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 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 only effective 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.

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

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 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>
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 3.41 of the Linux man-pages project. A description of the project, and information about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.