C预处理器重新定义依赖于包含顺序的冲突

Question

I just had a redefine conflict in the project I'm working on and while tracing down why it's not happening on all platforms (turned out to be to order of includes), I stumbled upon the following behavior which I cannot explain.

1. compiles without warnings

    #define LIST_HEAD(a) { int a = 0; }                                                                                                                     
    #include <sys/queue.h>                                                          

    int main() {                                                                    
        return 0;                                                               
    }

2. "macro redefined" warning

    #include <sys/queue.h>
    #define LIST_HEAD(a) { int a = 0; }                                                                                                       

    int main() {                                                                    
        return 0;                                                               
    }

I would expect both cases to produce the warning, since there're no checks in <sys/queue.h> that would prevent a redefine.

So why does the first case produces no warning, while the second one does? What I'm missing here?

Btw: I get the same results on my Mac with clang and my Linux box with gcc.

Answer 1

By default, this warning is suppressed in system headers. The code in <sys/queue.h> is considered to come from a system header because sys/queue.h was found by searching a path marked as containing system headers.

So in (2) you see the warning because it is generated within your code, while in (1) the warning is generated within queue.h , and so is suppressed. Add -Wsystem-headers to your compilation options, and you'll see the warning in both cases.