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| |
| <chapter id="mc-manual" xreflabel="Memcheck: a memory error detector"> |
| <title>Memcheck: a memory error detector</title> |
| |
| <para>To use this tool, you may specify <option>--tool=memcheck</option> |
| on the Valgrind command line. You don't have to, though, since Memcheck |
| is the default tool.</para> |
| |
| |
| <sect1 id="mc-manual.overview" xreflabel="Overview"> |
| <title>Overview</title> |
| |
| <para>Memcheck is a memory error detector. It can detect the following |
| problems that are common in C and C++ programs.</para> |
| |
| <itemizedlist> |
| <listitem> |
| <para>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.</para> |
| </listitem> |
| |
| <listitem> |
| <para>Using undefined values, i.e. values that have not been initialised, |
| or that have been derived from other undefined values.</para> |
| </listitem> |
| |
| <listitem> |
| <para>Incorrect freeing of heap memory, such as double-freeing heap |
| blocks, or mismatched use of |
| <function>malloc</function>/<computeroutput>new</computeroutput>/<computeroutput>new[]</computeroutput> |
| versus |
| <function>free</function>/<computeroutput>delete</computeroutput>/<computeroutput>delete[]</computeroutput></para> |
| </listitem> |
| |
| <listitem> |
| <para>Overlapping <computeroutput>src</computeroutput> and |
| <computeroutput>dst</computeroutput> pointers in |
| <computeroutput>memcpy</computeroutput> and related |
| functions.</para> |
| </listitem> |
| |
| <listitem> |
| <para>Memory leaks.</para> |
| </listitem> |
| </itemizedlist> |
| |
| <para>Problems like these can be difficult to find by other means, |
| often remaining undetected for long periods, then causing occasional, |
| difficult-to-diagnose crashes.</para> |
| |
| </sect1> |
| |
| |
| |
| <sect1 id="mc-manual.errormsgs" |
| xreflabel="Explanation of error messages from Memcheck"> |
| <title>Explanation of error messages from Memcheck</title> |
| |
| <para>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 <xref |
| linkend="mc-manual.machine"/>.</para> |
| |
| |
| <sect2 id="mc-manual.badrw" |
| xreflabel="Illegal read / Illegal write errors"> |
| <title>Illegal read / Illegal write errors</title> |
| |
| <para>For example:</para> |
| <programlisting><![CDATA[ |
| 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 |
| ]]></programlisting> |
| |
| <para>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 <filename>qpngio.cpp</filename>, |
| and so on.</para> |
| |
| <para>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 <option><xref |
| linkend="opt.read-var-info"/></option> option Memcheck will run more slowly |
| but may give a more detailed description of any illegal address.</para> |
| |
| <para>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.</para> |
| |
| <para>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.</para> |
| |
| </sect2> |
| |
| |
| |
| <sect2 id="mc-manual.uninitvals" |
| xreflabel="Use of uninitialised values"> |
| <title>Use of uninitialised values</title> |
| |
| <para>For example:</para> |
| <programlisting><![CDATA[ |
| 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) |
| ]]></programlisting> |
| |
| <para>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 |
| <function>printf</function> machinery of the C library. This error was |
| reported when running the following small program:</para> |
| <programlisting><![CDATA[ |
| int main() |
| { |
| int x; |
| printf ("x = %d\n", x); |
| }]]></programlisting> |
| |
| <para>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, <varname>x</varname> is uninitialised. Memcheck observes |
| the value being passed to <function>_IO_printf</function> and thence to |
| <function>_IO_vfprintf</function>, but makes no comment. However, |
| <function>_IO_vfprintf</function> has to examine the value of |
| <varname>x</varname> so it can turn it into the corresponding ASCII string, |
| and it is at this point that Memcheck complains.</para> |
| |
| <para>Sources of uninitialised data tend to be:</para> |
| <itemizedlist> |
| <listitem> |
| <para>Local variables in procedures which have not been initialised, |
| as in the example above.</para> |
| </listitem> |
| <listitem> |
| <para>The contents of heap blocks (allocated with |
| <function>malloc</function>, <function>new</function>, or a similar |
| function) before you (or a constructor) write something there. |
| </para> |
| </listitem> |
| </itemizedlist> |
| |
| <para>To see information on the sources of uninitialised data in your |
| program, use the <option>--track-origins=yes</option> option. This |
| makes Memcheck run more slowly, but can make it much easier to track down |
| the root causes of uninitialised value errors.</para> |
| |
| </sect2> |
| |
| |
| |
| <sect2 id="mc-manual.bad-syscall-args" |
| xreflabel="Use of uninitialised or unaddressable values in system |
| calls"> |
| <title>Use of uninitialised or unaddressable values in system |
| calls</title> |
| |
| <para>Memcheck checks all parameters to system calls: |
| <itemizedlist> |
| <listitem> |
| <para>It checks all the direct parameters themselves, whether they are |
| initialised.</para> |
| </listitem> |
| <listitem> |
| <para>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.</para> |
| </listitem> |
| <listitem> |
| <para>Also, if the system call needs to write to a user-supplied |
| buffer, Memcheck checks that the buffer is addressable.</para> |
| </listitem> |
| </itemizedlist> |
| </para> |
| |
| <para>After the system call, Memcheck updates its tracked information to |
| precisely reflect any changes in memory state caused by the system |
| call.</para> |
| |
| <para>Here's an example of two system calls with invalid parameters:</para> |
| <programlisting><![CDATA[ |
| #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]); |
| } |
| ]]></programlisting> |
| |
| <para>You get these complaints ...</para> |
| <programlisting><![CDATA[ |
| 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) |
| ]]></programlisting> |
| |
| <para>... because the program has (a) written uninitialised junk |
| from the heap block to the standard output, and (b) passed an |
| uninitialised value to <function>exit</function>. Note that the first |
| error refers to the memory pointed to by |
| <computeroutput>buf</computeroutput> (not |
| <computeroutput>buf</computeroutput> itself), but the second error |
| refers directly to <computeroutput>exit</computeroutput>'s argument |
| <computeroutput>arr2[0]</computeroutput>.</para> |
| |
| </sect2> |
| |
| |
| <sect2 id="mc-manual.badfrees" xreflabel="Illegal frees"> |
| <title>Illegal frees</title> |
| |
| <para>For example:</para> |
| <programlisting><![CDATA[ |
| 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) |
| ]]></programlisting> |
| |
| <para>Memcheck keeps track of the blocks allocated by your program |
| with <function>malloc</function>/<computeroutput>new</computeroutput>, |
| so it can know exactly whether or not the argument to |
