SEND(2) System Calls Manual SEND(2)
send, sendto, sendmsg - send a message from a socket
send(int s, const void *msg, size_t len, int flags);
sendto(int s, const void *msg, size_t len, int flags,
const struct sockaddr *to, socklen_t tolen);
sendmsg(int s, const struct msghdr *msg, int flags);
send(), sendto(), and sendmsg() are used to transmit a message to another
socket. send() may be used only when the socket is in a connected state,
while sendto() and sendmsg() 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 structure.
The call returns the number of characters sent, or -1 if an error
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
[EMSGSIZE] The socket requires that message be sent atomically,
and the size of the message to be sent made this
[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
[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
[EACCES] The SO_BROADCAST option 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
[ECONNREFUSED] The destination host rejected the message (or a
previous one). This error can only be returned by
[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:
[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
[EMFILE] 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),
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 4.2BSD.
OpenBSD 5.9 September 9, 2014 OpenBSD 5.9
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