SEND(3P)

PROLOG
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.

NAME
send — send a message on a socket

SYNOPSIS

#include <sys/socket.h>

ssize_t send(int socket, const void *buffer, size_t length, int flags);

DESCRIPTION
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.

RETURN VALUE
Upon successful completion, send() shall return the number of bytes sent. Otherwise, -1 shall be returned and errno set to indicate the error.

ERRORS
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.

EXAMPLES
None.

APPLICATION USAGE
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().

RATIONALE
None.

FUTURE DIRECTIONS
None.

SEE ALSO
connect(), getsockopt(), poll(), pselect(), recv(), recvfrom(), recvmsg(), sendmsg(), sendto(), setsockopt(), shutdown(), socket(), write()

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

COPYRIGHT
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 .

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