| <function>free</function>/<computeroutput>delete</computeroutput> 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.</para> |
| |
| </sect2> |
| |
| |
| <sect2 id="mc-manual.rudefn" |
| xreflabel="When a heap block is freed with an inappropriate deallocation |
| function"> |
| <title>When a heap block is freed with an inappropriate deallocation |
| function</title> |
| |
| <para>In the following example, a block allocated with |
| <function>new[]</function> has wrongly been deallocated with |
| <function>free</function>:</para> |
| <programlisting><![CDATA[ |
| 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) |
| ]]></programlisting> |
| |
| <para>In <literal>C++</literal> it's important to deallocate memory in a |
| way compatible with how it was allocated. The deal is:</para> |
| <itemizedlist> |
| <listitem> |
| <para>If allocated with |
| <function>malloc</function>, |
| <function>calloc</function>, |
| <function>realloc</function>, |
| <function>valloc</function> or |
| <function>memalign</function>, you must |
| deallocate with <function>free</function>.</para> |
| </listitem> |
| <listitem> |
| <para>If allocated with <function>new</function>, you must deallocate |
| with <function>delete</function>.</para> |
| </listitem> |
| <listitem> |
| <para>If allocated with <function>new[]</function>, you must |
| deallocate with <function>delete[]</function>.</para> |
| </listitem> |
| </itemizedlist> |
| |
| <para>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".</para> |
| |
| <para>The reason behind the requirement is as follows. In some C++ |
| implementations, <function>delete[]</function> must be used for |
| objects allocated by <function>new[]</function> 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. <function>delete</function> doesn't account for this and will get |
| confused, possibly corrupting the heap.</para> |
| |
| </sect2> |
| |
| |
| |
| <sect2 id="mc-manual.overlap" |
| xreflabel="Overlapping source and destination blocks"> |
| <title>Overlapping source and destination blocks</title> |
| |
| <para>The following C library functions copy some data from one |
| memory block to another (or something similar): |
| <function>memcpy</function>, |
| <function>strcpy</function>, |
| <function>strncpy</function>, |
| <function>strcat</function>, |
| <function>strncat</function>. |
| The blocks pointed to by their <computeroutput>src</computeroutput> and |
| <computeroutput>dst</computeroutput> 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. |
| </para> |
| |
| <para>For example:</para> |
| <programlisting><![CDATA[ |
| ==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) |
| ]]></programlisting> |
| |
| <para>You don't want the two blocks to overlap because one of them could |
| get partially overwritten by the copying.</para> |
| |
| <para>You might think that Memcheck is being overly pedantic reporting |
| this in the case where <computeroutput>dst</computeroutput> is less than |
| <computeroutput>src</computeroutput>. For example, the obvious way to |
| implement <function>memcpy</function> 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 <function>memcpy</function> zero |
| <computeroutput>dst</computeroutput> before copying, because zeroing the |
| destination's cache line(s) can improve performance.</para> |
| |
| <para>The moral of the story is: if you want to write truly portable |
| code, don't make any assumptions about the language |
| implementation.</para> |
| |
| </sect2> |
| |
| |
| <sect2 id="mc-manual.leaks" xreflabel="Memory leak detection"> |
| <title>Memory leak detection</title> |
| |
| <para>Memcheck keeps track of all heap blocks issued in response to |
| calls to |
| <function>malloc</function>/<function>new</function> et al. |
| So when the program exits, it knows which blocks have not been freed. |
| </para> |
| |
| <para>If <option>--leak-check</option> 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.</para> |
| |
| <para>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 |
| three ways we know of that an interior-pointer can occur:</para> |
| |
| <itemizedlist> |
| <listitem> |
| <para>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.</para> |
| </listitem> |
| |
| <listitem> |
| <para>It might be a random junk value in memory, entirely unrelated, just |
| a coincidence.</para> |
| </listitem> |
| |
| <listitem> |
| <para>It might be a pointer to an array of C++ objects (which possess |
| destructors) allocated with <computeroutput>new[]</computeroutput>. 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 <ulink url="http://theory.uwinnipeg.ca/gnu/gcc/gxxint_14.html">this |
| page</ulink> for more information.</para> |
| </listitem> |
| </itemizedlist> |
| |
| <para>With that in mind, consider the nine possible cases described by the |
| following figure.</para> |
| |
| <programlisting><![CDATA[ |
| Pointer chain AAA Category BBB Category |
| ------------- ------------ ------------ |
| (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 |
| |
| Category 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 |
| ]]></programlisting> |
| |
| <para>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 |
| categories.</para> |
| |
| |
| <itemizedlist> |
| |
| <listitem> |
| <para>"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. Because these |
| are very common and arguably not a problem, Memcheck won't report such |
| blocks individually unless <option>--show-reachable=yes</option> is |
| specified.</para> |
| </listitem> |
| |
| <listitem> |
| <para>"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.</para> |
| </listitem> |
| |
| <listitem> |
| <para>"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 |
| unless <option>--show-reachable=yes</option> is specified.</para> |
| </listitem> |
| |
| <listitem> |
| <para>"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.</para> |
| </listitem> |
| |
| </itemizedlist> |
| |
| <para>(Note: This mapping of the nine possible cases onto four categories 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.)</para> |
| |
| <para>Furthermore, if suppressions exists for a block, it will be reported |
| as "suppressed" no matter what which of the above four categories it belongs |
| to.</para> |
| |
| |
| <para>The following is an example leak summary.</para> |
| |
| <programlisting><![CDATA[ |
| 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. |
| ]]></programlisting> |
| |
| <para>If <option>--leak-check=full</option> 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 category and sufficiently similar stack traces |
| into a single "loss record". The |
| <option>--leak-resolution</option> 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.</para> |
| |
| <para>For example:</para> |
| <programlisting><![CDATA[ |
| 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) |
| ]]></programlisting> |
| |
| <para>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.</para> |
| |
| <para>If you specify <option>--show-reachable=yes</option>, |
| reachable and indirectly lost blocks will also be shown, as the following |
| two examples show.</para> |
| |
