What is this (( ))?

Question

While browsing through the gcc compiler source code (gcc/c-family/c-pragma.c) I see:

typedef struct GTY(()) align_stack {
  int                  alignment;
  tree                 id;
  struct align_stack * prev;
} align_stack;

and regardless of having lots of C programming years behind me, these bits: (()) are totally unknown to me yet. Can someone please explain what they mean? Google does not seem to find it.

Answer 1

They are GCC internal "magic", i.e. part of the compiler implementation itself.

See this page which talks about their use. The macro is used to mark types for garbage-collection purposes. There can be arguments too, see this page for details.

UPDATE:: As pointed out by Drew Dorman in a comment, the actual double parenthesis are not part of the "internalness" of the GNU implementation; they're commonly used when you want to collect an entire list of arguments into a single argument for the called macro. This can be useful sometimes when wrapping e.g. printf(), too. See this question, for more on this technique.

Answer 2

In general, it's used with macros to shield commas. Given #define foo(a,b), the macro invocation foo(1,2,3) would be illegal. Using an extra pair of parenthesis clarifies which comma is shielded: foo((1,2),3) versus foo(1,(2,3)).

In this case, the GTY can take multiple arguments, separated by commas, but all these commas must be shielded. That's why the inner () surround all arguments.