Question

Are Statement and Declarations in Expressions specific to GNU C ? Or this feature is also included in C99 standard ?

Answer 1

It’s a GCC extension. (See the GCC docs, e.g. here for gcc 4.3.3, for a full list of GCC extensions; and the C99 spec is available here.)

GCC will warn about such things if you use the -pedantic -std=c99 flags, e.g.:

$ cat foo.c
int main(void)
{
  return ({ int a = 0; a; });
}
$ gcc -pedantic -std=c99 -c foo.c
foo.c: In function 'main':
foo.c:3: warning: ISO C forbids braced-groups within expressions