| <programlisting><![CDATA[ |
| 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) |
| ]]></programlisting> |
| |
| <para>Because there are different kinds of leaks with different severities, an |
| interesting question is this: which leaks should be counted as true "errors" |
| and which should not? The answer to this question affects the numbers printed |
| in the <computeroutput>ERROR SUMMARY</computeroutput> line, and also the effect |
| of the <option>--error-exitcode</option> option. Memcheck uses the following |
| criteria:</para> |
| |
| <itemizedlist> |
| <listitem> |
| <para>First, a leak is only counted as a true "error" if |
| <option>--leak-check=full</option> is specified. In other words, an |
| unprinted leak is not considered a true "error". If this were not the |
| case, it would be possible to get a high error count but not have any |
| errors printed, which would be confusing.</para> |
| </listitem> |
| |
| <listitem> |
| <para>After that, definitely lost and possibly lost blocks are counted as |
| true "errors". Indirectly lost and still reachable blocks are not counted |
| as true "errors", even if <option>--show-reachable=yes</option> is |
| specified and they are printed; this is because such blocks don't need |
| direct fixing by the programmer. |
| </para> |
| </listitem> |
| </itemizedlist> |
| |
| </sect2> |
| |
| </sect1> |
| |
| |
| |
| <sect1 id="mc-manual.options" |
| xreflabel="Memcheck Command-Line Options"> |
| <title>Memcheck Command-Line Options</title> |
| |
| <!-- start of xi:include in the manpage --> |
| <variablelist id="mc.opts.list"> |
| |
| <varlistentry id="opt.leak-check" xreflabel="--leak-check"> |
| <term> |
| <option><![CDATA[--leak-check=<no|summary|yes|full> [default: summary] ]]></option> |
| </term> |
| <listitem> |
| <para>When enabled, search for memory leaks when the client |
| program finishes. If set to <varname>summary</varname>, it says how |
| many leaks occurred. If set to <varname>full</varname> or |
| <varname>yes</varname>, it also gives details of each individual |
| leak.</para> |
| </listitem> |
| </varlistentry> |
| |
| <varlistentry id="opt.show-possibly-lost" xreflabel="--show-possibly-lost"> |
| <term> |
| <option><![CDATA[--show-possibly-lost=<yes|no> [default: yes] ]]></option> |
| </term> |
| <listitem> |
| <para>When disabled, the memory leak detector will not show "possibly lost" blocks. |
| </para> |
| </listitem> |
| </varlistentry> |
| |
| <varlistentry id="opt.leak-resolution" xreflabel="--leak-resolution"> |
| <term> |
| <option><![CDATA[--leak-resolution=<low|med|high> [default: high] ]]></option> |
| </term> |
| <listitem> |
| <para>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 <varname>low</varname>, only the first |
| two entries need match. When <varname>med</varname>, four entries |
| have to match. When <varname>high</varname>, all entries need to |
| match.</para> |
| |
| <para>For hardcore leak debugging, you probably want to use |
| <option>--leak-resolution=high</option> together with |
| <option>--num-callers=40</option> or some such large number. |
| </para> |
| |
| <para>Note that the <option>--leak-resolution</option> setting |
| does not affect Memcheck's ability to find |
| leaks. It only changes how the results are presented.</para> |
| </listitem> |
| </varlistentry> |
| |
| <varlistentry id="opt.show-reachable" xreflabel="--show-reachable"> |
| <term> |
| <option><![CDATA[--show-reachable=<yes|no> [default: no] ]]></option> |
| </term> |
| <listitem> |
| <para>When disabled, the memory leak detector only shows "definitely |
| lost" and "possibly lost" blocks. When enabled, the leak detector also |
| shows "reachable" and "indirectly lost" blocks. (In other words, it |
| shows all blocks, except suppressed ones, so |
| <option>--show-all</option> would be a better name for |
| it.)</para> |
| </listitem> |
| </varlistentry> |
| |
| <varlistentry id="opt.undef-value-errors" xreflabel="--undef-value-errors"> |
| <term> |
| <option><![CDATA[--undef-value-errors=<yes|no> [default: yes] ]]></option> |
| </term> |
| <listitem> |
| <para>Controls whether Memcheck reports |
| uses of undefined value errors. Set this to |
| <varname>no</varname> if you don't want to see undefined value |
| errors. It also has the side effect of speeding up |
| Memcheck somewhat. |
| </para> |
| </listitem> |
| </varlistentry> |
| |
| <varlistentry id="opt.track-origins" xreflabel="--track-origins"> |
| <term> |
| <option><![CDATA[--track-origins=<yes|no> [default: no] ]]></option> |
| </term> |
| <listitem> |
| <para>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. |
| </para> |
| <para>When set |
| to <varname>yes</varname>, 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 <varname>brk</varname>). |
| </para> |
| <para>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. |
| </para> |
| <para>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. |
| </para> |
| <para>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. |
| </para> |
| <para>Note that the combination |
| <option>--track-origins=yes</option> |
| and <option>--undef-value-errors=no</option> is |
| nonsensical. Memcheck checks for and |
| rejects this combination at startup. |
| </para> |
| </listitem> |
| </varlistentry> |
| |
| <varlistentry id="opt.partial-loads-ok" xreflabel="--partial-loads-ok"> |
| <term> |
| <option><![CDATA[--partial-loads-ok=<yes|no> [default: no] ]]></option> |
| </term> |
| <listitem> |
| <para>Controls how Memcheck handles word-sized, |
| word-aligned loads from addresses for which some bytes are |
| addressable and others are not. When <varname>yes</varname>, 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.</para> |
| |
| <para>When <varname>no</varname>, 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.</para> |
| |
| <para>Note that code that behaves in this way is in violation of |
| the the ISO C/C++ standards, and should be considered broken. If |
| at all possible, such code should be fixed. This option should be |
| used only as a last resort.</para> |
| </listitem> |
| </varlistentry> |
| |
| <varlistentry id="opt.freelist-vol" xreflabel="--freelist-vol"> |
| <term> |
| <option><![CDATA[--freelist-vol=<number> [default: 20000000] ]]></option> |
| </term> |
| <listitem> |
| <para>When the client program releases memory using |
| <function>free</function> (in <literal>C</literal>) or |
| <computeroutput>delete</computeroutput> |
| (<literal>C++</literal>), 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.</para> |
| |
| <para>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.</para> |
| </listitem> |
| </varlistentry> |
| |
| <varlistentry id="opt.workaround-gcc296-bugs" xreflabel="--workaround-gcc296-bugs"> |
| <term> |
| <option><![CDATA[--workaround-gcc296-bugs=<yes|no> [default: no] ]]></option> |
| </term> |
| <listitem> |
| <para>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.</para> |
| |
| <para>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.</para> |
| </listitem> |
| </varlistentry> |
| |
