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 memory-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 −mcheck 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 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 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.

MCHECK_FREE 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 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 demonstrates 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 3.52 of the Linux man-pages project. A description of the project, and information about reporting bugs, can be found at−pages/.

  Copyright (c) 2012 by Michael Kerrisk <>

Permission is granted to make and distribute verbatim copies of this
manual provided the copyright notice and this permission notice are
preserved on all copies.

Permission is granted to copy and distribute modified versions of this
manual under the conditions for verbatim copying, provided that the
entire resulting derived work is distributed under the terms of a
permission notice identical to this one.

Since the Linux kernel and libraries are constantly changing, this
manual page may be incorrect or out-of-date.  The author(s) assume no
responsibility for errors or omissions, or for damages resulting from
the use of the information contained herein.  The author(s) may not
have taken the same level of care in the production of this manual,
which is licensed free of charge, as they might when working

Formatted or processed versions of this manual, if unaccompanied by
the source, must acknowledge the copyright and authors of this work.