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PROLOG | NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | EXAMPLES | APPLICATION USAGE | RATIONALE | FUTURE DIRECTIONS | SEE ALSO | COPYRIGHT |
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SEND(3P) POSIX Programmer's Manual SEND(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 — send a message on a socket
#include <sys/socket.h>
ssize_t send(int socket, const void *buffer, size_t length, int flags);
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. If the socket is a
connectionless-mode socket, the message shall be sent to the pre-
specified peer address.
The send() function takes the following arguments:
socket Specifies the socket file descriptor.
buffer Points to the buffer containing the message to send.
length Specifies the length of the message in bytes.
flags Specifies the type of message transmission. Values of
this argument are formed by logically OR'ing zero or
more of the following flags:
MSG_EOR Terminates a record (if supported by the
protocol).
MSG_OOB Sends out-of-band data on sockets that
support out-of-band communications. The
significance and semantics of out-of-
band data are protocol-specific.
MSG_NOSIGNAL 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 [EPIPE] error shall still
be returned.
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
transmitted.
Successful completion of a call to send() 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 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.
EBADF The socket argument is not a valid file descriptor.
ECONNRESET
A connection was forcibly closed by a peer.
EDESTADDRREQ
The socket is not connection-mode and no peer address is
set.
EINTR A signal interrupted send() before any data was
transmitted.
EMSGSIZE
The message is too large to be sent all at once, as the
socket requires.
ENOTCONN
The socket is not connected.
ENOTSOCK
The socket argument does not refer to a socket.
EOPNOTSUPP
The socket argument is associated with a socket that does
not support one or more of the values set in flags.
EPIPE 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.
The send() function may fail if:
EACCES The calling process does not have appropriate privileges.
EIO An I/O error occurred while reading from or writing to the
file system.
ENETDOWN
The local network interface used to reach the destination
is down.
ENETUNREACH
No route to the network is present.
ENOBUFS
Insufficient resources were available in the system to
perform the operation.
The following sections are informative.
None.
If the socket argument refers to a connection-mode socket, the
send() function is equivalent to sendto() (with any value for the
dest_addr and dest_len arguments, as they are ignored in this
case). If the socket argument refers to a socket and the flags
argument is 0, the send() function is equivalent to write().
None.
None.
connect(3p), getsockopt(3p), poll(3p), pselect(3p), recv(3p),
recvfrom(3p), recvmsg(3p), sendmsg(3p), sendto(3p),
setsockopt(3p), shutdown(3p), socket(3p), write(3p)
The Base Definitions volume of POSIX.1‐2017, sys_socket.h(0p)
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
http://www.opengroup.org/unix/online.html .
Any typographical or formatting errors that appear in this page
are most likely to have been introduced during the conversion of
the source files to man page format. To report such errors, see
https://www.kernel.org/doc/man-pages/reporting_bugs.html .
IEEE/The Open Group 2017 SEND(3P)
Pages that refer to this page: sys_socket.h(0p), connect(3p), recv(3p), recvfrom(3p), recvmsg(3p), sendmsg(3p), sendto(3p), shutdown(3p), socket(3p)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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