SEND(2)                     Linux Programmer's Manual                    SEND(2)

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

       #include <sys/socket.h>

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

       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 information).

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

       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

       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

              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 don't.

       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 process.

              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.

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

           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,

       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

       The msg_flags field is ignored.

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

       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 address.

              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

              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 set.

       EFAULT An invalid user space address was specified for an argument.

       EINTR  A signal occurred before any data was transmitted; see signal(7).

       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 impossible.

              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

       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

       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.

       4.4BSD, SVr4, POSIX.1-2001.  These interfaces 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.  The MSG_CONFIRM flag is a Linux

       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.

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

       Linux may return EPIPE instead of ENOTCONN.

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

       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)

       This page is part of release 5.13 of the Linux man-pages project.  A
       description of the project, information about reporting bugs, and the
       latest version of this page, can be found at

Linux                              2021-03-22                            SEND(2)