Name
mcheck, mcheck_check_all, mcheck_pedantic, mprobe - heap consistency checkingLibrary
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.Attributes
For an explanation of the terms used in this section, see attributes(7).| Interface | Attribute | Value |
| T} | Thread safety | MT-Unsafe race:mcheck const:malloc_hooks |
Standards
GNU.History
mcheck_pedantic() mcheck_check_all() glibc 2.2.
mcheck() mprobe() glibc 2.0.
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\en");
exit(EXIT_FAILURE);
}
p = malloc(1000);
fprintf(stderr, "About to free\en");
free(p);
fprintf(stderr, "\enAbout to free a second time\en");
free(p);
exit(EXIT_SUCCESS);
}