sendmsg

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.

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) */
};

Return Value

On success, these calls return the number of bytes 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.

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.

Conforming to

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.

Notes

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).

See Also

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

This page is part of release 5.04 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 https://www.kernel.org/doc/man-pages/.

Referenced By

ares_set_socket_functions(3), arp(7), biblesync(7), bpf-helpers(7), bpftool-cgroup(8), cmsg(3), ddp(7), getaddrinfo(3), getifaddrs(3), getrlimit(2), gnutls_transport_set_push_function(3), if_nameindex(3), ip(7), ipv6(7), kcapi_aead_stream_init_dec(3), kcapi_aead_stream_init_enc(3), kcapi_akcipher_stream_init_dec(3), kcapi_akcipher_stream_init_enc(3), kcapi_akcipher_stream_init_sgn(3), kcapi_akcipher_stream_init_vfy(3), kcapi_cipher_stream_init_dec(3), kcapi_cipher_stream_init_enc(3), lwres_getaddrinfo(3), netlink(7), packet(7), perlfunc(1), pthsem(3), raw(7), recvmmsg(2), rtime(3), rxrpc(7), sctp(7), select(2), select_tut(2), sendmmsg(2), signal(7), signal-safety(7), sockatmark(3), socket(2), socket(7), socketcall(2), splice(2), stress-ng(1), swtpm_ioctls(3), syscalls(2), tcp(7), tevent_queue(3), tevent_queue_tutorial(3), trafgen(8), udp(7), unix(7), vsock(7), xinetd.conf(5).

The man pages sendmsg(2) and sendto(2) are aliases of send(2).