NAME
send, sendto, sendmsg - send a message on a socket SYNOPSIS
#include
#include
ssizet send(int sockfd, const void *buf, sizet len, int flags); ssizet sendto(int sockfd, const void *buf, sizet len, int flags, const struct sockaddr *destaddr, socklent addrlen); ssizet 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 (SOCKSTREAM, SOCKSEQPACKET) socket, the arguments destaddr 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 destaddr with addrlen specifying its size. For sendmsg(), the address of the target is given by msg.msgname, with msg.msgnamelen 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.msgiov. 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‐ mitted. 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 is the bitwise OR of zero or more of the following flags. MSGCONFIRM (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 SOCKDGRAM and SOCKRAW sockets and currently implemented only for IPv4 and IPv6. See arp(7) for details. MSGDONTROUTE 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. MSGDONTWAIT (since Linux 2.2) Enables nonblocking operation; if the operation would block, EAGAIN or EWOULDBLOCK is returned (this can also be enabled using the ONONBLOCK flag with the FSETFL fcntl(2)). MSGEOR (since Linux 2.2) Terminates a record (when this notion is supported, as for sock‐ ets of type SOCKSEQPACKET). MSGMORE (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 TCPCORK 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 UDPCORK socket option described in udp(7).) MSGNOSIGNAL (since Linux 2.2) Requests not to send SIGPIPE on errors on stream oriented sock‐ ets when the other end breaks the connection. The EPIPE error is still returned. MSGOOB
Sends out-of-band data on sockets that support this notion (e.g., of type SOCKSTREAM); 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 *msgname; /* optional address */ socklent msgnamelen; /* size of address */ struct iovec *msgiov; /* scatter/gather array */
sizet msgiovlen; /* # elements in msgiov */ void *msgcontrol; /* ancillary data, see below */ sizet msgcontrollen; /* ancillary data buffer len */ int msgflags; /* flags on received message */ }; You may send control information using the msgcontrol and msgcon‐ trollen members. The maximum control buffer length the kernel can process is limited per socket by the value in /proc/sys/net/core/opt‐ memmax; 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. 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 pathresolution(7).) (For UDP sockets) An attempt was made to send to a net‐ work/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 pos‐ sibilities. 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 sig‐ nal(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 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. 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 MSGNOSIGNAL is set. CONFORMING TO
4.4BSD, SVr4, POSIX.1-2001. These function calls appeared in 4.2BSD.
POSIX.1-2001 describes only the MSGOOB and MSGEOR flags.
POSIX.1-2008 adds a specification of MSGNOSIGNAL. The MSGCONFIRM 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 sizet 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 msgcontrollen field of the msghdr structure should be typed as socklent, but glibc currently types it as sizet.
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). SEE ALSO fcntl(2), getsockopt(2), recv(2), select(2), sendfile(2), sendmmsg(2), shutdown(2), socket(2), write(2), cmsg(3), ip(7), socket(7), tcp(7), udp(7) COLOPHON
This page is part of release 3.53 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/.
Linux 2012-04-23 SEND(2)