sendtimeout

Sendtimeout is a socket option used to control how long a send operation can block when attempting to transmit data over a network socket. Its purpose is to prevent a program from waiting indefinitely if the underlying network or peer is slow to accept data. The setting applies to blocking send calls; it does not guarantee that all bytes will be sent, only that the call will not block longer than the specified timeout.

On POSIX systems, the option is SO_SNDTIMEO and is set with setsockopt(SOL_SOCKET, SO_SNDTIMEO, ...). The value is a struct timeval, typically specifying seconds and microseconds. If the timeout expires before the send completes, the call fails with an error such as EWOULDBLOCK or EAGAIN, depending on the platform.

Windows implementations of Winsock also expose SO_SNDTIMEO. The timeout is provided as a timeval structure, indicating the maximum duration the send will block. When the timeout elapses, the send operation fails with a corresponding would-block error.

Notes and caveats: the behavior of sendtimeout is platform- and protocol-dependent. On non-blocking sockets, timeouts are often ignored because send calls return immediately with EWOULDBLOCK. Some protocols or kernel implementations may not honor the option in all circumstances. The flag is advisory and should be used in conjunction with robust error handling and logic to manage partial sends and retries.

Related options include SO_RCVTIMEO for receive timeouts and various I/O multiplexing techniques (select, poll, epoll) used to implement overall timeouts in event-driven applications.