EWOULDBLOCK

EWOULDBLOCK is an errno value used by Unix-like systems to indicate that a non-blocking I/O operation would block if it were allowed to proceed. It is most commonly encountered when performing read or write operations on a file descriptor that has been opened with the O_NONBLOCK flag, or when working with non-blocking sockets, pipes, or fifos.

In practice, a non-blocking I/O call that cannot complete immediately returns an error of -1 with errno set to EWOULDBLOCK. Because EWOULDBLOCK is often implemented as the same value as EAGAIN, many systems treat these two errors equivalently. For portability, code frequently checks for both EWOULDBLOCK and EAGAIN when handling non-blocking I/O.

Typical usage involves non-blocking I/O alongside readiness notifications. Programs may use mechanisms such as select, poll, epoll, or kqueue to wait for a file descriptor to become ready for reading or writing, and then retry the operation. When a call returns EWOULDBLOCK, the appropriate response is usually to wait for the descriptor to become ready and retry later, or to implement a backoff strategy.

Notes:

- EWOULDBLOCK is most relevant to non-blocking file descriptors, including sockets, pipes, and fifos.

- On regular files, operations typically do not produce EWOULDBLOCK because they are not constrained by readiness in the same way as non-blocking I/O on sockets or pipes.

- For portability, many code bases check for both EWOULDBLOCK and EAGAIN.

See also EAGAIN, O_NONBLOCK, errno.