send(2) — Linux manual page


send(2)                    System Calls Manual                   send(2)

NAME         top

       send, sendto, sendmsg - send a message on a socket

LIBRARY         top

       Standard C library (libc, -lc)

SYNOPSIS         top

       #include <sys/socket.h>

       ssize_t send(int sockfd, const void buf[.len], size_t len, int flags);
       ssize_t sendto(int sockfd, const void buf[.len], size_t len, int flags,
                      const struct sockaddr *dest_addr, socklen_t addrlen);
       ssize_t sendmsg(int sockfd, const struct msghdr *msg, int flags);

DESCRIPTION         top

       The system calls send(), sendto(), and sendmsg() are used to
       transmit a message to another socket.

       The send() call may be used only when the socket is in a
       connected state (so that the intended recipient is known).  The
       only difference between send() and write(2) is the presence of
       flags.  With a zero flags argument, send() is equivalent to
       write(2).  Also, the following call

           send(sockfd, buf, len, flags);

       is equivalent to

           sendto(sockfd, buf, len, flags, NULL, 0);

       The argument sockfd is the file descriptor of the sending socket.

       If sendto() is used on a connection-mode (SOCK_STREAM,
       SOCK_SEQPACKET) socket, the arguments dest_addr and addrlen are
       ignored (and the error EISCONN may be returned when they are not
       NULL and 0), and the error ENOTCONN is returned when the socket
       was not actually connected.  Otherwise, the address of the target
       is given by dest_addr with addrlen specifying its size.  For
       sendmsg(), the address of the target is given by msg.msg_name,
       with msg.msg_namelen specifying its size.

       For send() and sendto(), the message is found in buf and has
       length len.  For sendmsg(), the message is pointed to by the
       elements of the array msg.msg_iov.  The sendmsg() call also
       allows sending ancillary data (also known as control

       If the message is too long to pass atomically through the
       underlying protocol, the error EMSGSIZE is returned, and the
       message is not transmitted.

       No indication of failure to deliver is implicit in a send().
       Locally detected errors are indicated by a return value of -1.

       When the message does not fit into the send buffer of the socket,
       send() normally blocks, unless the socket has been placed in
       nonblocking I/O mode.  In nonblocking mode it would fail with the
       error EAGAIN or EWOULDBLOCK in this case.  The select(2) call may
       be used to determine when it is possible to send more data.

   The flags argument
       The flags argument is the bitwise OR of zero or more of the
       following flags.

       MSG_CONFIRM (since Linux 2.3.15)
              Tell the link layer that forward progress happened: you
              got a successful reply from the other side.  If the link
              layer doesn't get this it will regularly reprobe the
              neighbor (e.g., via a unicast ARP).  Valid only on
              SOCK_DGRAM and SOCK_RAW sockets and currently implemented
              only for IPv4 and IPv6.  See arp(7) for details.

              Don't use a gateway to send out the packet, send to hosts
              only on directly connected networks.  This is usually used
              only by diagnostic or routing programs.  This is defined
              only for protocol families that route; packet sockets

       MSG_DONTWAIT (since Linux 2.2)
              Enables nonblocking operation; if the operation would
              block, EAGAIN or EWOULDBLOCK is returned.  This provides
              similar behavior to setting the O_NONBLOCK flag (via the
              fcntl(2) F_SETFL operation), but differs in that
              MSG_DONTWAIT is a per-call option, whereas O_NONBLOCK is a
              setting on the open file description (see open(2)), which
              will affect all threads in the calling process and as well
              as other processes that hold file descriptors referring to
              the same open file description.

       MSG_EOR (since Linux 2.2)
              Terminates a record (when this notion is supported, as for
              sockets of type SOCK_SEQPACKET).

       MSG_MORE (since Linux 2.4.4)
              The caller has more data to send.  This flag is used with
              TCP sockets to obtain the same effect as the TCP_CORK
              socket option (see tcp(7)), with the difference that this
              flag can be set on a per-call basis.

              Since Linux 2.6, this flag is also supported for UDP
              sockets, and informs the kernel to package all of the data
              sent in calls with this flag set into a single datagram
              which is transmitted only when a call is performed that
              does not specify this flag.  (See also the UDP_CORK socket
              option described in udp(7).)

       MSG_NOSIGNAL (since Linux 2.2)
              Don't generate a SIGPIPE signal if the peer on a stream-
              oriented socket has closed the connection.  The EPIPE
              error is still returned.  This provides similar behavior
              to using sigaction(2) to ignore SIGPIPE, but, whereas
              MSG_NOSIGNAL is a per-call feature, ignoring SIGPIPE sets
              a process attribute that affects all threads in the

              Sends out-of-band data on sockets that support this notion
              (e.g., of type SOCK_STREAM); the underlying protocol must
              also support out-of-band data.

