table of contents
|SEND(2)||System Calls Manual||SEND(2)|
send message(s) from a socket
Standard C Library (libc, -lc)
s, const void *msg,
s, const void *msg,
const struct sockaddr
s, const struct msghdr
s, struct mmsghdr *
restrict msgvec, size_t
sendmmsg() functions, and
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
state. The functions
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
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
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
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) 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 */
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.
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
MSG_NOSIGNAL is used to prevent
SIGPIPE generation when writing a socket that may be
See recv(2) for a description of the msghdr structure and the mmsghdr structure.
return the number of octets sent. The
call returns the number of messages sent. If an error occurred a value of -1
sendmmsg() functions and
system calls fail if:
- An invalid descriptor was specified.
- The destination address is a broadcast address, and
SO_BROADCASThas not been set on the socket.
- The socket is connection-mode but is not connected.
- The argument s is not a socket.
- An invalid user space address was specified for an argument.
- 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
MSG_DONTWAITis specified, 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 remote host was unreachable.
- A destination address was specified and the socket is already connected.
- 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.
- The remote host was down.
- The remote network was down.
- The process using a
SOCK_RAWsocket was jailed and the source address specified in the IP header did not match the IP address bound to the prison.
- The socket is unable to send anymore data
SBS_CANTSENDMOREhas been set on the socket). This typically means that the socket is not connected.
connect(2), fcntl(2), getsockopt(2), recv(2), select(2), socket(2), write(2), CMSG_DATA(3)
send() function appeared in
function appeared in FreeBSD 11.0.
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
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.
|April 27, 2020||Debian|