[小技巧] gcc attribute error 属性小试

gcc __attribute__  里有一个属性是 error 能够用于编译时报错。

參考：

https://gcc.gnu.org/onlinedocs/gcc-4.3.0/gcc/Function-Attributes.html

error ("message")

If this attribute is used on a function declaration and a call to such a functionis not eliminated through dead code elimination or other optimizations, an errorwhich
will include message will be diagnosed. This is usefulfor compile time checking, especially together with
__builtin_constant_pand inline functions where checking the inline function arguments is notpossible through
extern char [(condition) ? 1 : -1]; tricks. While it is possible to leave the function undefined and thus invokea link failure, when using this attribute the problem will be diagnosedearlier and with exact location of the call even in presence
of inlinefunctions or when not emitting debugging information.

写了一个简单的演示样例：

extern void foo(void) __attribute__((error("build failed"))); 

#define FOO 0
int main(void)
{
	if(FOO == 0)
		foo();

	return 1;
}

这里因为 FOO == 0 推断成立，所以会在编译时候直接报以下的错误：

/tmp/helloworld.c: In function 'main':

/tmp/helloworld.c:7:6: error: call to 'foo' declared with attribute error: build failed

这里文件名称／行号和错误的信息 "build failed" 都会输出。

另外。内核的 BUILD_BUG() 这个宏也是用了GCC这个属性。