What does EAGAIN mean?

Question

As in the title what does EAGAIN mean?

Frédéric Hamidi · Accepted Answer · 2014-08-18T21:38:19.670

97

EAGAIN is often raised when performing non-blocking I/O. It means "there is no data available right now, try again later".

It might (or might not) be the same as EWOULDBLOCK, which means "your thread would have to block in order to do that".

edited Aug 18 '14 at 21:38

answered Oct 30 '10 at 11:01

Frédéric Hamidi

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According to IEEE 1003.1, `EAGAIN` may be the same as `EWOULDBLOCK`. http://www.opengroup.org/onlinepubs/000095399/basedefs/errno.h.html – Fred Foo Oct 30 '10 at 11:09
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What I mean is: a portable program should not rely on them being distinct. – Fred Foo Oct 30 '10 at 11:12
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On Linux EAGAIN & EWOULDBLOCK are the same value but truly portable code should check both. – hookenz May 27 '14 at 21:16
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It might be because you reached a ulimit soft/hard resource such as threads, files or network connections. – Buzut Apr 28 '16 at 10:39

score 27 · Answer 2 · answered Mar 05 '13 at 15:26

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Using man 2 intro | less -Ip EAGAIN:

     35 EAGAIN Resource temporarily unavailable.  This is a temporary condi-
         tion and later calls to the same routine may complete normally.

answered Mar 05 '13 at 15:26

turfx

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Down to the core.. like it ;) – brunsgaard Mar 05 '15 at 00:41

score 7 · Answer 3 · answered Mar 05 '15 at 00:59

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What it means is less important. What it implies:

your system call failed
nothing happened (system calls are atomic, and this one just did not happen)
you could try it again (it could fail again, possibly with a different result)
or you could choose otherwise.

The whole thing about EAGAIN is that your process is not blocked inside the system call; it has the right to choose: either retry or do something useful.

answered Mar 05 '15 at 00:59

wildplasser

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Do you have a source to support the "system calls are atomic" claim? I find contradictory results – sehe Jul 10 '18 at 11:20

score 0 · Answer 4 · answered Aug 22 '23 at 07:12

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Excerpt errno(3) - Linux manual page:

EAGAIN Resource temporarily unavailable (may be the same value as EWOULDBLOCK) (POSIX.1-2001).

Use case: When epoll in ET mode, only wait after read or write return EAGAIN

answered Aug 22 '23 at 07:12

douyu

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score -1 · Answer 5 · answered Oct 30 '10 at 11:01

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According to this, it means "Operation would have caused the process to be suspended."

answered Oct 30 '10 at 11:01

thelost

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link gives HTTP 404 – rofrol Apr 10 '21 at 07:02

What does EAGAIN mean?

5 Answers5

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