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<html>
<head>
<title>Memcheck: a heavyweight memory checker</title>
</head>
<a name="mc-top"></a>
<h2>3&nbsp; <b>Memcheck</b>: a heavyweight memory checker</h2>
To use this skin, you may specify <code>--skin=memcheck</code> on the
Valgrind command line. But you don't have to, since this is the
default skin.
<h3>3.1&nbsp; Kinds of bugs that memcheck can find</h3>
Memcheck is Valgrind-1.0.X's checking mechanism bundled up into a skin.
All reads and writes of memory are checked, and calls to
malloc/new/free/delete are intercepted. As a result, memcheck can
detect the following problems:
<ul>
<li>Use of uninitialised memory</li>
<li>Reading/writing memory after it has been free'd</li>
<li>Reading/writing off the end of malloc'd blocks</li>
<li>Reading/writing inappropriate areas on the stack</li>
<li>Memory leaks -- where pointers to malloc'd blocks are lost
forever</li>
<li>Mismatched use of malloc/new/new [] vs free/delete/delete []</li>
<li>Overlapping <code>src</code> and <code>dst</code> pointers in
<code>memcpy()</code> and related functions</li>
<li>Some misuses of the POSIX pthreads API</li>
</ul>
<p>
<h3>3.2&nbsp; Command-line flags specific to memcheck</h3>
<ul>
<li><code>--leak-check=no</code> [default]<br>
<code>--leak-check=yes</code>
<p>When enabled, search for memory leaks when the client program
finishes. A memory leak means a malloc'd block, which has not
yet been free'd, but to which no pointer can be found. Such a
block can never be free'd by the program, since no pointer to it
exists. Leak checking is disabled by default because it tends
to generate dozens of error messages. </li><br><p>
<li><code>--show-reachable=no</code> [default]<br>
<code>--show-reachable=yes</code>
<p>When disabled, the memory leak detector only shows blocks for
which it cannot find a pointer to at all, or it can only find a
pointer to the middle of. These blocks are prime candidates for
memory leaks. When enabled, the leak detector also reports on
blocks which it could find a pointer to. Your program could, at
least in principle, have freed such blocks before exit.
Contrast this to blocks for which no pointer, or only an
interior pointer could be found: they are more likely to
indicate memory leaks, because you do not actually have a
pointer to the start of the block which you can hand to
<code>free</code>, even if you wanted to. </li><br><p>
<li><code>--leak-resolution=low</code> [default]<br>
<code>--leak-resolution=med</code> <br>
<code>--leak-resolution=high</code>
<p>When doing leak checking, determines how willing Memcheck is
to consider different backtraces to be the same. When set to
<code>low</code>, the default, only the first two entries need
match. When <code>med</code>, four entries have to match. When
<code>high</code>, all entries need to match.
<p>
For hardcore leak debugging, you probably want to use
<code>--leak-resolution=high</code> together with
<code>--num-callers=40</code> or some such large number. Note
however that this can give an overwhelming amount of
information, which is why the defaults are 4 callers and
low-resolution matching.
<p>
Note that the <code>--leak-resolution=</code> setting does not
affect Memcheck's ability to find leaks. It only changes how
the results are presented.
</li><br><p>
<li><code>--freelist-vol=&lt;number></code> [default: 1000000]
<p>When the client program releases memory using free (in C) or
delete (C++), 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 delay 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>
This flag specifies the maximum total size, in bytes, of the
blocks in the queue. The default value is one 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.</li><br><p>
<li><code>--workaround-gcc296-bugs=no</code> [default]<br>
<code>--workaround-gcc296-bugs=yes</code> <p>When enabled,
assume that reads and writes some small distance below the stack
pointer <code>%esp</code> 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 popular Linux
distributions (RedHat 7.X, Mandrake) and so you may well need to
use this flag. Do not use it if you do not have to, as it can
cause real errors to be overlooked. Another option is to use a
gcc/g++ which does not generate accesses below the stack
pointer. 2.95.3 seems to be a good choice in this respect.
<p>
Unfortunately (27 Feb 02) it looks like g++ 3.0.4 has a similar
bug, so you may need to issue this flag if you use 3.0.4. A
while later (early Apr 02) this is confirmed as a scheduling bug
in g++-3.0.4.
