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).

score 9 · Accepted Answer · edited May 23 '17 at 12:13

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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.

edited May 23 '17 at 12:13

Community

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answered Aug 13 '13 at 11:40

user4815162342

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Thanks for the answer and for the details of `_GNU_SOURCE` on the given link. – Don't You Worry Child Aug 14 '13 at 05:02
The given link also recommends to make `#define _GNU_SOURCE` "the very first thing in the file, preceded only by comments". – Alexander Pozdneev Dec 06 '16 at 11:09

Dayal rai · Answer 2 · 2013-08-13T11:52:24.210

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

edited Aug 13 '13 at 11:52

answered Aug 13 '13 at 11:41

Dayal rai

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

2 Answers2