send, sendto, sendmsg -- send a message from a socket
Standard C Library (libc, -lc)
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);
The send() function, and sendto() and sendmsg() system calls are used to
transmit a message to another socket. The send() function 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) 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 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 */
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_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, and is used to
implement Transaction TCP (see ttcp(4)). 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 error
The send() function and sendto() and sendmsg() system calls 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 an
[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 would block.
[ENOBUFS] The system was unable to allocate an internal buffer.
The operation may succeed when buffers become available.
[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 congestion.
[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
[EHOSTDOWN] The remote host was down.
[ENETDOWN] The remote network 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.
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 actually
been sent, the result being that the receiver gets a closed file
descriptor. It is left to the application to implement an acknowlegment
mechanism to prevent this from happening.
fcntl(2), getsockopt(2), recv(2), select(2), socket(2), write(2)
The send() function appeared in 4.2BSD.
FreeBSD 5.2.1 February 15, 1995 FreeBSD 5.2.1 [ Back ]