Updated: 2022/Sep/29
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SEND(2) System Calls Manual SEND(2)
NAME
send, sendto, sendmsg, sendmmsg - send a message from a socket
LIBRARY
Standard C Library (libc, -lc)
SYNOPSIS
#include <sys/socket.h>
ssize_t
send(int s, const void *msg, size_t len, int flags);
ssize_t
sendto(int s, const void *msg, size_t len, int flags,
const struct sockaddr *to, socklen_t tolen);
ssize_t
sendmsg(int s, const struct msghdr *msg, int flags);
int
sendmmsg(int s, struct mmsghdr *mmsg, unsigned int vlen,
unsigned int flags);
DESCRIPTION
send(), sendto(), sendmsg(), and sendmmsg() are used to transmit a
message to another socket. send() may be used only when the socket is in
a connected state, while sendto(), sendmsg() and sendmmsg() may be used
at any time.
The sendmmsg() call can be used to send multiple messages in the same
call using an array of mmsghdr elements with the following form, as
defined in <sys/socket.h>:
struct mmsghdr {
struct msghdr msg_hdr; /* the message to be sent */
unsigned int msg_len; /* number of bytes transmitted */
};
The msg_len member contains the number of bytes sent for each msg_hdr
member. The array has vlen elements, which is limited to 1024. If there
is an error, a number fewer than vlen may be returned, and the error may
be retrieved using getsockopt(2) with SO_ERROR.
The address of the target is given by to, with tolen specifying its size.
The length of the message is given by len. 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.
If no messages space is available at the socket to hold the message to be
transmitted, then send() normally blocks, unless the socket has been
placed in non-blocking I/O mode. The select(2) or poll(2) call may be
used to determine when it is possible to send more data. Unfortunately
this does not work when the interface queue which is used to send the
message is full, and the call returns ENOBUFS.
The flags parameter may include one or more of the following:
#define MSG_OOB 0x0001 /* process out-of-band data */
#define MSG_PEEK 0x0002 /* peek at incoming message */
#define MSG_DONTROUTE 0x0004 /* bypass routing, use direct interface */
#define MSG_EOR 0x0008 /* data completes record */
#define MSG_NOSIGNAL 0x0400 /* do not generate SIGPIPE on EOF */
The flag MSG_OOB is used to send "out-of-band" data on sockets that
support this notion (e.g. SOCK_STREAM); the underlying protocol must
also support "out-of-band" data. MSG_EOR is used to indicate a record
mark for protocols which support the concept. MSG_DONTROUTE is usually
used only by diagnostic or routing programs.
See recv(2) for a description of the msghdr structure. MSG_NOSIGNAL is
used to prevent SIGPIPE generation when writing a socket that may be
closed.
RETURN VALUES
The send(), sendto(), and sendmsg() calls return the number of characters
sent, or -1 if an error occurred. The sendmmsg() call returns the number
of messages sent, or -1 if an error occurred.
ERRORS
send(), sendto(), sendmsg(), and sendmmsg() fail if:
[EACCES] The SO_BROADCAST option is not set on the socket, and
a broadcast address was given as the destination.
[EAFNOSUPPORT] Addresses in the specified address family cannot be
used with this socket.
[EAGAIN|EWOULDBLOCK]
The socket is marked non-blocking and the requested
operation would block.
[EBADF] An invalid descriptor was specified.
[EDSTADDRREQ] In a non-connected socket a destination address has
not been specified.
[EFAULT] An invalid user space address was specified for a
parameter.
[EHOSTDOWN] The destination is a host on the local subnet and does
not respond to arp(4).
[EHOSTUNREACH] The destination for the message is unreachable.
[EINVAL] The total length of the I/O is more than can be
expressed by the ssize_t return value.
[EINVAL] The socket address length passed was outside the
allowable range.
[EMSGSIZE] The socket requires that message be sent atomically,
and the size of the message to be sent made this
impossible.
[ENOBUFS] The system was unable to allocate an internal buffer.
The operation may succeed when buffers become
available.
An alternative reason: 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.
[ENOTSOCK] The argument s is not a socket.
[EPIPE] In a connected socket the connection has been broken.
sendto() will also fail if:
[EISCONN] A destination address was specified and the socket is
already connected.
sendmsg() and sendmmsg() will also fail if:
[EMSGSIZE] The msg_iovlen member of the msg structure is less
than or equal to 0 or is greater than {IOV_MAX}.
SEE ALSO
fcntl(2), getsockopt(2), recv(2), select(2), socket(2), write(2)
HISTORY
The send() function call appeared in 4.2BSD. The sendmmsg() function
call appeared in Linux 3.0 and NetBSD 7.0.
NetBSD 10.99 March 27, 2021 NetBSD 10.99