NAME
send
, sendto
,
sendmsg
, sendmmsg
—
send a message from a socket
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, const struct mmsghdr
*mmsg, unsigned int
vlen, 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 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) system calls may be
used to determine when it is possible to send more data.
The flags parameter may include one or more of the following:
MSG_DONTROUTE
- bypass routing tables, silently ignored
MSG_DONTWAIT
- don't block
MSG_EOR
- terminate the record (SOCK_SEQPACKET only)
MSG_NOSIGNAL
- don't send
SIGPIPE
MSG_OOB
- process out-of-band data
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_NOSIGNAL
is used to request not to send the
SIGPIPE
signal if an attempt to send is made on a
socket that is shut down for writing or no longer connected.
See recv(2) for a description of the msghdr and mmsghdr structures.
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 before the first message has been sent.
ERRORS
send
(), sendto
(),
and sendmsg
() fail if:
- [
EBADF
] - An invalid descriptor was specified.
- [
ENOTSOCK
] - The argument s is not a socket.
- [
EFAULT
] - An invalid user space address was specified for a parameter.
- [
EMSGSIZE
] - The socket requires that message be sent atomically, and the size of the message to be sent made this impossible.
- [
EAGAIN
] - The socket is marked non-blocking or the
MSG_DONTWAIT
flag is set and the requested operation would block. - [
ENOBUFS
] - The system was unable to allocate an internal buffer. The operation may succeed when buffers become available.
- [
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.
- [
EACCES
] - The connection was blocked by pf(4), or
SO_BROADCAST
is not set on the socket and a broadcast address was given as the destination. - [
EHOSTUNREACH
] - The destination address specified an unreachable host.
- [
EINVAL
] - The flags parameter is invalid.
- [
EHOSTDOWN
] - The destination address specified a host that is down.
- [
ENETDOWN
] - The destination address specified a network that is down.
- [
ECONNREFUSED
] - The destination host rejected the message (or a previous one). This error can only be returned by connected sockets.
- [
ENOPROTOOPT
] - There was a problem sending the message. This error can only be returned by connected sockets.
- [
EDESTADDRREQ
] - The socket is not connected, and no destination address was specified.
- [
EPIPE
] - The socket is shut down for writing or not longer connected and the
MSG_NOSIGNAL
flag is set.
In addition, send
() and
sendto
() may return the following error:
- [
EINVAL
] - len was larger than
SSIZE_MAX
.
sendto
() and
sendmsg
() may return the following errors:
- [
EADDRNOTAVAIL
] - No suitable address is available on the local machine.
- [
EAFNOSUPPORT
] - Addresses in the specified address family cannot be used with this socket.
- [
EISCONN
] - The socket is already connected, and a destination address was specified.
sendmsg
() may return the following
errors:
- [
EINVAL
] - The sum of the iov_len values in the msg_iov array overflowed an ssize_t.
- [
EMSGSIZE
] - The msg_iovlen member of msg
was less than 0 or larger than
IOV_MAX
. - [
EMFILE
] - The message contains control information utilizing CMSG_DATA(3) to pass file descriptors, but too many file descriptors are already in-flight.
SEE ALSO
fcntl(2), getsockopt(2), poll(2), recv(2), select(2), socket(2), write(2), CMSG_DATA(3)
STANDARDS
The send
(),
sendto
(), and sendmsg
()
functions conform to IEEE Std 1003.1-2008
(“POSIX.1”). The MSG_DONTWAIT
and MSG_NOSIGNAL
flags are extensions to that
specification.
HISTORY
The send
() function call appeared in
4.1cBSD. The sendmmsg
()
syscall first appeared in Linux 3.0 and was added to
OpenBSD 7.2.