SEND(2) BSD System Calls Manual SEND(2)NAME
send, sendto, sendmsg — send a message from a socket
LIBRARY
Standard C Library (libc, -lc)
SYNOPSIS
#include <sys/types.h>
#include <sys/socket.h>
ssize_t
send(int s, const void *msgbuf, size_t len, int flags);
ssize_t
sendto(int s, const void *msgbuf, size_t len, int flags,
const struct sockaddr *to, socklen_t tolen);
ssize_t
sendmsg(int s, const struct msghdr *msg, int flags);
DESCRIPTIONSend(), 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 socket file descriptor is given by s. msgbuf points to a buffer con‐
taining the message. msg points to a msghdr structure. 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 atomi‐
cally 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) call may be used to
determine when it is possible to send more data.
The flags parameter may include one or more of the following:
#define MSG_OOB 0x1 /* process out-of-band data */
#define MSG_PEEK 0x2 /* peek at incoming message */
#define MSG_DONTROUTE 0x4 /* bypass routing, use direct interface */
#define MSG_EOR 0x8 /* data completes record */
#define MSG_EOF 0x100 /* data completes transaction */
#define MSG_NOSIGNAL 0x400 /* No SIGPIPE to unconnected socket stream */
The flag MSG_OOB is used to send “out-of-band” data on sockets that sup‐
port 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. 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
requests not to send the SIGPIPE signal if an attempt to send is made on
a stream-oriented socket that is no longer connected.
See recv(2) for a description of the msghdr structure.
RETURN VALUES
Upon successful completion the number of characters which were sent is
returned. Otherwise -1 is returned and the global variable errno is set
to indicate the error.
ERRORSSend(), sendto(), and sendmsg() 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.
[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
impossible.
[EAGAIN] The socket is marked non-blocking and the requested
operation would block.
[ENOBUFS] The system was unable to allocate an internal buffer.
The operation may succeed when buffers become avail‐
able.
[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 con‐
gestion.
[EHOSTUNREACH] The remote host was unreachable.
[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.
[EPIPE] The socket is unable to send anymore data
(SS_CANTSENDMORE has been set on the socket). This
typically means that the socket is not connected.
SEE ALSOfcntl(2), getsockopt(2), recv(2), select(2), socket(2), write(2)HISTORY
The send() function call appeared in 4.2BSD.
BUGS
Because sendmsg() doesn't 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 actu‐
ally 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.
BSD October 6, 2010 BSD