</li><br><p>
<li><code>--partial-loads-ok=yes</code> [the default]<br>
<code>--partial-loads-ok=no</code>
<p>Controls how Memcheck handles word (4-byte) loads from
addresses for which some bytes are addressible and others
are not. When <code>yes</code> (the default), such loads
do not elicit an address error. Instead, the loaded V bytes
corresponding to the illegal addresses indicate undefined, and
those corresponding to legal addresses are loaded from shadow
memory, as usual.
<p>
When <code>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 V bytes indicate valid data.
</li><br><p>
<li><code>--cleanup=no</code><br>
<code>--cleanup=yes</code> [default]
<p><b>This is a flag to help debug valgrind itself. It is of no
use to end-users.</b> When enabled, various improvments are
applied to the post-instrumented intermediate code, aimed at
removing redundant value checks.</li><br>
<p>
</ul>
<a name="errormsgs"></a>
<h3>3.3&nbsp; Explanation of error messages from Memcheck</h3>
Despite considerable sophistication under the hood, Memcheck can only
really detect two kinds of errors, use of illegal addresses, and use
of undefined values. Nevertheless, this is enough to help you
discover all sorts of memory-management nasties in your code. This
section presents a quick summary of what error messages mean. The
precise behaviour of the error-checking machinery is described in
<a href="#machine">this section</a>.
<h4>3.3.1&nbsp; Illegal read / Illegal write errors</h4>
For example:
<pre>
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__FP8QImageIO (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
qpngio.cpp, and so on.
<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 free'd at. Likewise, if it should turn out
to be just off the end of a malloc'd 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 malloc'd.
<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, %esp, and that isn't
allowed. In this particular case it's probably caused by gcc
generating invalid code, a known bug in various flavours of gcc.
<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.
<h4>3.3.2&nbsp; Use of uninitialised values</h4>
For example:
<pre>
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)
by 0x402A6E5E: __libc_start_main (libc-start.c:129)
</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 printf()
machinery of the C library. This error was reported when running the
following small program:
<pre>
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 to its heart's content. 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 this example, x is uninitialised. Memcheck observes the
value being passed to _IO_printf and thence to _IO_vfprintf, but makes
no comment. However, _IO_vfprintf has to examine the value of x so it
can turn it into the corresponding ASCII string, and it is at this
point that Memcheck complains.
<p>Sources of uninitialised data tend to be:
<ul>
<li>Local variables in procedures which have not been initialised,
as in the example above.</li><p>
<li>The contents of malloc'd blocks, before you write something
there. In C++, the new operator is a wrapper round malloc, so
if you create an object with new, its fields will be
uninitialised until you (or the constructor) fill them in, which
is only Right and Proper.</li>
</ul>
<h4>3.3.3&nbsp; Illegal frees</h4>
For example:
<pre>
Invalid free()
at 0x4004FFDF: free (vg_clientmalloc.c:577)
by 0x80484C7: main (tests/doublefree.c:10)
by 0x402A6E5E: __libc_start_main (libc-start.c:129)
by 0x80483B1: (within tests/doublefree)
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)
by 0x402A6E5E: __libc_start_main (libc-start.c:129)
by 0x80483B1: (within tests/doublefree)
</pre>
<p>Memcheck keeps track of the blocks allocated by your program with
malloc/new, so it can know exactly whether or not the argument to
free/delete 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 free'd. 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.
<h4>3.3.4&nbsp; When a block is freed with an inappropriate
deallocation function</h4>
In the following example, a block allocated with <code>new[]</code>
has wrongly been deallocated with <code>free</code>:
<pre>
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: __builtin_vec_new (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 &amp;) (olefilter.cc:272)
</pre>
The following was told to me be the KDE 3 developers. I didn't know
any of it myself. They also implemented the check itself.
<p>
In C++ it's important to deallocate memory in a way compatible with
how it was allocated. The deal is:
<ul>
<li>If allocated with <code>malloc</code>, <code>calloc</code>,
<code>realloc</code>, <code>valloc</code> or
<code>memalign</code>, you must deallocate with <code>free</code>.
<li>If allocated with <code>new[]</code>, you must deallocate with
<code>delete[]</code>.
<li>If allocated with <code>new</code>, you must deallocate with
<code>delete</code>.
