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
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
returned, and the message is not transmitted.
No indication of failure to deliver is implicit in a send().
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
placed in non-blocking I/O mode. The select(2) or poll(2)
may be used to determine when it is possible to send more
The flags parameter may include one or more of the following:
#define MSG_OOB 0x1 /* process out-of-band data */
#define MSG_DONTROUTE 0x4 /* bypass routing, use direct
The flag MSG_OOB is used to send ``out-of-band'' data on
support this notion (e.g., SOCK_STREAM); the underlying protocol must also
support ``out-of-band'' data. MSG_DONTROUTE is usually
used only by
diagnostic or routing programs.
See recv(2) for a description of the msghdr structure.
The call returns the number of characters sent, or -1 if an
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 parameter.
[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 and the requested operation
[ENOBUFS] The system was unable to allocate an internal
operation may succeed when buffers become
[ENOBUFS] The output queue for a network interface was
generally indicates that the interface has
but may be caused by transient congestion.
[EACCES] The SO_BROADCAST option is not set on the
socket, and a
broadcast address was given as the destination.
The destination address specified an unreachable host.
[EINVAL] The flags parameter is invalid.
[EHOSTDOWN] The destination address specified a host that
[ENETDOWN] The destination address specified a network
that is down.
The destination host rejected the message (or
one). This error can only be returned by connected sockets.
There was a problem sending the message. This
only be returned by connected sockets.
The socket is not connected, and no destination address was
[EISCONN] The socket is already connected, and a destination address
In addition, send() and sendto() may return the following
[EINVAL] len was larger than SSIZE_MAX.
Also, sendmsg() may return the following errors:
[EINVAL] The sum of the iov_len values in the msg_iov
[EMSGSIZE] The msg_iovlen member of msg was less than 0
or larger than
Addresses in the specified address family cannot be used
with this socket.
fcntl(2), getsockopt(2), poll(2), recv(2), select(2), socket(2), write(2)
The send() function call appeared in 4.2BSD.
OpenBSD 3.6 July 28, 1998
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