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SEND(2) System Calls Manual SEND(2)

send, sendto, sendmsg, sendmmsg — send a message from a socket

#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);

(), (), (), and () are used to transmit a message to another socket. send() may be used only when the socket is in a 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 (). 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 () 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:

bypass routing tables, silently ignored
don't block
terminate the record (SOCK_SEQPACKET only)
don't send SIGPIPE
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.

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.

send(), sendto(), and sendmsg() fail if:

[]
An invalid descriptor was specified.
[]
The argument s is not a socket.
[]
An invalid user space address was specified for a parameter.
[]
The socket requires that message be sent atomically, and the size of the message to be sent made this impossible.
[]
The socket is marked non-blocking or the MSG_DONTWAIT flag is set and the requested operation would block.
[]
The system was unable to allocate an internal buffer. The operation may succeed when buffers become available.
[]
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.
[]
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.
[]
The destination address specified an unreachable host.
[]
The flags parameter is invalid.
[]
The destination address specified a host that is down.
[]
The destination address specified a network that is down.
[]
The destination host rejected the message (or a previous one). This error can only be returned by connected sockets.
[]
There was a problem sending the message. This error can only be returned by connected sockets.
[]
The socket is not connected, and no destination address was specified.
[]
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:

[]
len was larger than SSIZE_MAX.

sendto() and sendmsg() may return the following errors:

[]
No suitable address is available on the local machine.
[]
Addresses in the specified address family cannot be used with this socket.
[]
The socket is already connected, and a destination address was specified.

sendmsg() may return the following errors:

[]
The sum of the iov_len values in the msg_iov array overflowed an .
[]
The msg_iovlen member of msg was less than 0 or larger than IOV_MAX.
[]
The message contains control information utilizing CMSG_DATA(3) to pass file descriptors, but too many file descriptors are already in-flight.

fcntl(2), getsockopt(2), poll(2), recv(2), select(2), socket(2), write(2), CMSG_DATA(3)

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.

The send() function call appeared in 4.1cBSD. The sendmmsg() syscall first appeared in Linux 3.0 and was added to OpenBSD 7.2.

September 9, 2022 OpenBSD-7.6