__attribute__((noreturn))的用法

外文地址：http://www.unixwiz.net/techtips/gnu-c-attributes.html

__attribute__ noreturn    表示没有返回值

This attribute tells the compiler that the function won't ever return, and this can be used to suppress errors about code paths not being reached. The C library functions abort() and exit() are both declared with this attribute:

这个属性告诉编译器函数不会返回，这可以用来抑制关于未达到代码路径的错误。 C库函数abort（）和exit（）都使用此属性声明：

extern void exit(int)   __attribute__((noreturn));
extern void abort(void) __attribute__((noreturn));

Once tagged this way, the compiler can keep track of paths through the code and suppress errors that won't ever happen due to the flow of control never returning after the function call.

In this example, two nearly-identical C source files refer to an "exitnow()" function that never returns, but without the __attribute__tag, the compiler issues a warning. The compiler is correct here, because it has no way of knowing that control doesn't return.

$ cat test1.c
extern void exitnow();

int foo(int n)
{
        if ( n > 0 )
	{
                exitnow();
		/* control never reaches this point */
	}
        else
                return 0;
}

$ cc -c -Wall test1.c
test1.c: In function `foo':
test1.c:9: warning: this function may return with or without a value

But when we add __attribute__, the compiler suppresses the spurious warning:

$ cat test2.c
extern void exitnow() __attribute__((noreturn));

int foo(int n)
{
        if ( n > 0 )
                exitnow();
        else
                return 0;
}

$ cc -c -Wall test2.c
no warnings!