What does EAGAIN mean?

Question

As in the title what does EAGAIN mean?

Solution 1

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".

Solution 2

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.

Solution 3

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.

Comments

[Fred Foo]

According to IEEE 1003.1, EAGAIN may be the same as EWOULDBLOCK. opengroup.org/onlinepubs/000095399/basedefs/errno.h.html

[Fred Foo]

What I mean is: a portable program should not rely on them being distinct.

[hookenz]

On Linux EAGAIN & EWOULDBLOCK are the same value but truly portable code should check both.

[brunsgaard]

Down to the core.. like it ;)

[Buzut]

It might be because you reached a ulimit soft/hard resource such as threads, files or network connections.

[sehe]

Do you have a source to support the "system calls are atomic" claim? I find contradictory results

[rofrol]

link gives HTTP 404