sendto(2) — Linux manual page


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

NAME         top

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

SYNOPSIS         top

       #include <sys/types.h>
       #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);

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 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 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 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 ad‐
       dress.  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_con‐
       trol 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 informa‐
       tion 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 appropriately.

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 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 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 set.

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

       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 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 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

       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.

CONFORMING TO         top

       4.4BSD, SVr4, POSIX.1-2001.  These interfaces first appeared in

       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 extension.

NOTES         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.

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

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)

COLOPHON         top

       This page is part of release 5.09 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                            2020-11-01                          SEND(2)

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