这是什么(()) ？

在浏览 gcc 编译器源代码(gcc/c-family/c-praga.c)时，我看到:

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

不管我有多少年的 C 编程经验，这些比特: (())对我来说是完全陌生的。有人能解释一下这是什么意思吗？谷歌似乎没有找到它。

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最佳答案

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.

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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.