otherwise typically reports for MPI |
| applications.</p> |
| <p>The wrappers also take the opportunity to carefully check |
| size and definedness of buffers passed as arguments to MPI functions, hence |
| detecting errors such as passing undefined data to |
| <code class="computeroutput">PMPI_Send</code>, or receiving data into a |
| buffer which is too small.</p> |
| <p>Unlike most of the rest of Valgrind, the wrapper library is subject to a |
| BSD-style license, so you can link it into any code base you like. |
| See the top of <code class="computeroutput">mpi/libmpiwrap.c</code> |
| for license details.</p> |
| <div class="sect2"> |
| <div class="titlepage"><div><div><h3 class="title"> |
| <a name="mc-manual.mpiwrap.build"></a>4.9.1. Building and installing the wrappers</h3></div></div></div> |
| <p> The wrapper library will be built automatically if possible. |
| Valgrind's configure script will look for a suitable |
| <code class="computeroutput">mpicc</code> to build it with. This must be |
| the same <code class="computeroutput">mpicc</code> you use to build the |
| MPI application you want to debug. By default, Valgrind tries |
| <code class="computeroutput">mpicc</code>, but you can specify a |
| different one by using the configure-time option |
| <code class="option">--with-mpicc</code>. Currently the |
| wrappers are only buildable with |
| <code class="computeroutput">mpicc</code>s which are based on GNU |
| GCC or Intel's C++ Compiler.</p> |
| <p>Check that the configure script prints a line like this:</p> |
| <pre class="programlisting"> |
| checking for usable MPI2-compliant mpicc and mpi.h... yes, mpicc |
| </pre> |
| <p>If it says <code class="computeroutput">... no</code>, your |
| <code class="computeroutput">mpicc</code> has failed to compile and link |
| a test MPI2 program.</p> |
| <p>If the configure test succeeds, continue in the usual way with |
| <code class="computeroutput">make</code> and <code class="computeroutput">make |
| install</code>. The final install tree should then contain |
| <code class="computeroutput">libmpiwrap-<platform>.so</code>. |
| </p> |
| <p>Compile up a test MPI program (eg, MPI hello-world) and try |
| this:</p> |
| <pre class="programlisting"> |
| LD_PRELOAD=$prefix/lib/valgrind/libmpiwrap-<platform>.so \ |
| mpirun [args] $prefix/bin/valgrind ./hello |
| </pre> |
| <p>You should see something similar to the following</p> |
| <pre class="programlisting"> |
| valgrind MPI wrappers 31901: Active for pid 31901 |
| valgrind MPI wrappers 31901: Try MPIWRAP_DEBUG=help for possible options |
| </pre> |
| <p>repeated for every process in the group. If you do not see |
| these, there is an build/installation problem of some kind.</p> |
| <p> The MPI functions to be wrapped are assumed to be in an ELF |
| shared object with soname matching |
| <code class="computeroutput">libmpi.so*</code>. This is known to be |
| correct at least for Open MPI and Quadrics MPI, and can easily be |
| changed if required.</p> |
| </div> |
| <div class="sect2"> |
| <div class="titlepage"><div><div><h3 class="title"> |
| <a name="mc-manual.mpiwrap.gettingstarted"></a>4.9.2. Getting started</h3></div></div></div> |
| <p>Compile your MPI application as usual, taking care to link it |
| using the same <code class="computeroutput">mpicc</code> that your |
| Valgrind build was configured with.</p> |
| <p> |
| Use the following basic scheme to run your application on Valgrind with |
| the wrappers engaged:</p> |
| <pre class="programlisting"> |
| MPIWRAP_DEBUG=[wrapper-args] \ |
| LD_PRELOAD=$prefix/lib/valgrind/libmpiwrap-<platform>.so \ |
| mpirun [mpirun-args] \ |
| $prefix/bin/valgrind [valgrind-args] \ |
| [application] [app-args] |
| </pre> |
| <p>As an alternative to |
| <code class="computeroutput">LD_PRELOAD</code>ing |
| <code class="computeroutput">libmpiwrap-<platform>.so</code>, you can |
