__attribute__

__attribute__ noreturn

This attribute tells the compiler that the function won't ever return,and this can be used to suppress errors about code paths not beingreached. The C library functionsabort() and exit() are bothdeclared 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 throughthe code and suppress errors that won't ever happen due to the flowof 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 compileris correct here, because it has no way of knowing that controldoesn't return.

$ cat test1.cextern void exitnow();int foo(int n){        if ( n > 0 ){                exitnow();/* control never reaches this point */}        else                return 0;}$ cc -c -Wall test1.ctest1.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 spuriouswarning:

$ cat test2.cextern void exitnow() __attribute__((noreturn));int foo(int n){        if ( n > 0 )                exitnow();        else                return 0;}$ cc -c -Wall test2.cno warnings!

参考：https://www.cnblogs.com/astwish/p/3460618.html

           http://www.unixwiz.net/techtips/gnu-c-attributes.html#noreturn