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


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


     #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);


     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
         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.  Oth-
     erwise, 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 trans-
     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 nonblock-
     ing 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 deter-
     mine 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 proto-
            col 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 behav-
            ior  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 sock-
            ets 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 fea-
            ture, 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 fol-
         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 tar-
     get  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 using the msg_control and msg_con-
     trollen members.  The maximum control  buffer  length  the  kernel  can
     process  is  limited per socket by the value in /proc/sys/net/core/opt-
     mem_max; see socket(7).
     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 appropriately.


     These  are  some  standard errors generated by the socket layer.  Addi-
     tional errors may be generated and returned from the underlying  proto-
     col 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  net-
            work/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 pos-
     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  sig-
     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  gener-
            ally  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.


     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 extension.


     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
     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 4.16 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 2017-09-15 SEND(2)

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