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



NAME
       mcheck,	mcheck_check_all,  mcheck_pedantic,  mprobe - heap consistency
       checking

SYNOPSIS
       #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);

DESCRIPTION
       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
       beforehand.

       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.

       MCHECK_HEAD
	      Memory preceding an allocated block was clobbered.

       MCHECK_TAIL
	      Memory following an allocated block was clobbered.

       MCHECK_FREE
	      A block of memory was freed twice.

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

VERSIONS
       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

ATTRIBUTES
       For   an	  explanation	of   the  terms	 used  in  this	 section,  see
       attributes(7).

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

CONFORMING TO
       These functions are GNU extensions.

NOTES
       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.

EXAMPLE
       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>

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

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

	       exit(EXIT_FAILURE);
	   }

	   p = malloc(1000);

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

	   exit(EXIT_SUCCESS);
       }

SEE ALSO
       malloc(3), mallopt(3), mtrace(3)

COLOPHON
       This  page  is  part of release 4.10 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
       https://www.kernel.org/doc/man-pages/.



GNU				  2015-05-07			     MCHECK(3)