send
, sendto
,
sendmsg
, sendmmsg
—
send message(s) from a socket
Standard C Library (libc, -lc)
#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);
ssize_t
sendmmsg
(int
s, struct mmsghdr *
restrict msgvec, size_t
vlen, int
flags);
The send
() and sendmmsg
()
functions, and sendto
() and
sendmsg
() system calls are used to transmit one or
more messages (with the sendmmsg
() call) to another
socket. The send
() function may be used only when the
socket is in a connected state. The functions
sendto
(), sendmsg
() and
sendmmsg
() may be used at any time if the socket is
connectionless-mode. If the socket is connection-mode, the protocol must
support implied connect (currently
tcp(4) is
the only protocol with support) or the socket must be in a connected state
before use.
The address of the target is given by to
with tolen specifying its size, or the equivalent
msg_name and msg_namelen in
struct msghdr. If the socket is in a connected state,
the target address passed to sendto
(),
sendmsg
() or sendmmsg
() is
ignored. 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.
The sendmmsg
() function sends multiple
messages at a call. They are given by the msgvec
vector along with vlen specifying the vector size. The
number of octets sent per each message is placed in the
msg_len field of each processed element of the vector
after transmission.
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)
system call may be used to determine when it is possible to send more
data.
The flags argument may include one or more
of the following:
#define MSG_OOB 0x00001 /* process out-of-band data */
#define MSG_DONTROUTE 0x00004 /* bypass routing, use direct interface */
#define MSG_EOR 0x00008 /* data completes record */
#define MSG_DONTWAIT 0x00080 /* do not block */
#define MSG_EOF 0x00100 /* data completes transaction */
#define MSG_NOSIGNAL 0x20000 /* 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. The
MSG_DONTWAIT
flag request the call to return when it
would block otherwise. MSG_EOF
requests that the
sender side of a socket be shut down, and that an appropriate indication be
sent at the end of the specified data; this flag is only implemented for
SOCK_STREAM
sockets in the
PF_INET
protocol family.
MSG_DONTROUTE
is usually used only by diagnostic or
routing programs. MSG_NOSIGNAL
is used to prevent
SIGPIPE
generation when writing a socket that may be
closed.
See
recv(2)
for a description of the msghdr structure and the
mmsghdr structure.
The send
(), sendto
() and
sendmsg
() calls return the number of octets sent. The
sendmmsg
() call returns the number of messages sent.
If an error occurred a value of -1 is returned.
The send
() and sendmmsg
()
functions and sendto
() and
sendmsg
() system calls fail if:
- [
EBADF
]
- An invalid descriptor was specified.
- [
EACCES
]
- The destination address is a broadcast address, and
SO_BROADCAST
has not been set on the socket.
- [
ENOTCONN
]
- The socket is connection-mode but is not connected.
- [
ENOTSOCK
]
- The argument s is not a socket.
- [
EFAULT
]
- An invalid user space address was specified for an argument.
- [
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
MSG_DONTWAIT
is specified, 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.
- [
EHOSTUNREACH
]
- The remote host was unreachable.
- [
EISCONN
]
- A destination address was specified and the socket is already
connected.
- [
ECONNREFUSED
]
- The socket received an ICMP destination unreachable message from the last
message sent. This typically means that the receiver is not listening on
the remote port.
- [
EHOSTDOWN
]
- The remote host was down.
- [
ENETDOWN
]
- The remote network was down.
- [
EADDRNOTAVAIL
]
- The process using a
SOCK_RAW
socket was jailed and
the source address specified in the IP header did not match the IP address
bound to the prison.
- [
EPIPE
]
- The socket is unable to send anymore data
(
SBS_CANTSENDMORE
has been set on the socket).
This typically means that the socket is not connected.
The send
() function appeared in
4.2BSD. The sendmmsg
()
function appeared in FreeBSD 11.0.
Because sendmsg
() does not necessarily block until the
data has been transferred, it is possible to transfer an open file descriptor
across an AF_UNIX
domain socket (see
recv(2)),
then close
() it before it has actually been sent, the
result being that the receiver gets a closed file descriptor. It is left to
the application to implement an acknowledgment mechanism to prevent this from
happening.