| <varlistentry id="opt.ignore-ranges" xreflabel="--ignore-ranges"> |
| <term> |
| <option><![CDATA[--ignore-ranges=0xPP-0xQQ[,0xRR-0xSS] ]]></option> |
| </term> |
| <listitem> |
| <para>Any ranges listed in this option (and multiple ranges can be |
| specified, separated by commas) will be ignored by Memcheck's |
| addressability checking.</para> |
| </listitem> |
| </varlistentry> |
| |
| <varlistentry id="opt.malloc-fill" xreflabel="--malloc-fill"> |
| <term> |
| <option><![CDATA[--malloc-fill=<hexnumber> ]]></option> |
| </term> |
| <listitem> |
| <para>Fills blocks allocated |
| by <computeroutput>malloc</computeroutput>, |
| <computeroutput>new</computeroutput>, etc, but not |
| by <computeroutput>calloc</computeroutput>, 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. |
| </para> |
| </listitem> |
| </varlistentry> |
| |
| <varlistentry id="opt.free-fill" xreflabel="--free-fill"> |
| <term> |
| <option><![CDATA[--free-fill=<hexnumber> ]]></option> |
| </term> |
| <listitem> |
| <para>Fills blocks freed |
| by <computeroutput>free</computeroutput>, |
| <computeroutput>delete</computeroutput>, 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. |
| </para> |
| </listitem> |
| </varlistentry> |
| |
| </variablelist> |
| <!-- end of xi:include in the manpage --> |
| |
| </sect1> |
| |
| |
| <sect1 id="mc-manual.suppfiles" xreflabel="Writing suppression files"> |
| <title>Writing suppression files</title> |
| |
| <para>The basic suppression format is described in |
| <xref linkend="manual-core.suppress"/>.</para> |
| |
| <para>The suppression-type (second) line should have the form:</para> |
| <programlisting><![CDATA[ |
| Memcheck:suppression_type]]></programlisting> |
| |
| <para>The Memcheck suppression types are as follows:</para> |
| |
| <itemizedlist> |
| <listitem> |
| <para><varname>Value1</varname>, |
| <varname>Value2</varname>, |
| <varname>Value4</varname>, |
| <varname>Value8</varname>, |
| <varname>Value16</varname>, |
| meaning an uninitialised-value error when |
| using a value of 1, 2, 4, 8 or 16 bytes.</para> |
| </listitem> |
| |
| <listitem> |
| <para><varname>Cond</varname> (or its old |
| name, <varname>Value0</varname>), meaning use |
| of an uninitialised CPU condition code.</para> |
| </listitem> |
| |
| <listitem> |
| <para><varname>Addr1</varname>, |
| <varname>Addr2</varname>, |
| <varname>Addr4</varname>, |
| <varname>Addr8</varname>, |
| <varname>Addr16</varname>, |
| meaning an invalid address during a |
| memory access of 1, 2, 4, 8 or 16 bytes respectively.</para> |
| </listitem> |
| |
| <listitem> |
| <para><varname>Jump</varname>, meaning an |
| jump to an unaddressable location error.</para> |
| </listitem> |
| |
| <listitem> |
| <para><varname>Param</varname>, meaning an |
| invalid system call parameter error.</para> |
| </listitem> |
| |
| <listitem> |
| <para><varname>Free</varname>, meaning an |
| invalid or mismatching free.</para> |
| </listitem> |
| |
| <listitem> |
| <para><varname>Overlap</varname>, meaning a |
| <computeroutput>src</computeroutput> / |
| <computeroutput>dst</computeroutput> overlap in |
| <function>memcpy</function> or a similar function.</para> |
| </listitem> |
| |
| <listitem> |
| <para><varname>Leak</varname>, meaning |
| a memory leak.</para> |
| </listitem> |
| |
| </itemizedlist> |
| |
| <para><computeroutput>Param</computeroutput> errors have an extra |
| information line at this point, which is the name of the offending |
| system call parameter. No other error kinds have this extra |
| line.</para> |
| |
| <para>The first line of the calling context: for <varname>ValueN</varname> |
| and <varname>AddrN</varname> errors, it is either the name of the function |
| in which the error occurred, or, failing that, the full path of the |
| <filename>.so</filename> file |
| or executable containing the error location. For <varname>Free</varname> errors, is the name |
| of the function doing the freeing (eg, <function>free</function>, |
| <function>__builtin_vec_delete</function>, etc). For |
| <varname>Overlap</varname> errors, is the name of the function with the |
| overlapping arguments (eg. <function>memcpy</function>, |
| <function>strcpy</function>, etc).</para> |
| |
| <para>Lastly, there's the rest of the calling context.</para> |
| |
| </sect1> |
| |
| |
| |
| <sect1 id="mc-manual.machine" |
| xreflabel="Details of Memcheck's checking machinery"> |
| <title>Details of Memcheck's checking machinery</title> |
| |
| <para>Read this section if you want to know, in detail, exactly |
| what and how Memcheck is checking.</para> |
| |
| |
| <sect2 id="mc-manual.value" xreflabel="Valid-value (V) bit"> |
| <title>Valid-value (V) bits</title> |
| |
| <para>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.</para> |
| |
| <para>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.</para> |
| |
| <para>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.</para> |
| |
| <para>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.</para> |
| |
| <para>Here's an (admittedly nonsensical) example:</para> |
| <programlisting><![CDATA[ |
| int i, j; |
| int a[10], b[10]; |
| for ( i = 0; i < 10; i++ ) { |
| j = a[i]; |
| b[i] = j; |
| }]]></programlisting> |
| |
| <para>Memcheck emits no complaints about this, since it merely copies |
| uninitialised values from <varname>a[]</varname> into |
| <varname>b[]</varname>, and doesn't use them in a way which could |
| affect the behaviour of the program. However, if |
| the loop is changed to:</para> |
| <programlisting><![CDATA[ |
| for ( i = 0; i < 10; i++ ) { |
| j += a[i]; |
| } |
| if ( j == 77 ) |
| printf("hello there\n"); |
| ]]></programlisting> |
| |
| <para>then Memcheck will complain, at the |
| <computeroutput>if</computeroutput>, that the condition depends on |
| uninitialised values. Note that it <command>doesn't</command> complain |
| at the <varname>j += a[i];</varname>, 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 <function>printf</function> -- an |
| observable action of your program -- that Memcheck complains.</para> |
| |
| <para>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.</para> |
| |
| <para>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.</para> |
| |
| <para>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.</para> |
| |
| <para>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:</para> |
| <programlisting><![CDATA[ |
| struct S { int x; char c; }; |
| struct S s1, s2; |
| s1.x = 42; |
| s1.c = 'z'; |
| s2 = s1; |
| ]]></programlisting> |
| |
| <para>The question to ask is: how large is <varname>struct S</varname>, |
| in bytes? An <varname>int</varname> is 4 bytes and a |
| <varname>char</varname> one byte, so perhaps a <varname>struct |
| S</varname> occupies 5 bytes? Wrong. All non-toy compilers we know |
| of will round the size of <varname>struct S</varname> 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 |
| <varname>struct S</varname>'s on some architectures.</para> |
| |
| <para>So <varname>s1</varname> occupies 8 bytes, yet only 5 of them will |
| be initialised. For the assignment <varname>s2 = s1</varname>, GCC |
