On Mon, 27 Aug 2007 13:57:42 -0700
Joe Perches <[EMAIL PROTECTED]> wrote:
> On Mon, 2007-08-27 at 13:41 -0700, David Miller wrote:
> > From: Johannes Berg <[EMAIL PROTECTED]>
> > Date: Mon, 27 Aug 2007 12:54:09 +0200
> > > #define MAC_FMT "%s"
> > > #define MAC_ARG(a) ({char __buf[18]; print_mac(a, __buf); __buf;})
>
> > 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As Dave said, you are passing out a variable which is no longer valid outside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 <[EMAIL PROTECTED]>
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