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

Provided by: manpages-dev_3.54-1ubuntu1_all

NAME

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

SYNOPSIS

       #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

       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 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).  Only valid 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 can also be enabled using the O_NONBLOCK flag with the F_SETFL fcntl(2)).

       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)
              Requests  not  to  send SIGPIPE on errors on stream oriented sockets when the other end breaks the
              connection.  The EPIPE error is still returned.

       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.

       The  definition  of  the msghdr structure follows.  See recv(2) and below for an exact description of its
       fields.

           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 on received message */
           };

       You may send control information 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).

RETURN VALUE

       On success, these calls return the number of characters sent.  On error, -1 is returned, and errno is set
       appropriately.

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.

EBADF An invalid descriptor was specified.

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 argument sockfd is not 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.

CONFORMING TO

       4.4BSD, SVr4, POSIX.1-2001.  These function calls 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 extension.

NOTES

       The  prototypes given above follow the Single UNIX Specification, as glibc2 also does; the flags argument
       was int in 4.x BSD, but unsigned int in libc4 and libc5; the len argument was int in 4.x BSD  and  libc4,
       but size_t in libc5; the addrlen argument was int in 4.x BSD and libc4 and libc5.  See also accept(2).

       According to POSIX.1-2001, the msg_controllen field of the msghdr structure should be typed as socklen_t,
       but glibc currently types it 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

       Linux may return EPIPE instead of ENOTCONN.

EXAMPLE

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

COLOPHON

       This page is part of release 3.54 of the Linux man-pages project.  A  description  of  the  project,  and
       information about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.