errno 11 (EAGAIN): meaning and diagnosis

Status / error code:		Category:

Constant name:
Source:		Operating System:
Description: a nonblocking socket cannot make progress now; wait for readiness and retry.

Could be also:
Constant	Type	OS
ERROR_BAD_FORMAT	Win32 error	Windows
KERN_ALREADY_IN_SET	Kern return	Mac
ippStsResFloor	Intel Ipp Status	Any
EDEADLK	errno	Mac
EAGAIN	errno	Solaris
EAGAIN	errno	Windows
NO_EXCEPTION_HANDLING_SUPPORT	BugCheck Code	Windows

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EAGAIN

On a socket, EAGAIN is normally a backpressure signal rather than a broken connection. A nonblocking receive has no data available yet, or a nonblocking send cannot currently accept more bytes into the local output buffer. The same code can also be returned when a socket receive timeout expires before data arrives.

Portable code must also consider EWOULDBLOCK. POSIX allows the two names to have different numeric values, even though many systems define them as the same value. An event loop should stop the current read or write pass when it receives either result, wait for the appropriate readiness event, and then retry. Repeating the system call in a tight loop only burns CPU.

What to check

Confirm whether O_NONBLOCK, MSG_DONTWAIT, or a socket timeout is in use.
For writes, preserve unsent bytes and resume from the correct offset after the socket becomes writable.
For reads, distinguish EAGAIN from a zero-length return, which has different stream-socket semantics.

On Linux, an unbound Internet datagram socket can also report EAGAIN when every ephemeral port is in use, so the failing call and socket type matter.

Linux recv(2) · Linux send(2) · POSIX error definitions

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