Memory allocation fails. But why does it crash? Or does it?

Question

I was experimenting with realloc, giving it larger and larger sizes and checking whether the same block was reused or not:

int main ( void )
{
  char * newstr, * prevstr = NULL;
  size_t newsize, prevsize = 0;
  printf ("We play with realloc\n");
  while (1) {
    newsize = prevsize + 1 + prevsize/3; /* add 33% */
    newstr = realloc(prevstr, newsize);
    if (newstr == NULL) {
      printf ("could not alloc newsize=%zu. Sorry\n", newsize);
      break;
    } else {
      printf ("newsize=%zu successfully alloc'ed\n", newsize);
      if (newstr == prevstr) {
        printf ("newstr = prevstr:\tSame block reused\n");
      } else {
        printf ("newstr != prevstr:\tNew block alloc'ed\n");
      }
      prevstr = newstr; prevsize = newsize;
    }
  }
  return (EXIT_SUCCESS);
}

As expected, one eventually reaches a point where the size is too large and realloc cannot answer the request. According to the manual, realloc should return NULL and set errno = ENOMEM when it does not succeed.

This is not what happens when I run the above code on my machine, a Mac with "Darwin Kernel Version 15.0.0". Instead of returning NULL, the code crashes and says

malloc: *** mach_vm_map(size=153288611651584) failed (error code=3)
*** error: can't allocate region
*** set a breakpoint in malloc_error_break to debug
could not alloc newsize=153288611651277. Sorry

Is this normal? Something I did not understand when reading the man page?

This is not crucial for my code at the moment but I can imagine situations where I'd like to test whether memory can be alloc'ed without risking a crash. Is there a standard way of testing whether alloc will work without risking such crash?

ADDED after mystery is solved (see answers below): there is no crash, just some system error message from malloc that gets in the way of the expected output. See below on how to avoid that.

Answer 1

As the comments have stated, there is no crash on allocation failure, only an error message.

If the message annoys you, you can turn it off by re-directing malloc's log to /dev/null, like this:

export MallocLogFile=/dev/null

Output before setting the environment variable looks like this:

newstr = prevstr:   Same block reused
a.out(4275,0x7fff7146e000) malloc: *** mach_vm_map(size=153288611651584) failed (error code=3)
*** error: can't allocate region
*** set a breakpoint in malloc_error_break to debug
could not alloc newsize=153288611651277. Sorry

After setting the variable the output looks like this:

newstr = prevstr:   Same block reused
could not alloc newsize=153288611651277. Sorry

Note: This is highly Mac-specific. See documentation for description of other environment variables that tweak the details of malloc's operation.

Answer 2

Code did not crash as "could not alloc newsize=153288611651277. Sorry" output occurred - just an additional message was printed. @Blagovest Buyukliev

The additional message is likely sent out on stderr rather than stdout. @Eugene Sh.