EAGAIN

EAGAIN is an errno value defined by POSIX that indicates the resource or operation is temporarily unavailable and should be retried later. It is most commonly encountered with non-blocking I/O or asynchronous operations, where a call would block if it waited for the resource.

In practice, EAGAIN appears when a non-blocking file descriptor cannot complete an operation immediately. For example, reading from a non-blocking socket with no data available, or writing when the output buffer is full, may return -1 and set errno to EAGAIN. In these cases the correct response is typically to try again later or to wait for the descriptor to become ready (using mechanisms such as select, poll, or epoll).

Relation to EWOULDBLOCK: on many systems, EWOULDBLOCK is defined as an alias of EAGAIN, sharing the same numeric value and meaning. This reflects historical usage where socket operations could set EWOULDBLOCK for non-blocking operations; POSIX treats EAGAIN as the standard name, while EWOULDBLOCK remains a compatibility synonym in some environments.

Handling and patterns: code that performs non-blocking I/O often checks for EAGAIN (or EWOULDBLOCK) and then retries the operation after awaiting readiness. It is important to distinguish EAGAIN from other errors such as EINTR (interrupted by a signal); EINTR typically indicates the operation should be retried, but the appropriate response depends on context and the specific system call.

Platform considerations: while Unix-like systems generally use EAGAIN, Windows handles non-blocking socket operations with a separate error code (WSAEWOULDBLOCK) rather than EAGAIN. Regular files on many systems do not usually return EAGAIN, since they are typically always ready for I/O. See also: EINTR, EPOLL, SELECT, WSAEWOULDBLOCK.