| generates code to copy all 8 bytes wholesale into <varname>s2</varname> |
| 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 |
| <varname>s1</varname> into <varname>s2</varname> any way it likes, and a |
| warning will only be emitted if the uninitialised values are later |
| used.</para> |
| |
| </sect2> |
| |
| |
| <sect2 id="mc-manual.vaddress" xreflabel=" Valid-address (A) bits"> |
| <title>Valid-address (A) bits</title> |
| |
| <para>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.</para> |
| |
| <para>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 or the data |
| at that location -- that's the job of the V bits -- only whether or not |
| the location may be accessed.</para> |
| |
| <para>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.</para> |
| |
| <para>So how do the A bits get set/cleared? Like this:</para> |
| |
| <itemizedlist> |
| <listitem> |
| <para>When the program starts, all the global data areas are |
| marked as accessible.</para> |
| </listitem> |
| |
| <listitem> |
| <para>When the program does |
| <function>malloc</function>/<computeroutput>new</computeroutput>, |
| 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.</para> |
| </listitem> |
| |
| <listitem> |
| <para>When the stack pointer register (<literal>SP</literal>) moves |
| up or down, A bits are set. The rule is that the area from |
| <literal>SP</literal> up to the base of the stack is marked as |
| accessible, and below <literal>SP</literal> 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 |
| <literal>SP</literal> 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.</para> |
| </listitem> |
| |
| <listitem> |
| <para>When doing system calls, A bits are changed appropriately. |
| For example, <literal>mmap</literal> |
| magically makes files appear in the process' |
| address space, so the A bits must be updated if <literal>mmap</literal> |
| succeeds.</para> |
| </listitem> |
| |
| <listitem> |
| <para>Optionally, your program can tell Memcheck about such changes |
| explicitly, using the client request mechanism described |
| above.</para> |
| </listitem> |
| |
| </itemizedlist> |
| |
| </sect2> |
| |
| |
| <sect2 id="mc-manual.together" xreflabel="Putting it all together"> |
| <title>Putting it all together</title> |
| |
| <para>Memcheck's checking machinery can be summarised as |
| follows:</para> |
| |
| <itemizedlist> |
| <listitem> |
| <para>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%.</para> |
| </listitem> |
| |
| <listitem> |
| <para>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.</para> |
| </listitem> |
| |
| <listitem> |
| <para>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.</para> |
| </listitem> |
| |
| <listitem> |
| <para>When a register is written out to memory, the V bits for that |
| register are written back to memory too.</para> |
| </listitem> |
| |
| <listitem> |
| <para>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.</para> |
| </listitem> |
| |
| <listitem> |
| <para>When values in CPU registers are used for any other purpose, |
| Memcheck computes the V bits for the result, but does not check |
| them.</para> |
| </listitem> |
| |
| <listitem> |
| <para>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.</para> |
| </listitem> |
| |
| <listitem> |
| <para>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.</para> |
| |
| <para>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.</para> |
| |
| <para>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 <option>--partial-loads-ok</option> for details. |
| </para> |
| </listitem> |
| |
| </itemizedlist> |
| |
| |
| <para>Memcheck intercepts calls to <function>malloc</function>, |
| <function>calloc</function>, <function>realloc</function>, |
| <function>valloc</function>, <function>memalign</function>, |
| <function>free</function>, <computeroutput>new</computeroutput>, |
| <computeroutput>new[]</computeroutput>, |
| <computeroutput>delete</computeroutput> and |
| <computeroutput>delete[]</computeroutput>. The behaviour you get |
| is:</para> |
| |
| <itemizedlist> |
| |
| <listitem> |
| <para><function>malloc</function>/<function>new</function>/<computeroutput>new[]</computeroutput>: |
| 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.</para> |
| </listitem> |
| |
| <listitem> |
| <para><function>calloc</function>: returned memory is marked both |
| addressable and valid, since <function>calloc</function> clears |
| the area to zero.</para> |
| </listitem> |
| |
| <listitem> |
| <para><function>realloc</function>: if the new size is larger than |
| the old, the new section is addressable but invalid, as with |
| <function>malloc</function>. If the new size is smaller, the |
| dropped-off section is marked as unaddressable. You may only pass to |
| <function>realloc</function> a pointer previously issued to you by |
| <function>malloc</function>/<function>calloc</function>/<function>realloc</function>.</para> |
| </listitem> |
| |
| <listitem> |
| <para><function>free</function>/<computeroutput>delete</computeroutput>/<computeroutput>delete[]</computeroutput>: |
| 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.</para> |
| </listitem> |
| |
| </itemizedlist> |
| |
| </sect2> |
| </sect1> |
| |
| |
| |
| <sect1 id="mc-manual.clientreqs" xreflabel="Client requests"> |
| <title>Client Requests</title> |
| |
| <para>The following client requests are defined in |
| <filename>memcheck.h</filename>. |
| See <filename>memcheck.h</filename> for exact details of their |
| arguments.</para> |
| |
| <itemizedlist> |
| |
| <listitem> |
| <para><varname>VALGRIND_MAKE_MEM_NOACCESS</varname>, |
| <varname>VALGRIND_MAKE_MEM_UNDEFINED</varname> and |
| <varname>VALGRIND_MAKE_MEM_DEFINED</varname>. |
| These mark address ranges as completely inaccessible, |
| accessible but containing undefined data, and accessible and |
| containing defined data, respectively.</para> |
| </listitem> |
| |
| <listitem> |
| <para><varname>VALGRIND_MAKE_MEM_DEFINED_IF_ADDRESSABLE</varname>. |
| This is just like <varname>VALGRIND_MAKE_MEM_DEFINED</varname> but only |
| affects those bytes that are already addressable.</para> |
| </listitem> |
| |
| <listitem> |
| <para><varname>VALGRIND_CHECK_MEM_IS_ADDRESSABLE</varname> and |
| <varname>VALGRIND_CHECK_MEM_IS_DEFINED</varname>: 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.</para> |
| </listitem> |
| |
| <listitem> |
| <para><varname>VALGRIND_CHECK_VALUE_IS_DEFINED</varname>: 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.</para> |
| </listitem> |
| |
| <listitem> |
| <para><varname>VALGRIND_DO_LEAK_CHECK</varname>: does a full memory leak |
| check (like <option>--leak-check=full</option>) right now. |
| This is useful for incrementally checking for leaks between arbitrary |
| places in the program's execution. It has no return value.</para> |
| </listitem> |
| |
| <listitem> |
| <para><varname>VALGRIND_DO_QUICK_LEAK_CHECK</varname>: like |
| <varname>VALGRIND_DO_LEAK_CHECK</varname>, except it produces only a leak |
| summary (like <option>--leak-check=summary</option>). |
