linking with libmcheck ruins output of malloc_usable_size()

[Peter Jay Salzman]

Proof of concept code:



#include <malloc.h>
#include <stdio.h>
#include <stdlib.h>
#include <mcheck.h>
// On my system, size_t is an unsigned int.

int main(void)
{
   char *p = (char *) malloc(sizeof(char));

   if (p == NULL)
      abort();

   printf("I requested %u bytes.\n", sizeof(char));
   printf("p was allocated %u bytes.\n", malloc_usable_size(p));

   return 0;
}



Output on glibc 2.1:


   $ gcc -W -Wall try.c
   $ ./a.out
   I requested 1 bytes.
   p was allocated 12 bytes.


   $ gcc -W -Wall try.c -lmcheck
   $ ./a.out
   I requested 1 bytes.
   p was allocated 0 bytes.



Output on glibc 2.3:


   $ gcc -W -Wall try.c
   $ ./a.out
   I requested 1 bytes.
   p was allocated 12 bytes.


   $ gcc -W -Wall try.c -lmcheck
   $ ./a.out
   I requested 1 bytes.
   p was allocated 4141754496 bytes.


Either way, the output of malloc_usable_size() is wrong when the program
is linked with libmcheck.  I WISH I had 4141754496 bytes of memory!  :-)

Please cc me, since I'm not on the glibc-bug list.

Thanks guys!
Pete

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