Provided by: freebsd-manpages_12.2-2_all 
NAME
send, sendto, sendmsg, sendmmsg — send message(s) 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);
ssize_t
sendmmsg(int s, struct mmsghdr * restrict msgvec, size_t vlen, int flags);
DESCRIPTION
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.
RETURN VALUES
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.
ERRORS
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.
SEE ALSO
connect(2), fcntl(2), getsockopt(2), recv(2), select(2), socket(2), write(2), CMSG_DATA(3)
HISTORY
The send() function appeared in 4.2BSD. The sendmmsg() function appeared in FreeBSD 11.0.
BUGS
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.