Re: [PATCH net-2.6.24] introduce MAC_FMT/MAC_ARG

On Mon, 27 Aug 2007 13:57:42 -0700
Joe Perches [off-list ref] wrote:

On Mon, 2007-08-27 at 13:41 -0700, David Miller wrote:

quoted

From: Johannes Berg <johannes@sipsolutions.net>
Date: Mon, 27 Aug 2007 12:54:09 +0200

quoted

#define MAC_FMT "%s"
#define MAC_ARG(a) ({char __buf[18]; print_mac(a, __buf); __buf;})

quoted

I don't think this works.

$ cat test_fmt.c
#include <stdio.h>
#include <stdlib.h>

#define MAC_FMT "%s"
#define MAC_ARG(a) ({char __buf[18]; print_mac(a, __buf); __buf;})

int print_mac(const char* p, char* b)
{
  return sprintf(b, "%02x:%02x:%02x:%02x:%02x:%02x",
		 p[0], p[1], p[2], p[3], p[4], p[5]);
}

int main(int argc, char** argv)
{
  char m1[6] = {1,2,3,4,5,6};
  char m2[6] = {6,5,4,3,2,1};

  printf("m1: " MAC_FMT " m2: " MAC_FMT "\n", MAC_ARG(m1), MAC_ARG(m2));
  return 0;
}

$ gcc test_fmt.c
$ ./a.out
m1: 01:02:03:04:05:06 m2: 06:05:04:03:02:01

As Dave said, you are passing out a variable which is no longer valid outside
of it's scope. GCC today may accidentally allow it or it might work, but it
is only because of a GCC bug. If I recall discussions about some of the
recent kernel space bloat, GCC doesn't reuse space for variables declared
in subblocks.

I.e:
int foo(int x) {
	if (x) {
	    char block1[1024];
		...
	} else {
	    char block2[128];
	}

}

Compiler should be able to use same stack space for block1/block2 and only grow
stack by 1K. But it probably isn't that smart.




-- 
Stephen Hemminger [off-list ref]