attribute noreturn

最新推荐文章于 2023-08-05 19:34:57 发布

最新推荐文章于 2023-08-05 19:34:57 发布 · 105 阅读

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#C #C++ #C#

本文介绍如何在C语言中使用__attribute__((noreturn))属性来标记那些不会返回的函数，如abort()和exit()等。通过这种方式，可以告诉编译器这些函数永远不会返回，从而避免关于未到达的代码路径的错误警告。

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:

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!