| It has no return value.</para> |
| </listitem> |
| |
| <listitem> |
| <para><varname>VALGRIND_COUNT_LEAKS</varname>: 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 <varname>VALGRIND_DO_LEAK_CHECK</varname> or |
| <varname>VALGRIND_DO_QUICK_LEAK_CHECK</varname>.</para> |
| </listitem> |
| |
| <listitem> |
| <para><varname>VALGRIND_COUNT_LEAK_BLOCKS</varname>: identical to |
| <varname>VALGRIND_COUNT_LEAKS</varname> except that it returns the |
| number of blocks rather than the number of bytes in each |
| category.</para> |
| </listitem> |
| |
| <listitem> |
| <para><varname>VALGRIND_GET_VBITS</varname> and |
| <varname>VALGRIND_SET_VBITS</varname>: 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 |
| <varname>VALGRIND_GET_VBITS</varname>. Only for those who really |
| know what they are doing.</para> |
| </listitem> |
| |
| <listitem> |
| <para><varname>VALGRIND_CREATE_BLOCK</varname> and |
| <varname>VALGRIND_DISCARD</varname>. <varname>VALGRIND_CREATE_BLOCK</varname> |
| 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. |
| </para> |
| |
| <para>At some point you may want Memcheck to stop reporting errors |
| in terms of the block named |
| by <varname>VALGRIND_CREATE_BLOCK</varname>. To make this |
| possible, <varname>VALGRIND_CREATE_BLOCK</varname> returns a |
| "block handle", which is a C <varname>int</varname> value. You |
| can pass this block handle to <varname>VALGRIND_DISCARD</varname>. |
| After doing so, Valgrind will no longer relate addressing errors |
| in the specified range to the block. Passing invalid handles to |
| <varname>VALGRIND_DISCARD</varname> is harmless. |
| </para> |
| </listitem> |
| |
| </itemizedlist> |
| |
| </sect1> |
| |
| |
| |
| |
| <sect1 id="mc-manual.mempools" xreflabel="Memory Pools"> |
| <title>Memory Pools: describing and working with custom allocators</title> |
| |
| <para>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.</para> |
| |
| <para>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:</para> |
| |
| <itemizedlist> |
| <listitem> |
| <para>Custom allocation involves a set of independent "memory pools". |
| </para> |
| </listitem> |
| <listitem> |
| <para>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.</para> |
| </listitem> |
| <listitem> |
| <para>Typically a pool's anchor address is the address of a |
| book-keeping "header" structure.</para> |
| </listitem> |
| <listitem> |
| <para>Typically the pool's chunks are drawn from a contiguous |
| "superblock" acquired through the system |
| <function>malloc</function> or |
| <function>mmap</function>.</para> |
| </listitem> |
| |
| </itemizedlist> |
| |
| <para>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:</para> |
| |
| <programlisting><![CDATA[ |
| "pool" |
| (anchor address) |
| | |
| v |
| +--------+---+ |
| | header | o | |
| +--------+-|-+ |
| | |
| v superblock |
| +------+---+--------------+---+------------------+ |
| | |rzB| allocation |rzB| | |
| +------+---+--------------+---+------------------+ |
| ^ ^ |
| | | |
| "addr" "addr"+"size" |
| ]]></programlisting> |
| |
| <para> |
| 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".</para> |
| |
| <para> |
| 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 |
| <varname>VALGRIND_MAKE_MEM_NOACCESS</varname> client request.</para> |
| |
| <para> |
| 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.</para> |
| |
| <para> |
| 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:</para> |
| |
| <itemizedlist> |
| |
| <listitem> |
| <para> |
| <varname>VALGRIND_CREATE_MEMPOOL(pool, rzB, is_zeroed)</varname>: |
| This request registers the address <varname>pool</varname> as the anchor |
| address for a memory pool. It also provides a size |
| <varname>rzB</varname>, specifying how large the redzones placed around |
| chunks allocated from the pool should be. Finally, it provides an |
| <varname>is_zeroed</varname> argument that specifies whether the pool's |
| chunks are zeroed (more precisely: defined) when allocated. |
| </para> |
| <para> |
| 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. |
| </para> |
| </listitem> |
| |
| <listitem> |
| <para><varname>VALGRIND_DESTROY_MEMPOOL(pool)</varname>: |
| 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. |
| </para> |
| </listitem> |
| |
| <listitem> |
| <para><varname>VALGRIND_MEMPOOL_ALLOC(pool, addr, size)</varname>: |
| This request informs Memcheck that a <varname>size</varname>-byte chunk |
| has been allocated at <varname>addr</varname>, and associates the chunk with the |
| specified |
| <varname>pool</varname>. If the pool was created with nonzero |
| <varname>rzB</varname> redzones, Memcheck will mark the |
| <varname>rzB</varname> bytes before and after the chunk as NOACCESS. If |
| the pool was created with the <varname>is_zeroed</varname> argument set, |
| Memcheck will mark the chunk as DEFINED, otherwise Memcheck will mark |
| the chunk as UNDEFINED. |
| </para> |
| </listitem> |
| |
| <listitem> |
| <para><varname>VALGRIND_MEMPOOL_FREE(pool, addr)</varname>: |
| This request informs Memcheck that the chunk at <varname>addr</varname> |
| should no longer be considered allocated. Memcheck will mark the chunk |
| associated with <varname>addr</varname> as NOACCESS, and delete its |
| record of the chunk's existence. |
| </para> |
| </listitem> |
| |
| <listitem> |
| <para><varname>VALGRIND_MEMPOOL_TRIM(pool, addr, size)</varname>: |
| This request trims the chunks associated with <varname>pool</varname>. |
| The request only operates on chunks associated with |
| <varname>pool</varname>. Trimming is formally defined as:</para> |
| <itemizedlist> |
| <listitem> |
| <para> All chunks entirely inside the range |
| <varname>addr..(addr+size-1)</varname> are preserved.</para> |
| </listitem> |
| <listitem> |
| <para>All chunks entirely outside the range |
| <varname>addr..(addr+size-1)</varname> are discarded, as though |
| <varname>VALGRIND_MEMPOOL_FREE</varname> was called on them. </para> |
| </listitem> |
| <listitem> |
| <para>All other chunks must intersect with the range |
| <varname>addr..(addr+size-1)</varname>; areas outside the |
| intersection are marked as NOACCESS, as though they had been |
| independently freed with |
| <varname>VALGRIND_MEMPOOL_FREE</varname>.</para> |
| </listitem> |
| </itemizedlist> |
| <para>This is a somewhat rare request, but can be useful in |
| implementing the type of mass-free operations common in custom |
| LIFO allocators.</para> |
| </listitem> |
| |
| <listitem> |
| <para><varname>VALGRIND_MOVE_MEMPOOL(poolA, poolB)</varname>: This |
| request informs Memcheck that the pool previously anchored at |
| address <varname>poolA</varname> has moved to anchor address |
| <varname>poolB</varname>. This is a rare request, typically only needed |
| if you <function>realloc</function> the header of a mempool.</para> |
| <para>No memory-status bits are altered by this request.</para> |
| </listitem> |
| |
| <listitem> |
| <para> |
| <varname>VALGRIND_MEMPOOL_CHANGE(pool, addrA, addrB, |
| size)</varname>: This request informs Memcheck that the chunk |
