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| <h5 class="subsubsection">3.2.2.9 Heap Consistency Checking</h5> |
| |
| <p><a name="index-heap-consistency-checking-279"></a><a name="index-consistency-checking_002c-of-heap-280"></a> |
| You can ask <code>malloc</code> to check the consistency of dynamic memory by |
| using the <code>mcheck</code> function. This function is a GNU extension, |
| declared in <samp><span class="file">mcheck.h</span></samp>. |
| <a name="index-mcheck_002eh-281"></a> |
| <!-- mcheck.h --> |
| <!-- GNU --> |
| |
| <div class="defun"> |
| — Function: int <b>mcheck</b> (<var>void </var>(<var>*abortfn</var>) (<var>enum mcheck_status status</var>))<var><a name="index-mcheck-282"></a></var><br> |
| <blockquote><p>Calling <code>mcheck</code> tells <code>malloc</code> to perform occasional |
| consistency checks. These will catch things such as writing |
| past the end of a block that was allocated with <code>malloc</code>. |
| |
| <p>The <var>abortfn</var> argument is the function to call when an inconsistency |
| is found. If you supply a null pointer, then <code>mcheck</code> uses a |
| default function which prints a message and calls <code>abort</code> |
| (see <a href="Aborting-a-Program.html#Aborting-a-Program">Aborting a Program</a>). The function you supply is called with |
| one argument, which says what sort of inconsistency was detected; its |
| type is described below. |
| |
| <p>It is too late to begin allocation checking once you have allocated |
| anything with <code>malloc</code>. So <code>mcheck</code> does nothing in that |
| case. The function returns <code>-1</code> if you call it too late, and |
| <code>0</code> otherwise (when it is successful). |
| |
| <p>The easiest way to arrange to call <code>mcheck</code> early enough is to use |
| the option ‘<samp><span class="samp">-lmcheck</span></samp>’ when you link your program; then you don't |
| need to modify your program source at all. Alternatively you might use |
| a debugger to insert a call to <code>mcheck</code> whenever the program is |
| started, for example these gdb commands will automatically call <code>mcheck</code> |
| whenever the program starts: |
| |
| <pre class="smallexample"> (gdb) break main |
| Breakpoint 1, main (argc=2, argv=0xbffff964) at whatever.c:10 |
| (gdb) command 1 |
| Type commands for when breakpoint 1 is hit, one per line. |
| End with a line saying just "end". |
| >call mcheck(0) |
| >continue |
| >end |
| (gdb) ... |
| </pre> |
| <p>This will however only work if no initialization function of any object |
| involved calls any of the <code>malloc</code> functions since <code>mcheck</code> |
| must be called before the first such function. |
| |
| </blockquote></div> |
| |
| <div class="defun"> |
| — Function: enum mcheck_status <b>mprobe</b> (<var>void *pointer</var>)<var><a name="index-mprobe-283"></a></var><br> |
| <blockquote><p>The <code>mprobe</code> function lets you explicitly check for inconsistencies |
| in a particular allocated block. You must have already called |
| <code>mcheck</code> at the beginning of the program, to do its occasional |
| checks; calling <code>mprobe</code> requests an additional consistency check |
| to be done at the time of the call. |
| |
| <p>The argument <var>pointer</var> must be a pointer returned by <code>malloc</code> |
| or <code>realloc</code>. <code>mprobe</code> returns a value that says what |
| inconsistency, if any, was found. The values are described below. |
| </p></blockquote></div> |
| |
| <div class="defun"> |
| — Data Type: <b>enum mcheck_status</b><var><a name="index-enum-mcheck_005fstatus-284"></a></var><br> |
| <blockquote><p>This enumerated type describes what kind of inconsistency was detected |
| in an allocated block, if any. Here are the possible values: |
| |
| <dl> |
| <dt><code>MCHECK_DISABLED</code><dd><code>mcheck</code> was not called before the first allocation. |
| No consistency checking can be done. |
| <br><dt><code>MCHECK_OK</code><dd>No inconsistency detected. |
| <br><dt><code>MCHECK_HEAD</code><dd>The data immediately before the block was modified. |
| This commonly happens when an array index or pointer |
| is decremented too far. |
| <br><dt><code>MCHECK_TAIL</code><dd>The data immediately after the block was modified. |
| This commonly happens when an array index or pointer |
| is incremented too far. |
| <br><dt><code>MCHECK_FREE</code><dd>The block was already freed. |
| </dl> |
| </p></blockquote></div> |
| |
| <p>Another possibility to check for and guard against bugs in the use of |
| <code>malloc</code>, <code>realloc</code> and <code>free</code> is to set the environment |
| variable <code>MALLOC_CHECK_</code>. When <code>MALLOC_CHECK_</code> is set, a |
| special (less efficient) implementation is used which is designed to be |
| tolerant against simple errors, such as double calls of <code>free</code> with |
| the same argument, or overruns of a single byte (off-by-one bugs). Not |
| all such errors can be protected against, however, and memory leaks can |
| result. If <code>MALLOC_CHECK_</code> is set to <code>0</code>, any detected heap |
| corruption is silently ignored; if set to <code>1</code>, a diagnostic is |
| printed on <code>stderr</code>; if set to <code>2</code>, <code>abort</code> is called |
| immediately. This can be useful because otherwise a crash may happen |
| much later, and the true cause for the problem is then very hard to |
| track down. |
| |
| <p>There is one problem with <code>MALLOC_CHECK_</code>: in SUID or SGID binaries |
| it could possibly be exploited since diverging from the normal programs |
| behavior it now writes something to the standard error descriptor. |
| Therefore the use of <code>MALLOC_CHECK_</code> is disabled by default for |
| SUID and SGID binaries. It can be enabled again by the system |
| administrator by adding a file <samp><span class="file">/etc/suid-debug</span></samp> (the content is |
| not important it could be empty). |
| |
| <p>So, what's the difference between using <code>MALLOC_CHECK_</code> and linking |
| with ‘<samp><span class="samp">-lmcheck</span></samp>’? <code>MALLOC_CHECK_</code> is orthogonal with respect to |
| ‘<samp><span class="samp">-lmcheck</span></samp>’. ‘<samp><span class="samp">-lmcheck</span></samp>’ has been added for backward |
| compatibility. Both <code>MALLOC_CHECK_</code> and ‘<samp><span class="samp">-lmcheck</span></samp>’ should |
| uncover the same bugs - but using <code>MALLOC_CHECK_</code> you don't need to |
| recompile your application. |
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