Section: POSIX Programmer's Manual (3P)
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.
--- send a message on a socket
ssize_t send(int socket, const void *buffer, size_t length, int flags);
function shall initiate transmission of a message from the specified
socket to its peer. The
function shall send a message only when the socket is connected. If
the socket is a connectionless-mode socket, the message shall be sent
to the pre-specified peer address.
function takes the following arguments:
Specifies the socket file descriptor.
Points to the buffer containing the message to send.
Specifies the length of the message in bytes.
Specifies the type of message transmission. Values of this argument are
formed by logically OR'ing zero or more of the following flags:
Terminates a record (if supported by the protocol).
Sends out-of-band data on sockets that support out-of-band
communications. The significance and semantics of out-of-band data are
Requests not to send the SIGPIPE signal if an attempt to send is made
on a stream-oriented socket that is no longer connected. The
error shall still be returned.
The length of the message to be sent is specified by the
argument. If the message is too long to pass through the underlying
shall fail and no data shall be transmitted.
Successful completion of a call to
does not guarantee delivery of the message. A return value of -1
indicates only locally-detected errors.
If space is not available at the sending socket to hold the message to
be transmitted, and the socket file descriptor does not have O_NONBLOCK
shall block until space is available. If space is not available at the
sending socket to hold the message to be transmitted, and the socket
file descriptor does have O_NONBLOCK set,
shall fail. The
functions can be used to determine when it is possible to send more
The socket in use may require the process to have appropriate
privileges to use the
Upon successful completion,
shall return the number of bytes sent. Otherwise, -1 shall be
set to indicate the error.
function shall fail if:
- EAGAIN or EWOULDBLOCK
The socket's file descriptor is marked O_NONBLOCK and the requested
operation would block.
argument is not a valid file descriptor.
A connection was forcibly closed by a peer.
The socket is not connection-mode and no peer address is set.
A signal interrupted
before any data was transmitted.
The message is too large to be sent all at once, as the socket requires.
The socket is not connected.
argument does not refer to a socket.
argument is associated with a socket that does not support one or more
of the values set in
The socket is shut down for writing, or the socket is connection-mode
and is no longer connected. In the latter case, and if the socket is of
type SOCK_STREAM or SOCK_SEQPACKET and the MSG_NOSIGNAL flag is not set,
the SIGPIPE signal is generated to the calling thread.
function may fail if:
The calling process does not have appropriate privileges.
An I/O error occurred while reading from or writing to the file system.
The local network interface used to reach the destination is down.
No route to the network is present.
Insufficient resources were available in the system to perform the
The following sections are informative.
argument refers to a connection-mode socket, the
function is equivalent to
(with any value for the
arguments, as they are ignored in this case). If the
argument refers to a socket and the
argument is 0, the
function is equivalent to
The Base Definitions volume of POSIX.1-2017,
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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