Ubuntu Manpage: send, sendto, sendmsg - send a message on a socket

NAME

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

LIBRARY

       Standard C library (libc, -lc)

SYNOPSIS

       #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

       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 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.

       MSG_DONTROUTE
              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
              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.

       MSG_OOB
              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.

   sendmsg()
       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

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

ERRORS

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.

EAGAIN or EWOULDBLOCK
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).

EALREADY
Another Fast Open is in progress.

EBADF sockfd is not a valid open file descriptor.

ECONNRESET
Connection reset by peer.

EDESTADDRREQ
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.

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

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

ENOBUFS
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.

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

ENOTSOCK
The file descriptor sockfd does not refer to a socket.

EOPNOTSUPP
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

       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

       POSIX.1-2008.

       MSG_CONFIRM is a Linux extension.

HISTORY

       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

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

BUGS

       Linux may return EPIPE instead of ENOTCONN.

EXAMPLES

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