| simply link it to your application if desired. This should not disturb |
| native behaviour of your application in any way.</p> |
| </div> |
| <div class="sect2"> |
| <div class="titlepage"><div><div><h3 class="title"> |
| <a name="mc-manual.mpiwrap.controlling"></a>4.9.3. Controlling the wrapper library</h3></div></div></div> |
| <p>Environment variable |
| <code class="computeroutput">MPIWRAP_DEBUG</code> is consulted at |
| startup. The default behaviour is to print a starting banner</p> |
| <pre class="programlisting"> |
| valgrind MPI wrappers 16386: Active for pid 16386 |
| valgrind MPI wrappers 16386: Try MPIWRAP_DEBUG=help for possible options |
| </pre> |
| <p> and then be relatively quiet.</p> |
| <p>You can give a list of comma-separated options in |
| <code class="computeroutput">MPIWRAP_DEBUG</code>. These are</p> |
| <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "> |
| <li class="listitem"><p><code class="computeroutput">verbose</code>: |
| show entries/exits of all wrappers. Also show extra |
| debugging info, such as the status of outstanding |
| <code class="computeroutput">MPI_Request</code>s resulting |
| from uncompleted <code class="computeroutput">MPI_Irecv</code>s.</p></li> |
| <li class="listitem"><p><code class="computeroutput">quiet</code>: |
| opposite of <code class="computeroutput">verbose</code>, only print |
| anything when the wrappers want |
| to report a detected programming error, or in case of catastrophic |
| failure of the wrappers.</p></li> |
| <li class="listitem"><p><code class="computeroutput">warn</code>: |
| by default, functions which lack proper wrappers |
| are not commented on, just silently |
| ignored. This causes a warning to be printed for each unwrapped |
| function used, up to a maximum of three warnings per function.</p></li> |
| <li class="listitem"><p><code class="computeroutput">strict</code>: |
| print an error message and abort the program if |
| a function lacking a wrapper is used.</p></li> |
| </ul></div> |
| <p> If you want to use Valgrind's XML output facility |
| (<code class="option">--xml=yes</code>), you should pass |
| <code class="computeroutput">quiet</code> in |
| <code class="computeroutput">MPIWRAP_DEBUG</code> so as to get rid of any |
| extraneous printing from the wrappers.</p> |
| </div> |
| <div class="sect2"> |
| <div class="titlepage"><div><div><h3 class="title"> |
| <a name="mc-manual.mpiwrap.limitations.functions"></a>4.9.4. Functions</h3></div></div></div> |
| <p>All MPI2 functions except |
| <code class="computeroutput">MPI_Wtick</code>, |
| <code class="computeroutput">MPI_Wtime</code> and |
| <code class="computeroutput">MPI_Pcontrol</code> have wrappers. The |
| first two are not wrapped because they return a |
| <code class="computeroutput">double</code>, which Valgrind's |
| function-wrap mechanism cannot handle (but it could easily be |
| extended to do so). <code class="computeroutput">MPI_Pcontrol</code> cannot be |
| wrapped as it has variable arity: |
| <code class="computeroutput">int MPI_Pcontrol(const int level, ...)</code></p> |
| <p>Most functions are wrapped with a default wrapper which does |
| nothing except complain or abort if it is called, depending on |
| settings in <code class="computeroutput">MPIWRAP_DEBUG</code> listed |
| above. The following functions have "real", do-something-useful |
| wrappers:</p> |
| <pre class="programlisting"> |
| PMPI_Send PMPI_Bsend PMPI_Ssend PMPI_Rsend |
| |
| PMPI_Recv PMPI_Get_count |
| |
| PMPI_Isend PMPI_Ibsend PMPI_Issend PMPI_Irsend |
| |
| PMPI_Irecv |
| PMPI_Wait PMPI_Waitall |
| PMPI_Test PMPI_Testall |
| |
| PMPI_Iprobe PMPI_Probe |
| |
| PMPI_Cancel |
| |
| PMPI_Sendrecv |
| |
| PMPI_Type_commit PMPI_Type_free |
| |
| PMPI_Pack PMPI_Unpack |
| |
| PMPI_Bcast PMPI_Gather PMPI_Scatter PMPI_Alltoall |
| PMPI_Reduce PMPI_Allreduce PMPI_Op_create |
| |
| PMPI_Comm_create PMPI_Comm_dup PMPI_Comm_free PMPI_Comm_rank PMPI_Comm_size |
