Checking if a pointer is allocated memory or not

Question

Can we check whether a pointer passed to a function is allocated with memory or not in C?

I have wriiten my own function in C which accepts a character pointer -

Answer 1

The below code is what I have used once to check if some pointer tries to access illegal memory. The mechanism is to induce a SIGSEGV. The SEGV signal was redirected to a private function earlier, which uses longjmp to get back to the program. It is kind of a hack but it works.

The code can be improved (use 'sigaction' instead of 'signal' etc), but it is just to give an idea. Also it is portable to other Unix versions, for Windows I am not sure. Note that the SIGSEGV signal should not be used somewhere else in your program.

#include 
#include 
#include 
#include 

jmp_buf jump;

void segv (int sig)
{
  longjmp (jump, 1); 
}

int memcheck (void *x) 
{
  volatile char c;
  int illegal = 0;

  signal (SIGSEGV, segv);

  if (!setjmp (jump))
    c = *(char *) (x);
  else
    illegal = 1;

  signal (SIGSEGV, SIG_DFL);

  return (illegal);
}

int main (int argc, char *argv[])
{
  int *i, *j; 

  i = malloc (1);

  if (memcheck (i))
    printf ("i points to illegal memory\n");
  if (memcheck (j))
    printf ("j points to illegal memory\n");

  free (i);

  return (0);
}