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

       mcheck,  mcheck_check_all,  mcheck_pedantic,  mprobe - heap consistency

       #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  mal-
       loc(3)  family  of memory-allocation functions.  These hooks cause cer-
       tain 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 imme-
       diately 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 diffi-
       cult  to  ensure, linking the program with -lmcheck inserts an implicit
       call to mcheck() (with a NULL argument) before the first call to a mem-
       ory-allocation function.

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

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

       If the system detects an inconsistency in the heap, the caller-supplied
       function pointed to by abortfunc is invoked  with  a  single  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 func-
              tion 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

       For   an   explanation   of   the  terms  used  in  this  section,  see

       |Interface                    | Attribute     | Value                 |
       |mcheck(), mcheck_pedantic(), | Thread safety | MT-Unsafe race:mcheck |
       |mcheck_check_all(), mprobe() |               | const:malloc_hooks    |

       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 MALLOC_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  demon-
       strates what happens when running the program:

           $ ./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 4.15 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

GNU                               2017-09-15                         MCHECK(3)
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