| |
| PMPI_Error_string |
| PMPI_Init PMPI_Initialized PMPI_Finalize |
| </pre> |
| <p> A few functions such as |
| <code class="computeroutput">PMPI_Address</code> are listed as |
| <code class="computeroutput">HAS_NO_WRAPPER</code>. They have no wrapper |
| at all as there is nothing worth checking, and giving a no-op wrapper |
| would reduce performance for no reason.</p> |
| <p> Note that the wrapper library itself can itself generate large |
| numbers of calls to the MPI implementation, especially when walking |
| complex types. The most common functions called are |
| <code class="computeroutput">PMPI_Extent</code>, |
| <code class="computeroutput">PMPI_Type_get_envelope</code>, |
| <code class="computeroutput">PMPI_Type_get_contents</code>, and |
| <code class="computeroutput">PMPI_Type_free</code>. </p> |
| </div> |
| <div class="sect2"> |
| <div class="titlepage"><div><div><h3 class="title"> |
| <a name="mc-manual.mpiwrap.limitations.types"></a>4.9.5. Types</h3></div></div></div> |
| <p> MPI-1.1 structured types are supported, and walked exactly. |
| The currently supported combiners are |
| <code class="computeroutput">MPI_COMBINER_NAMED</code>, |
| <code class="computeroutput">MPI_COMBINER_CONTIGUOUS</code>, |
| <code class="computeroutput">MPI_COMBINER_VECTOR</code>, |
| <code class="computeroutput">MPI_COMBINER_HVECTOR</code> |
| <code class="computeroutput">MPI_COMBINER_INDEXED</code>, |
| <code class="computeroutput">MPI_COMBINER_HINDEXED</code> and |
| <code class="computeroutput">MPI_COMBINER_STRUCT</code>. This should |
| cover all MPI-1.1 types. The mechanism (function |
| <code class="computeroutput">walk_type</code>) should extend easily to |
| cover MPI2 combiners.</p> |
| <p>MPI defines some named structured types |
| (<code class="computeroutput">MPI_FLOAT_INT</code>, |
| <code class="computeroutput">MPI_DOUBLE_INT</code>, |
| <code class="computeroutput">MPI_LONG_INT</code>, |
| <code class="computeroutput">MPI_2INT</code>, |
| <code class="computeroutput">MPI_SHORT_INT</code>, |
| <code class="computeroutput">MPI_LONG_DOUBLE_INT</code>) which are pairs |
| of some basic type and a C <code class="computeroutput">int</code>. |
| Unfortunately the MPI specification makes it impossible to look inside |
| these types and see where the fields are. Therefore these wrappers |
| assume the types are laid out as <code class="computeroutput">struct { float val; |
| int loc; }</code> (for |
| <code class="computeroutput">MPI_FLOAT_INT</code>), etc, and act |
| accordingly. This appears to be correct at least for Open MPI 1.0.2 |
| and for Quadrics MPI.</p> |
| <p>If <code class="computeroutput">strict</code> is an option specified |
| in <code class="computeroutput">MPIWRAP_DEBUG</code>, the application |
| will abort if an unhandled type is encountered. Otherwise, the |
| application will print a warning message and continue.</p> |
| <p>Some effort is made to mark/check memory ranges corresponding to |
| arrays of values in a single pass. This is important for performance |
| since asking Valgrind to mark/check any range, no matter how small, |
| carries quite a large constant cost. This optimisation is applied to |
| arrays of primitive types (<code class="computeroutput">double</code>, |
| <code class="computeroutput">float</code>, |
| <code class="computeroutput">int</code>, |
| <code class="computeroutput">long</code>, <code class="computeroutput">long |
| long</code>, <code class="computeroutput">short</code>, |
| <code class="computeroutput">char</code>, and <code class="computeroutput">long |
| double</code> on platforms where <code class="computeroutput">sizeof(long |
| double) == 8</code>). For arrays of all other types, the |
| wrappers handle each element individually and so there can be a very |
| large performance cost.</p> |
| </div> |
| <div class="sect2"> |
| <div class="titlepage"><div><div><h3 class="title"> |