       MSG_FASTOPEN (since Linux 3.7)
              Attempts TCP Fast Open (RFC7413) and sends data in the SYN
              like a combination of connect(2) and write(2), by
              performing an implicit connect(2) operation.  It blocks
              until the data is buffered and the handshake has
              completed.  For a non-blocking socket, it returns the
              number of bytes buffered and sent in the SYN packet.  If
              the cookie is not available locally, it returns
              EINPROGRESS, and sends a SYN with a Fast Open cookie
              request automatically.  The caller needs to write the data
              again when the socket is connected.  On errors, it sets
              the same errno as connect(2) if the handshake fails.  This
              flag requires enabling TCP Fast Open client support on
              sysctl net.ipv4.tcp_fastopen.

              Refer to TCP_FASTOPEN_CONNECT socket option in tcp(7) for
              an alternative approach.

       The definition of the msghdr structure employed by sendmsg() is
       as follows:

           struct msghdr {
               void         *msg_name;       /* Optional address */
               socklen_t     msg_namelen;    /* Size of address */
               struct iovec *msg_iov;        /* Scatter/gather array */
               size_t        msg_iovlen;     /* # elements in msg_iov */
               void         *msg_control;    /* Ancillary data, see below */
               size_t        msg_controllen; /* Ancillary data buffer len */
               int           msg_flags;      /* Flags (unused) */

       The msg_name field is used on an unconnected socket to specify
       the target address for a datagram.  It points to a buffer
       containing the address; the msg_namelen field should be set to
       the size of the address.  For a connected socket, these fields
       should be specified as NULL and 0, respectively.

       The msg_iov and msg_iovlen fields specify scatter-gather
       locations, as for writev(2).

       You may send control information (ancillary data) using the
       msg_control and msg_controllen members.  The maximum control
       buffer length the kernel can process is limited per socket by the
       value in /proc/sys/net/core/optmem_max; see socket(7).  For
       further information on the use of ancillary data in various
       socket domains, see unix(7) and ip(7).

       The msg_flags field is ignored.

RETURN VALUE         top

       On success, these calls return the number of bytes sent.  On
       error, -1 is returned, and errno is set to indicate the error.

ERRORS         top

       These are some standard errors generated by the socket layer.
       Additional errors may be generated and returned from the
       underlying protocol modules; see their respective manual pages.

       EACCES (For UNIX domain sockets, which are identified by
              pathname) Write permission is denied on the destination
              socket file, or search permission is denied for one of the
              directories the path prefix.  (See path_resolution(7).)

              (For UDP sockets) An attempt was made to send to a
              network/broadcast address as though it was a unicast

              The socket is marked nonblocking and the requested
              operation would block.  POSIX.1-2001 allows either error
              to be returned for this case, and does not require these
              constants to have the same value, so a portable
              application should check for both possibilities.

       EAGAIN (Internet domain datagram sockets) The socket referred to
              by sockfd had not previously been bound to an address and,
              upon attempting to bind it to an ephemeral port, it was
              determined that all port numbers in the ephemeral port
              range are currently in use.  See the discussion of
              /proc/sys/net/ipv4/ip_local_port_range in ip(7).

              Another Fast Open is in progress.

       EBADF  sockfd is not a valid open file descriptor.

              Connection reset by peer.

              The socket is not connection-mode, and no peer address is

       EFAULT An invalid user space address was specified for an

       EINTR  A signal occurred before any data was transmitted; see

       EINVAL Invalid argument passed.

              The connection-mode socket was connected already but a
              recipient was specified.  (Now either this error is
              returned, or the recipient specification is ignored.)

              The socket type requires that message be sent atomically,
              and the size of the message to be sent made this

              The output queue for a network interface was full.  This
              generally indicates that the interface has stopped
              sending, but may be caused by transient congestion.
              (Normally, this does not occur in Linux.  Packets are just
              silently dropped when a device queue overflows.)

       ENOMEM No memory available.

              The socket is not connected, and no target has been given.

              The file descriptor sockfd does not refer to a socket.

              Some bit in the flags argument is inappropriate for the
              socket type.

       EPIPE  The local end has been shut down on a connection oriented
              socket.  In this case, the process will also receive a
              SIGPIPE unless MSG_NOSIGNAL is set.

VERSIONS         top

       According to POSIX.1-2001, the msg_controllen field of the msghdr
       structure should be typed as socklen_t, and the msg_iovlen field
       should be typed as int, but glibc currently types both as size_t.

STANDARDS         top


       MSG_CONFIRM is a Linux extension.

HISTORY         top

       4.4BSD, SVr4, POSIX.1-2001.  (first appeared in 4.2BSD).

       POSIX.1-2001 describes only the MSG_OOB and MSG_EOR flags.
       POSIX.1-2008 adds a specification of MSG_NOSIGNAL.

NOTES         top

       See sendmmsg(2) for information about a Linux-specific system
       call that can be used to transmit multiple datagrams in a single

BUGS         top

       Linux may return EPIPE instead of ENOTCONN.

EXAMPLES         top

       An example of the use of sendto() is shown in getaddrinfo(3).

SEE ALSO         top

       fcntl(2), getsockopt(2), recv(2), select(2), sendfile(2),
       sendmmsg(2), shutdown(2), socket(2), write(2), cmsg(3), ip(7),
       ipv6(7), socket(7), tcp(7), udp(7), unix(7)

Linux man-pages (unreleased)     (date)                          send(2)

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