setsockopt

Setsockopt is a function in the sockets API used to configure options for a socket. It allows enabling, disabling, or tuning features that affect socket behavior or performance, at the socket level (SOL_SOCKET) or at a protocol level (for example IPPROTO_TCP or IPPROTO_IP).

The standard signature in C is: int setsockopt(int sockfd, int level, int optname, const void *optval, socklen_t optlen); On success it returns 0; on failure it returns -1 and errno is set to indicate the error.

Parameters: sockfd identifies the socket. level selects the protocol layer or SOL_SOCKET. optname specifies the option to configure. optval points to the option value and optlen is the size of that value. For most socket options, optval is a pointer to an int, but some options use a structure (for example struct linger for SO_LINGER or a timeout structure for SO_SNDTIMEO and SO_RCVTIMEO).

Common socket-level options include SO_REUSEADDR, SO_KEEPALIVE, SO_BROADCAST, SO_LINGER, SO_SNDBUF, SO_RCVBUF, SO_SNDLOWAT, and the timeouts SO_SNDTIMEO and SO_RCVTIMEO. Protocol-specific options include TCP_NODELAY under IPPROTO_TCP.

Notes: The option value is copied by the implementation, and optlen must reflect the type and size. The function can fail for reasons such as EBADF (bad socket), ENOTSOCK, EINVAL, ENOPROTOOPT, EACCES/EPERM, or EFAULT.

See also: getsockopt, socket, and the relevant protocol documentation.