</ul>
The worst thing is that on Linux apparently it doesn't matter if you
do muddle these up, and it all seems to work ok, 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>
Pascal Massimino adds the following clarification:
<code>delete[]</code> must be called associated with a
<code>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. This implies a
variable-sized overhead in what's returned by <code>new</code> or
<code>new[]</code>. It rather surprising how compilers [Ed:
runtime-support libraries?] are robust to mismatch in
<code>new</code>/<code>delete</code>
<code>new[]</code>/<code>delete[]</code>.
<h4>3.3.5&nbsp; Passing system call parameters with inadequate
read/write permissions</h4>
Memcheck checks all parameters to system calls. If a system call
needs to read from a buffer provided by your program, Memcheck checks
that the entire buffer is addressible and has valid data, ie, it is
readable. And if the system call needs to write to a user-supplied
buffer, Memcheck checks that the buffer is addressible. After the
system call, Memcheck updates its administrative information to
precisely reflect any changes in memory permissions caused by the
system call.
<p>Here's an example of a system call with an invalid parameter:
<pre>
#include &lt;stdlib.h>
#include &lt;unistd.h>
int main( void )
{
char* arr = malloc(10);
(void) write( 1 /* stdout */, arr, 10 );
return 0;
}
</pre>
<p>You get this complaint ...
<pre>
Syscall param write(buf) contains uninitialised or unaddressable byte(s)
at 0x4035E072: __libc_write
by 0x402A6E5E: __libc_start_main (libc-start.c:129)
by 0x80483B1: (within tests/badwrite)
by &lt;bogus frame pointer> ???
Address 0x3807E6D0 is 0 bytes inside a block of size 10 alloc'd
at 0x4004FEE6: malloc (ut_clientmalloc.c:539)
by 0x80484A0: main (tests/badwrite.c:6)
by 0x402A6E5E: __libc_start_main (libc-start.c:129)
by 0x80483B1: (within tests/badwrite)
</pre>
<p>... because the program has tried to write uninitialised junk from
the malloc'd block to the standard output.
<h4>3.3.6&nbsp; Overlapping source and destination blocks</h4>
The following C library functions copy some data from one memory block
to another (or something similar): <code>memcpy()</code>,
<code>strcpy()</code>, <code>strncpy()</code>, <code>strcat()</code>,
<code>strncat()</code>. The blocks pointed to by their <code>src</code> and
<code>dst</code> pointers aren't allowed to overlap. Memcheck checks
for this.
<p>
For example:
<pre>
==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)
==27492== by 0x40246335: __libc_start_main (../sysdeps/generic/libc-start.c:129)
==27492== by 0x8048470: (within /auto/homes/njn25/grind/head6/memcheck/tests/overlap)
==27492==
</pre>
<p>
You don't want the two blocks to overlap because one of them could get
partially trashed by the copying.
<a name="suppfiles"></a>
<h3>3.4&nbsp; Writing suppressions files</h3>
The basic suppression format was described in <a
href="coregrind_core.html#suppress">this section</a>.
<p>
The suppression (2nd) line should have the form:
<pre>
Memcheck:suppression_type
</pre>
Or, since some of the suppressions are shared with the Addrcheck skin:
<pre>
Memcheck,Addrcheck:suppression_type
</pre>
<p>
The Memcheck suppression types are as follows:
<code>Value1</code>,
<code>Value2</code>,
<code>Value4</code>,
<code>Value8</code>,
<code>Value16</code>,
meaning an uninitialised-value error when
using a value of 1, 2, 4, 8 or 16 bytes.
Or
<code>Cond</code> (or its old name, <code>Value0</code>),
meaning use of an uninitialised CPU condition code. Or:
<code>Addr1</code>,
<code>Addr2</code>,
<code>Addr4</code>,
<code>Addr8</code>,
<code>Addr16</code>,
meaning an invalid address during a
memory access of 1, 2, 4, 8 or 16 bytes respectively. Or
<code>Param</code>,
meaning an invalid system call parameter error. Or
<code>Free</code>, meaning an invalid or mismatching free.
<code>Overlap</code>, meaning a <code>src</code>/<code>dst</code>
overlap in <code>memcpy() or a similar function</code>. Last but not least,
you can suppress leak reports with <code>Leak</code>. Leak suppression was
added in valgrind-1.9.3, I believe.
