The send() function shall initiate transmission of a message
from the specified socket to its peer. The send()
function shall send a message only when the socket is connected (including
when the peer of a connectionless socket has been set
The send() 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
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
The length of the message to be sent is specified by the length
argument. If the message is too long to pass through the
underlying protocol, send() shall fail and no data shall be
Successful completion of a call to send() does not guarantee
delivery of the message. A return value of -1 indicates only
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 set, send() 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, send() shall fail. The select() and
poll() functions can be used to
determine when it is possible to send more data.
The socket in use may require the process to have appropriate privileges
to use the send() function.
Upon successful completion, send() shall return the number of
bytes sent. Otherwise, -1 shall be returned and
errno set to indicate the error.
The send() function shall fail if:
EAGAIN or EWOULDBLOCK
The socket's file descriptor is marked O_NONBLOCK and the requested
operation would block.
The socket 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 send() 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 or otherwise has not had the peer pre-specified.
The socket argument does not refer to a socket.
The 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, the SIGPIPE signal is generated
to the calling thread.
The send() function may fail if:
The calling process does not have the 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.
The send() function is equivalent to sendto() with a null
dest_len argument, and to write() if no flags are used.
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 6, Copyright (C) 2001-2003 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