Thursday, January 8, 2009

Making NULL-pointer reference legal

Normally, NULL pointer reference results in access violation and is illegal. However, it is possible to make NULL pointer reference legal on several operating systems. There are two ways to do that: (a) using mmap(2) with MAP_FIXED flag (works on Linux and Mac OS X), and (b) using shmat(2) with the SHM_RND flag (works on Linux). Consider the following program:
#include "stdio.h"
#include "sys/mman.h"

int main()
{
void* p = mmap(
NULL, 4096, PROT_READ | PROT_WRITE,
MAP_FIXED | MAP_PRIVATE | MAP_ANON, -1, 0);

if (p == NULL)
printf("*(NULL) = %x.\n", *((int*) NULL));
else
perror("mmap(2)");

return 0;
}
The program, when run on Linux and Mac OS X, prints *(NULL) = 0, and that's the result of the program doing a NULL pointer reference. This doesn't work on AIX.
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