<p>
The extra information line: for Param errors, is the name of the offending
system call parameter.
No other error kinds have this extra line.
<p>
The first line of the calling context: for Value and Addr errors, it is either
the name of the function in which the error occurred, or, failing that, the
full path of the .so file or executable containing the error location. For
Free errors, is the name of the function doing the freeing (eg,
<code>free</code>, <code>__builtin_vec_delete</code>, etc). For Overlap
errors, is the name of the function with the overlapping arguments (eg.
<code>memcpy()</code>, <code>strcpy()</code>, etc).
<p>
Lastly, there's the rest of the calling context.
<p>
<a name="machine"></a>
<h3>3.5&nbsp; Details of Memcheck's checking machinery</h3>
Read this section if you want to know, in detail, exactly what and how
Memcheck is checking.
<a name="vvalue"></a>
<h4>3.5.1&nbsp; Valid-value (V) bits</h4>
It is simplest to think of Memcheck implementing a synthetic Intel x86
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>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>In short, each bit in the system has an associated V bit, which
follows it around everywhere, even inside the CPU. Yes, the CPU's
(integer and <code>%eflags</code>) registers have their own V bit
vectors.
<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 the outcome of your program's computation, the
associated V bits are immediately checked. If any of these indicate
that the value is undefined, an error is reported.
<p>Here's an (admittedly nonsensical) example:
<pre>
int i, j;
int a[10], b[10];
for (i = 0; i &lt; 10; i++) {
j = a[i];
b[i] = j;
}
</pre>
<p>Memcheck emits no complaints about this, since it merely copies
uninitialised values from <code>a[]</code> into <code>b[]</code>, and
doesn't use them in any way. However, if the loop is changed to
<pre>
for (i = 0; i &lt; 10; i++) {
j += a[i];
}
if (j == 77)
printf("hello there\n");
</pre>
then Valgrind will complain, at the <code>if</code>, that the
condition depends on uninitialised values. Note that it
<b>doesn't</b> complain at the <code>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>printf</code> -- an observable action of your program -- that
Memcheck complains.
<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>Checks on definedness only occur in two places: when a value is
used to generate a memory address, and where control flow decision
needs to be made. Also, when a system call is detected, valgrind
checks definedness of parameters as required.
<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 effect,
we say that undefined values are non-infectious.
<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:
<pre>
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>struct S</code>, in
bytes? An int is 4 bytes and a char one byte, so perhaps a struct S
occupies 5 bytes? Wrong. All (non-toy) compilers we know of will
round the size of <code>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 subscripting arrays of <code>struct
S</code>'s.
<p>So s1 occupies 8 bytes, yet only 5 of them will be initialised.
For the assignment <code>s2 = s1</code>, gcc generates code to copy
all 8 bytes wholesale into <code>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 semantics described above is
necessary. This allows gcc to copy <code>s1</code> into
<code>s2</code> any way it likes, and a warning will only be emitted
if the uninitialised values are later used.
<p>One final twist to this story. The above scheme allows garbage to
pass through the CPU's integer registers without complaint. It does
this by giving the integer registers V tags, passing these around in
the expected way. This complicated and computationally expensive to
do, but is necessary. Memcheck is more simplistic about
floating-point loads and stores. In particular, V bits for data read
as a result of floating-point loads are checked at the load
instruction. So if your program uses the floating-point registers to
do memory-to-memory copies, you will get complaints about
uninitialised values. Fortunately, I have not yet encountered a
program which (ab)uses the floating-point registers in this way.
<a name="vaddress"></a>
<h4>3.5.2&nbsp; Valid-address (A) bits</h4>
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 issue.
<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 or the
data at that location -- that's the job of the V bits -- only whether
or not the location may be accessed.
<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>So how do the A bits get set/cleared? Like this:
<ul>
<li>When the program starts, all the global data areas are marked as
accessible.</li><br>
<p>
<li>When the program does malloc/new, the A bits for the 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.</li><br>
<p>
<li>When the stack pointer register (%esp) moves up or down, A bits
are set. The rule is that the area from %esp up to the base of
the stack is marked as accessible, and below %esp 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 %esp 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.</li><br>
<p>
<li>When doing system calls, A bits are changed appropriately. For
example, mmap() magically makes files appear in the process's
address space, so the A bits must be updated if mmap()
succeeds.</li><br>
<p>
<li>Optionally, your program can tell Valgrind about such changes
explicitly, using the client request mechanism described above.
