The program below demonstrates the use of M_CHECK_ACTION. If the
program is supplied with an (integer) command-line argument, then
that argument is used to set the M_CHECK_ACTION parameter. The
program then allocates a block of memory, and frees it twice (an
error).
The following shell session shows what happens when we run this
program under glibc, with the default value for M_CHECK_ACTION:
$ ./a.out
main(): returned from first free() call
*** glibc detected *** ./a.out: double free or corruption (top): 0x09d30008 ***
======= Backtrace: =========
/lib/libc.so.6(+0x6c501)[0x523501]
/lib/libc.so.6(+0x6dd70)[0x524d70]
/lib/libc.so.6(cfree+0x6d)[0x527e5d]
./a.out[0x80485db]
/lib/libc.so.6(__libc_start_main+0xe7)[0x4cdce7]
./a.out[0x8048471]
======= Memory map: ========
001e4000-001fe000 r-xp 00000000 08:06 1083555 /lib/libgcc_s.so.1
001fe000-001ff000 r--p 00019000 08:06 1083555 /lib/libgcc_s.so.1
[some lines omitted]
b7814000-b7817000 rw-p 00000000 00:00 0
bff53000-bff74000 rw-p 00000000 00:00 0 [stack]
Aborted (core dumped)
The following runs show the results when employing other values
for M_CHECK_ACTION:
$ ./a.out 1 # Diagnose error and continue
main(): returned from first free() call
*** glibc detected *** ./a.out: double free or corruption (top): 0x09cbe008 ***
main(): returned from second free() call
$ ./a.out 2 # Abort without error message
main(): returned from first free() call
Aborted (core dumped)
$ ./a.out 0 # Ignore error and continue
main(): returned from first free() call
main(): returned from second free() call
The next run shows how to set the same parameter using the
MALLOC_CHECK_ environment variable:
$ MALLOC_CHECK_=1 ./a.out
main(): returned from first free() call
*** glibc detected *** ./a.out: free(): invalid pointer: 0x092c2008 ***
main(): returned from second free() call
Program source
#include <malloc.h>
#include <stdio.h>
#include <stdlib.h>
int
main(int argc, char *argv[])
{
char *p;
if (argc > 1) {
if (mallopt(M_CHECK_ACTION, atoi(argv[1])) != 1) {
fprintf(stderr, "mallopt() failed");
exit(EXIT_FAILURE);
}
}
p = malloc(1000);
if (p == NULL) {
fprintf(stderr, "malloc() failed");
exit(EXIT_FAILURE);
}
free(p);
printf("main(): returned from first free() call\n");
free(p);
printf("main(): returned from second free() call\n");
exit(EXIT_SUCCESS);
}
