Why do we need to define _GNU_SOURCE macro before using POLLRDHUP event flag in poll() function?

Question

I am using poll() function in my program, I read this link that to use POLLRDHUP flag you have to define _GNU_SOURCE before all header file inclusion. I needed this flag so that polling the socket can tell me whether the client has hung up or not, so that I can terminate the corresponding thread.

By the way I am writing a C program for an 'echo' server which can handle multiple clients concurrently, and I am using GCC 4.1.2 on OpenSuse Linux Enterprise Server 10.3 (x86_64).

Answer 1

POLLRDHUP is a non-standard extension (it is missing from POSIX). To prevent polluting the namespace, non-standard extensions are not visible unless you explicitly request them by defining _GNU_SOURCE.

More details on _GNU_SOURCE can be found in previous StackOverflow answers such as this one.

Answer 2

_GNU_SOURCE is a feature test macros which is useful for creating portable applications, by preventing nonstandard definitions from being exposed.