| previously allocated at address <varname>addrA</varname> within |
| <varname>pool</varname> has been moved and/or resized, and should be |
| changed to cover the region <varname>addrB..(addrB+size-1)</varname>. This |
| is a rare request, typically only needed if you |
| <function>realloc</function> a superblock or wish to extend a chunk |
| without changing its memory-status bits. |
| </para> |
| <para>No memory-status bits are altered by this request. |
| </para> |
| </listitem> |
| |
| <listitem> |
| <para><varname>VALGRIND_MEMPOOL_EXISTS(pool)</varname>: |
| This request informs the caller whether or not Memcheck is currently |
| tracking a mempool at anchor address <varname>pool</varname>. 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. |
| </para> |
| </listitem> |
| |
| </itemizedlist> |
| |
| </sect1> |
| |
| |
| |
| |
| |
| |
| |
| <sect1 id="mc-manual.mpiwrap" xreflabel="MPI Wrappers"> |
| <title>Debugging MPI Parallel Programs with Valgrind</title> |
| |
| <para>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 |
| <computeroutput>PMPI_*</computeroutput> interface. When incorporated |
| into the application's address space, either by direct linking or by |
| <computeroutput>LD_PRELOAD</computeroutput>, the wrappers intercept |
| calls to <computeroutput>PMPI_Send</computeroutput>, |
| <computeroutput>PMPI_Recv</computeroutput>, 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.</para> |
| |
| <para>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 |
| <computeroutput>PMPI_Send</computeroutput>, or receiving data into a |
| buffer which is too small.</para> |
| |
| <para>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 <computeroutput>mpi/libmpiwrap.c</computeroutput> |
| for license details.</para> |
| |
| |
| <sect2 id="mc-manual.mpiwrap.build" xreflabel="Building MPI Wrappers"> |
| <title>Building and installing the wrappers</title> |
| |
| <para> The wrapper library will be built automatically if possible. |
| Valgrind's configure script will look for a suitable |
| <computeroutput>mpicc</computeroutput> to build it with. This must be |
| the same <computeroutput>mpicc</computeroutput> you use to build the |
| MPI application you want to debug. By default, Valgrind tries |
| <computeroutput>mpicc</computeroutput>, but you can specify a |
| different one by using the configure-time option |
| <option>--with-mpicc</option>. Currently the |
| wrappers are only buildable with |
| <computeroutput>mpicc</computeroutput>s which are based on GNU |
| GCC or Intel's C++ Compiler.</para> |
| |
| <para>Check that the configure script prints a line like this:</para> |
| |
| <programlisting><![CDATA[ |
| checking for usable MPI2-compliant mpicc and mpi.h... yes, mpicc |
| ]]></programlisting> |
| |
| <para>If it says <computeroutput>... no</computeroutput>, your |
| <computeroutput>mpicc</computeroutput> has failed to compile and link |
| a test MPI2 program.</para> |
| |
| <para>If the configure test succeeds, continue in the usual way with |
| <computeroutput>make</computeroutput> and <computeroutput>make |
| install</computeroutput>. The final install tree should then contain |
| <computeroutput>libmpiwrap-<platform>.so</computeroutput>. |
| </para> |
| |
| <para>Compile up a test MPI program (eg, MPI hello-world) and try |
| this:</para> |
| |
| <programlisting><![CDATA[ |
| LD_PRELOAD=$prefix/lib/valgrind/libmpiwrap-<platform>.so \ |
| mpirun [args] $prefix/bin/valgrind ./hello |
| ]]></programlisting> |
| |
| <para>You should see something similar to the following</para> |
| |
| <programlisting><![CDATA[ |
| valgrind MPI wrappers 31901: Active for pid 31901 |
| valgrind MPI wrappers 31901: Try MPIWRAP_DEBUG=help for possible options |
| ]]></programlisting> |
| |
| <para>repeated for every process in the group. If you do not see |
| these, there is an build/installation problem of some kind.</para> |
| |
| <para> The MPI functions to be wrapped are assumed to be in an ELF |
| shared object with soname matching |
| <computeroutput>libmpi.so*</computeroutput>. This is known to be |
| correct at least for Open MPI and Quadrics MPI, and can easily be |
| changed if required.</para> |
| </sect2> |
| |
| |
| <sect2 id="mc-manual.mpiwrap.gettingstarted" |
| xreflabel="Getting started with MPI Wrappers"> |
| <title>Getting started</title> |
| |
| <para>Compile your MPI application as usual, taking care to link it |
| using the same <computeroutput>mpicc</computeroutput> that your |
| Valgrind build was configured with.</para> |
| |
| <para> |
| Use the following basic scheme to run your application on Valgrind with |
| the wrappers engaged:</para> |
| |
| <programlisting><![CDATA[ |
| MPIWRAP_DEBUG=[wrapper-args] \ |
| LD_PRELOAD=$prefix/lib/valgrind/libmpiwrap-<platform>.so \ |
| mpirun [mpirun-args] \ |
| $prefix/bin/valgrind [valgrind-args] \ |
| [application] [app-args] |
| ]]></programlisting> |
| |
| <para>As an alternative to |
| <computeroutput>LD_PRELOAD</computeroutput>ing |
| <computeroutput>libmpiwrap-<platform>.so</computeroutput>, you can |
| simply link it to your application if desired. This should not disturb |
| native behaviour of your application in any way.</para> |
| </sect2> |
| |
| |
| <sect2 id="mc-manual.mpiwrap.controlling" |
| xreflabel="Controlling the MPI Wrappers"> |
| <title>Controlling the wrapper library</title> |
| |
| <para>Environment variable |
| <computeroutput>MPIWRAP_DEBUG</computeroutput> is consulted at |
| startup. The default behaviour is to print a starting banner</para> |
| |
| <programlisting><![CDATA[ |
| valgrind MPI wrappers 16386: Active for pid 16386 |
| valgrind MPI wrappers 16386: Try MPIWRAP_DEBUG=help for possible options |
| ]]></programlisting> |
| |
| <para> and then be relatively quiet.</para> |
| |
| <para>You can give a list of comma-separated options in |
| <computeroutput>MPIWRAP_DEBUG</computeroutput>. These are</para> |
| |
| <itemizedlist> |
| <listitem> |
| <para><computeroutput>verbose</computeroutput>: |
| show entries/exits of all wrappers. Also show extra |
| debugging info, such as the status of outstanding |
| <computeroutput>MPI_Request</computeroutput>s resulting |
| from uncompleted <computeroutput>MPI_Irecv</computeroutput>s.</para> |
| </listitem> |
| <listitem> |
| <para><computeroutput>quiet</computeroutput>: |
| opposite of <computeroutput>verbose</computeroutput>, only print |
| anything when the wrappers want |
| to report a detected programming error, or in case of catastrophic |
| failure of the wrappers.</para> |
| </listitem> |
| <listitem> |
| <para><computeroutput>warn</computeroutput>: |
| 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.</para> |
| </listitem> |
| <listitem> |
| <para><computeroutput>strict</computeroutput>: |
| print an error message and abort the program if |
| a function lacking a wrapper is used.</para> |
| </listitem> |
| </itemizedlist> |
| |
| <para> If you want to use Valgrind's XML output facility |
| (<option>--xml=yes</option>), you should pass |
| <computeroutput>quiet</computeroutput> in |
