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MCHECK(3)                           Linux Programmer's Manual                           MCHECK(3)

       mcheck, mcheck_check_all, mcheck_pedantic, mprobe - heap consistency checking

       #include <mcheck.h>

       int mcheck(void (*abortfunc)(enum mcheck_status mstatus));

       int mcheck_pedantic(void (*abortfunc)(enum mcheck_status mstatus));

       void mcheck_check_all(void);

       enum mcheck_status mprobe(void *ptr);

       The  mcheck()  function installs a set of debugging hooks for the malloc(3) family of mem‐
       ory-allocation functions.  These hooks cause certain consistency checks to be performed on
       the  state  of the heap.  The checks can detect application errors such as freeing a block
       of memory more than once or corrupting the bookkeeping data  structures  that  immediately
       precede a block of allocated memory.

       To  be  effective, the mcheck() function must be called before the first call to malloc(3)
       or a related function.  In cases where this is difficult to ensure,  linking  the  program
       with -lmcheck inserts an implicit call to mcheck() (with a NULL argument) before the first
       call to a memory-allocation function.

       The mcheck_pedantic() function is similar to mcheck(), but performs checks  on  all  allo‐
       cated  blocks whenever one of the memory-allocation functions is called.  This can be very

       The mcheck_check_all() function causes an immediate check on all allocated  blocks.   This
       call is effective only if mcheck() is called beforehand.

       If  the  system detects an inconsistency in the heap, the caller-supplied function pointed
       to by abortfunc is invoked with a single argument argument, mstatus, that  indicates  what
       type  of  inconsistency  was detected.  If abortfunc is NULL, a default function prints an
       error message on stderr and calls abort(3).

       The mprobe() function performs a consistency  check  on  the  block  of  allocated  memory
       pointed  to by ptr.  The mcheck() function should be called beforehand (otherwise mprobe()
       returns MCHECK_DISABLED).

       The following list describes the values returned by mprobe()  or  passed  as  the  mstatus
       argument when abortfunc is invoked:

       MCHECK_DISABLED (mprobe() only)
              mcheck()  was  not  called  before the first memory allocation function was called.
              Consistency checking is not possible.

       MCHECK_OK (mprobe() only)
              No inconsistency detected.

              Memory preceding an allocated block was clobbered.

              Memory following an allocated block was clobbered.

              A block of memory was freed twice.

       mcheck() and mcheck_pedantic() return 0 on success, or -1 on error.

       The mcheck_pedantic() and mcheck_check_all() functions are available since glibc 2.2.  The
       mcheck() and mprobe() functions are present since at least glibc 2.0

       These functions are GNU extensions.

       Linking  a  program  with  -lmcheck  and  using  the  MALLOC_CHECK_  environment  variable
       (described in mallopt(3)) cause the same kinds of errors to be detected.  But, using  MAL‐
       LOC_CHECK_ does not require the application to be relinked.

       The  program  below  calls  mcheck() with a NULL argument and then frees the same block of
       memory twice.  The following shell session demonstrates what happens when running the pro‐

           $ ./a.out
           About to free

           About to free a second time
           block freed twice
           Aborted (core dumped)

   Program source

       #include <stdlib.h>
       #include <stdio.h>
       #include <mcheck.h>

       main(int argc, char *argv[])
           char *p;

           if (mcheck(NULL) != 0) {
               fprintf(stderr, "mcheck() failed\n");


           p = malloc(1000);

           fprintf(stderr, "About to free\n");
           fprintf(stderr, "\nAbout to free a second time\n");


       malloc(3), mallopt(3), mtrace(3)

       This  page  is  part of release 3.74 of the Linux man-pages project.  A description of the
       project, information about reporting bugs, and the latest version of  this  page,  can  be
       found at http://www.kernel.org/doc/man-pages/.

GNU                                         2014-01-11                                  MCHECK(3)

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