MCHECK(3)

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

LIBRARY
Standard C library (libc, -lc)

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 -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 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, 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

ATTRIBUTES
For an explanation of the terms used in this section, see attributes(7).
Interface | Attribute | Value
 | Thread safety | MT-Unsafe race:mcheck const:malloc_hooks

STANDARDS
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.

EXAMPLES
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 <mcheck.h>
    #include <stdio.h>
    #include <stdlib.h>
    int
    main(void)
    {
        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)