| <computeroutput>MPIWRAP_DEBUG</computeroutput> so as to get rid of any |
| extraneous printing from the wrappers.</para> |
| |
| </sect2> |
| |
| |
| <sect2 id="mc-manual.mpiwrap.limitations.functions" |
| xreflabel="Functions: Abilities and Limitations"> |
| <title>Functions</title> |
| |
| <para>All MPI2 functions except |
| <computeroutput>MPI_Wtick</computeroutput>, |
| <computeroutput>MPI_Wtime</computeroutput> and |
| <computeroutput>MPI_Pcontrol</computeroutput> have wrappers. The |
| first two are not wrapped because they return a |
| <computeroutput>double</computeroutput>, which Valgrind's |
| function-wrap mechanism cannot handle (but it could easily be |
| extended to do so). <computeroutput>MPI_Pcontrol</computeroutput> cannot be |
| wrapped as it has variable arity: |
| <computeroutput>int MPI_Pcontrol(const int level, ...)</computeroutput></para> |
| |
| <para>Most functions are wrapped with a default wrapper which does |
| nothing except complain or abort if it is called, depending on |
| settings in <computeroutput>MPIWRAP_DEBUG</computeroutput> listed |
| above. The following functions have "real", do-something-useful |
| wrappers:</para> |
| |
| <programlisting><![CDATA[ |
| 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 |
| ]]></programlisting> |
| |
| <para> A few functions such as |
| <computeroutput>PMPI_Address</computeroutput> are listed as |
| <computeroutput>HAS_NO_WRAPPER</computeroutput>. They have no wrapper |
| at all as there is nothing worth checking, and giving a no-op wrapper |
| would reduce performance for no reason.</para> |
| |
| <para> 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 |
| <computeroutput>PMPI_Extent</computeroutput>, |
| <computeroutput>PMPI_Type_get_envelope</computeroutput>, |
| <computeroutput>PMPI_Type_get_contents</computeroutput>, and |
| <computeroutput>PMPI_Type_free</computeroutput>. </para> |
| </sect2> |
| |
| <sect2 id="mc-manual.mpiwrap.limitations.types" |
| xreflabel="Types: Abilities and Limitations"> |
| <title>Types</title> |
| |
| <para> MPI-1.1 structured types are supported, and walked exactly. |
| The currently supported combiners are |
| <computeroutput>MPI_COMBINER_NAMED</computeroutput>, |
| <computeroutput>MPI_COMBINER_CONTIGUOUS</computeroutput>, |
| <computeroutput>MPI_COMBINER_VECTOR</computeroutput>, |
| <computeroutput>MPI_COMBINER_HVECTOR</computeroutput> |
| <computeroutput>MPI_COMBINER_INDEXED</computeroutput>, |
| <computeroutput>MPI_COMBINER_HINDEXED</computeroutput> and |
| <computeroutput>MPI_COMBINER_STRUCT</computeroutput>. This should |
| cover all MPI-1.1 types. The mechanism (function |
| <computeroutput>walk_type</computeroutput>) should extend easily to |
| cover MPI2 combiners.</para> |
| |
| <para>MPI defines some named structured types |
| (<computeroutput>MPI_FLOAT_INT</computeroutput>, |
| <computeroutput>MPI_DOUBLE_INT</computeroutput>, |
| <computeroutput>MPI_LONG_INT</computeroutput>, |
| <computeroutput>MPI_2INT</computeroutput>, |
| <computeroutput>MPI_SHORT_INT</computeroutput>, |
| <computeroutput>MPI_LONG_DOUBLE_INT</computeroutput>) which are pairs |
| of some basic type and a C <computeroutput>int</computeroutput>. |
| 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 <computeroutput>struct { float val; |
| int loc; }</computeroutput> (for |
| <computeroutput>MPI_FLOAT_INT</computeroutput>), etc, and act |
| accordingly. This appears to be correct at least for Open MPI 1.0.2 |
| and for Quadrics MPI.</para> |
| |
| <para>If <computeroutput>strict</computeroutput> is an option specified |
| in <computeroutput>MPIWRAP_DEBUG</computeroutput>, the application |
| will abort if an unhandled type is encountered. Otherwise, the |
| application will print a warning message and continue.</para> |
| |
| <para>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 (<computeroutput>double</computeroutput>, |
| <computeroutput>float</computeroutput>, |
| <computeroutput>int</computeroutput>, |
| <computeroutput>long</computeroutput>, <computeroutput>long |
| long</computeroutput>, <computeroutput>short</computeroutput>, |
| <computeroutput>char</computeroutput>, and <computeroutput>long |
| double</computeroutput> on platforms where <computeroutput>sizeof(long |
| double) == 8</computeroutput>). For arrays of all other types, the |
| wrappers handle each element individually and so there can be a very |
| large performance cost.</para> |
| |
| </sect2> |
| |
| |
| <sect2 id="mc-manual.mpiwrap.writingwrappers" |
| xreflabel="Writing new MPI Wrappers"> |
| <title>Writing new wrappers</title> |
| |
| <para> |
| For the most part the wrappers are straightforward. The only |
| significant complexity arises with nonblocking receives.</para> |
| |
| <para>The issue is that <computeroutput>MPI_Irecv</computeroutput> |
| states the recv buffer and returns immediately, giving a handle |
| (<computeroutput>MPI_Request</computeroutput>) for the transaction. |
| Later the user will have to poll for completion with |
| <computeroutput>MPI_Wait</computeroutput> etc, and when the |
| transaction completes successfully, the wrappers have to paint the |
| recv buffer. But the recv buffer details are not presented to |
| <computeroutput>MPI_Wait</computeroutput> -- only the handle is. The |
| library therefore maintains a shadow table which associates |
| uncompleted <computeroutput>MPI_Request</computeroutput>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.</para> |
| |
| <para>Access to the table is guarded by a (POSIX pthreads) lock, so as |
| to make the library thread-safe.</para> |
| |
| <para>The table is allocated with |
| <computeroutput>malloc</computeroutput> and never |
| <computeroutput>free</computeroutput>d, so it will show up in leak |
| checks.</para> |
| |
| <para>Writing new wrappers should be fairly easy. The source file is |
| <computeroutput>mpi/libmpiwrap.c</computeroutput>. 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).</para> |
| </sect2> |
| |
| <sect2 id="mc-manual.mpiwrap.whattoexpect" |
| xreflabel="What to expect with MPI Wrappers"> |
| <title>What to expect when using the wrappers</title> |
| |
| <para>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.</para> |
| |
| <para>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 |
| <computeroutput>MPI_Recv</computeroutput>.</para> |
| |
| <para>Functions which are not wrapped may increase the false |
| error rate. A possible approach is to run with |
| <computeroutput>MPI_DEBUG</computeroutput> containing |
| <computeroutput>warn</computeroutput>. This will show you functions |
| which lack proper wrappers but which are nevertheless used. You can |
| then write wrappers for them. |
| </para> |
| |
| <para>A known source of potential false errors are the |
| <computeroutput>PMPI_Reduce</computeroutput> 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 |
| <computeroutput>PMPI_Reduce</computeroutput> call. As a result you |
| may see false positives reported in your reduction function.</para> |
| |
| </sect2> |
| |
| </sect1> |
| |
| |
| |
| |
| |
| </chapter> |