| <a name="mc-manual.mpiwrap.writingwrappers"></a>4.9.6. Writing new wrappers</h3></div></div></div> |
| <p> |
| For the most part the wrappers are straightforward. The only |
| significant complexity arises with nonblocking receives.</p> |
| <p>The issue is that <code class="computeroutput">MPI_Irecv</code> |
| states the recv buffer and returns immediately, giving a handle |
| (<code class="computeroutput">MPI_Request</code>) for the transaction. |
| Later the user will have to poll for completion with |
| <code class="computeroutput">MPI_Wait</code> etc, and when the |
| transaction completes successfully, the wrappers have to paint the |
| recv buffer. But the recv buffer details are not presented to |
| <code class="computeroutput">MPI_Wait</code> -- only the handle is. The |
| library therefore maintains a shadow table which associates |
| uncompleted <code class="computeroutput">MPI_Request</code>s with the |
| corresponding buffer address/count/type. When an operation completes, |
| the table is searched for the associated address/count/type info, and |
| memory is marked accordingly.</p> |
| <p>Access to the table is guarded by a (POSIX pthreads) lock, so as |
| to make the library thread-safe.</p> |
| <p>The table is allocated with |
| <code class="computeroutput">malloc</code> and never |
| <code class="computeroutput">free</code>d, so it will show up in leak |
| checks.</p> |
| <p>Writing new wrappers should be fairly easy. The source file is |
| <code class="computeroutput">mpi/libmpiwrap.c</code>. If possible, |
| find an existing wrapper for a function of similar behaviour to the |
| one you want to wrap, and use it as a starting point. The wrappers |
| are organised in sections in the same order as the MPI 1.1 spec, to |
| aid navigation. When adding a wrapper, remember to comment out the |
| definition of the default wrapper in the long list of defaults at the |
| bottom of the file (do not remove it, just comment it out).</p> |
| </div> |
| <div class="sect2"> |
| <div class="titlepage"><div><div><h3 class="title"> |
| <a name="mc-manual.mpiwrap.whattoexpect"></a>4.9.7. What to expect when using the wrappers</h3></div></div></div> |
| <p>The wrappers should reduce Memcheck's false-error rate on MPI |
| applications. Because the wrapping is done at the MPI interface, |
| there will still potentially be a large number of errors reported in |
| the MPI implementation below the interface. The best you can do is |
| try to suppress them.</p> |
| <p>You may also find that the input-side (buffer |
| length/definedness) checks find errors in your MPI use, for example |
| passing too short a buffer to |
| <code class="computeroutput">MPI_Recv</code>.</p> |
| <p>Functions which are not wrapped may increase the false |
| error rate. A possible approach is to run with |
| <code class="computeroutput">MPI_DEBUG</code> containing |
| <code class="computeroutput">warn</code>. This will show you functions |
| which lack proper wrappers but which are nevertheless used. You can |
| then write wrappers for them. |
| </p> |
| <p>A known source of potential false errors are the |
| <code class="computeroutput">PMPI_Reduce</code> family of functions, when |
| using a custom (user-defined) reduction function. In a reduction |
| operation, each node notionally sends data to a "central point" which |
| uses the specified reduction function to merge the data items into a |
| single item. Hence, in general, data is passed between nodes and fed |
| to the reduction function, but the wrapper library cannot mark the |
| transferred data as initialised before it is handed to the reduction |
| function, because all that happens "inside" the |
| <code class="computeroutput">PMPI_Reduce</code> call. As a result you |
| may see false positives reported in your reduction function.</p> |
| </div> |
| </div> |
| </div> |
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