</ul>
<a name="together"></a>
<h4>3.5.3&nbsp; Putting it all together</h4>
Memcheck's checking machinery can be summarised as follows:
<ul>
<li>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.</li><br>
<p>
<li>When memory is read or written, the relevant A bits are
consulted. If they indicate an invalid address, Valgrind emits
an Invalid read or Invalid write error.</li><br>
<p>
<li>When memory is read into the CPU's integer registers, the
relevant V bits are fetched from memory and stored in the
simulated CPU. They are not consulted.</li><br>
<p>
<li>When an integer register is written out to memory, the V bits
for that register are written back to memory too.</li><br>
<p>
<li>When memory is read into the CPU's floating point registers, the
relevant V bits are read from memory and they are immediately
checked. If any are invalid, an uninitialised value error is
emitted. This precludes using the floating-point registers to
copy possibly-uninitialised memory, but simplifies Valgrind in
that it does not have to track the validity status of the
floating-point registers.</li><br>
<p>
<li>As a result, when a floating-point register is written to
memory, the associated V bits are set to indicate a valid
value.</li><br>
<p>
<li>When values in integer 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.</li><br>
<p>
<li>When values in integer CPU registers are used for any other
purpose, Valgrind computes the V bits for the result, but does
not check them.</li><br>
<p>
<li>One 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.</li><br>
<p>
<li>When values are loaded from memory, valgrind 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>
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 unaddressible 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>
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 flag <code>--partial-loads-ok</code> for details.
</li><br>
</ul>
Memcheck intercepts calls to malloc, calloc, realloc, valloc,
memalign, free, new and delete. The behaviour you get is:
<ul>
<li>malloc/new: the returned memory is marked as addressible but not
having valid values. This means you have to write on it before
you can read it.</li><br>
<p>
<li>calloc: returned memory is marked both addressible and valid,
since calloc() clears the area to zero.</li><br>
<p>
<li>realloc: if the new size is larger than the old, the new section
is addressible but invalid, as with malloc.</li><br>
<p>
<li>If the new size is smaller, the dropped-off section is marked as
unaddressible. You may only pass to realloc a pointer
previously issued to you by malloc/calloc/realloc.</li><br>
<p>
<li>free/delete: you may only pass to free a pointer previously
issued to you by malloc/calloc/realloc, or the value
NULL. Otherwise, Valgrind complains. If the pointer is indeed
valid, Valgrind marks the entire area it points at as
unaddressible, 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.</li><br>
</ul>
<a name="leaks"></a>
<h3>3.6&nbsp; Memory leak detection</h3>
Memcheck keeps track of all memory blocks issued in response to calls
to malloc/calloc/realloc/new. So when the program exits, it knows
which blocks are still outstanding -- have not been returned, in other
words. Ideally, you want your program to have no blocks still in use
at exit. But many programs do.
<p>For each such block, Memcheck scans the entire address space of the
process, looking for pointers to the block. One of three situations
may result:
<ul>
<li>A pointer to the start of the block is found. This usually
indicates programming sloppiness; since the block is still
pointed at, the programmer could, at least in principle, free'd
it before program exit.</li><br>
<p>
<li>A pointer to the interior of the block is found. The pointer
might originally have pointed to the start and have been moved
along, or it might be entirely unrelated. Memcheck deems such a
block as "dubious", that is, possibly leaked,
because it's unclear whether or
not a pointer to it still exists.</li><br>
<p>
<li>The worst outcome is that no pointer to the block can be found.
The block is classified as "leaked", because the
programmer could not possibly have free'd it at program exit,
since no pointer to it exists. This might be a symptom of
having lost the pointer at some earlier point in the
program.</li>
</ul>
Memcheck reports summaries about leaked and dubious blocks.
For each such block, it will also tell you where the block was
allocated. This should help you figure out why the pointer to it has
been lost. In general, you should attempt to ensure your programs do
not have any leaked or dubious blocks at exit.
<p>The precise area of memory in which Memcheck searches for pointers
is: all naturally-aligned 4-byte words for which all A bits indicate
addressibility and all V bits indicated that the stored value is
actually valid.