This manual page is part of the POSIX Programmer's Manual. The Linux implementation of this interface may differ (consult the corresponding Linux manual page for details of Linux behavior), or the interface may not be implemented on Linux.

setsockopt — set the socket options

#include <sys/socket.h>
int setsockopt(int socket, int level, int option_name,
    const void *option_value, socklen_t option_len);

The setsockopt() function shall set the option specified by the option_name argument, at the protocol level specified by the level argument, to the value pointed to by the option_value argument for the socket associated with the file descriptor specified by the socket argument.

The level argument specifies the protocol level at which the option resides. To set options at the socket level, specify the level argument as SOL_SOCKET. To set options at other levels, supply the appropriate level identifier for the protocol controlling the option. For example, to indicate that an option is interpreted by the TCP (Transport Control Protocol), set level to IPPROTO_TCP as defined in the <netinet/in.h> header.

The option_name argument specifies a single option to set. It can be one of the socket-level options defined in <sys_socket.h> and described in Section 2.10.16, Use of Options. If option_name is equal to SO_RCVTIMEO or SO_SNDTIMEO and the implementation supports setting the option, it is unspecified whether the struct timeval pointed to by option_value is stored as provided by this function or is rounded up to align with the resolution of the clock being used. If setsockopt() is called with option_name equal to SO_ACCEPTCONN, SO_ERROR, or SO_TYPE, the behavior is unspecified.

Upon successful completion, setsockopt() shall return 0. Otherwise, -1 shall be returned and errno set to indicate the error.

The setsockopt() function shall fail if:

The socket argument is not a valid file descriptor.
The send and receive timeout values are too big to fit into the timeout fields in the socket structure.
The specified option is invalid at the specified socket level or the socket has been shut down.
The socket is already connected, and a specified option cannot be set while the socket is connected.

The option is not supported by the protocol.
The socket argument does not refer to a socket.

The setsockopt() function may fail if:

There was insufficient memory available for the operation to complete.
Insufficient resources are available in the system to complete the call.

The following sections are informative.


The setsockopt() function provides an application program with the means to control socket behavior. An application program can use setsockopt() to allocate buffer space, control timeouts, or permit socket data broadcasts. The <sys/socket.h> header defines the socket-level options available to setsockopt().

Options may exist at multiple protocol levels. The SO_ options are always present at the uppermost socket level.



Section 2.10, Sockets, bind(), endprotoent(), getsockopt(), socket()

The Base Definitions volume of POSIX.1‐2017, <netinet_in.h>, <sys_socket.h>

Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1-2017, Standard for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base Specifications Issue 7, 2018 Edition, Copyright (C) 2018 by the Institute of Electrical and Electronics Engineers, Inc and The Open Group. In the event of any discrepancy between this version and the original IEEE and The Open Group Standard, the original IEEE and The Open Group Standard is the referee document. The original Standard can be obtained online at .

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2017 IEEE/The Open Group