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SEND(2) | 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
*msg, size_t
len, int
flags);
ssize_t
sendto
(int
s, const void
*msg, size_t
len, int
flags, const
struct sockaddr *to,
socklen_t
tolen);
ssize_t
sendmsg
(int
s, const struct
msghdr *msg, int
flags);
DESCRIPTION¶
Thesend
() 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 0x00001 /* process out-of-band data */ #define MSG_DONTROUTE 0x00004 /* bypass routing, use direct interface */ #define MSG_EOR 0x00008 /* data completes record */ #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.
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
is used to prevent
SIGPIPE
generation when writing a socket
that may be closed.
See recv(2) for a description of the
msghdr structure.
RETURN VALUES¶
The call returns the number of characters sent, or -1 if an error occurred.ERRORS¶
Thesend
() 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 argument.
- [
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 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.
- [
EISCONN
] - A destination address was specified and the socket is already connected.
- [
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.
- [
ENETDOWN
] - The remote network was down.
- [
EADDRNOTAVAIL
] - The process using a
SOCK_RAW
socket was jailed and the source address specified in the IP header did not match the IP address bound to the prison. - [
EPIPE
] - The socket is unable to send anymore data
(
SBS_CANTSENDMORE
has been set on the socket). This typically means that the socket is not connected.
SEE ALSO¶
fcntl(2), getsockopt(2), recv(2), select(2), socket(2), write(2)HISTORY¶
Thesend
() function appeared in
4.2BSD.
BUGS¶
Becausesendmsg
() 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
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 acknowledgment mechanism to prevent
this from happening.February 5, 2009 | Debian |