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Manpage of SEND
SEND
Section: Linux Programmer's Manual (2)
Updated: July 1999
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NAME
send, sendto, sendmsg - send a message from a socket
SYNOPSIS
#include <sys/types.h>
#include <sys/socket.h>
int send(int s, const void *msg, size_t len,
int flags);
int sendto(int s, const void *msg, size_t len,
int flags, const struct sockaddr *to, socklen_t tolen);
int sendmsg(int s, const struct msghdr *msg,
int flags);
DESCRIPTION
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.
When the message does not fit into the send buffer of the socket,
send
normally blocks, unless the socket has been placed in non-blocking I/O
mode. In non-blocking mode it would return
EAGAIN
in this case.
The
select(2)
call may be used to determine when it is possible to send more data.
The
flags
parameter is a flagword and can contain the following flags:
- MSG_OOB
-
Sends
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_DONTROUTE
-
Dont't use a gateway to send out the packet, only send to hosts on
directly connected networks. This is usually used only
by diagnostic or routing programs. This is only defined for protocol
families that route; packet sockets don't.
- MSG_DONTWAIT
-
Enables non-blocking operation; if the operation would block,
EAGAIN
is returned (this can also be enabled using the
O_NONBLOCK
with the
F_SETFL
fcntl(2)).
- MSG_NOSIGNAL
-
Requests not to send
SIGPIPE
on errors on stream oriented sockets when the other end breaks the
connection. The
EPIPE
error is still returned.
- MSG_CONFIRM (Linux 2.3+ only)
-
Tell the link layer that forward process happened: you got a successful
reply from the other side. If the link layer doesn't get this
it'll regularly reprobe the neighbour (e.g. via a unicast ARP).
Only valid on
SOCK_DGRAM
and
SOCK_RAW
sockets and currently only implemented for IPv4 and IPv6. See
arp(7)
for details.
The definition of the
msghdr
structure follows. See
recv(2)
and below for an exact description of its fields.
-
-
struct msghdr {
void * msg_name; /* optional address */
socklen_t msg_namelen; /* size of address */
struct iovec * msg_iov; /* scatter/gather array */
size_t msg_iovlen; /* # elements in msg_iov */
void * msg_control; /* ancillary data, see below */
socklen_t msg_controllen; /* ancillary data buffer len */
int msg_flags; /* flags on received message */
};
You may send control information using the
msg_control
and
msg_controllen
members. The maximum control buffer length the kernel can process is limited
per socket by the
net.core.optmem_max
sysctl; see
socket(7).
RETURN VALUE
The calls return the number of characters sent, or -1
if an error occurred.
ERRORS
These are some standard errors generated by the socket layer. Additional errors
may be generated and returned from the underlying protocol modules; see their
respective manual pages.
- 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 impossible.
- EAGAIN or EWOULDBLOCK
-
The socket is marked non-blocking and the requested operation
would block.
- 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.
(This cannot occur in Linux, packets are just silently dropped
when a device queue overflows.)
- EINTR
-
A signal occurred.
- ENOMEM
-
No memory available.
- EINVAL
-
Invalid argument passed.
- EPIPE
-
The local end has been shut down on a connection oriented socket.
In this case the process
will also receive a
SIGPIPE
unless
MSG_NOSIGNAL
is set.
CONFORMING TO
4.4BSD, SVr4, POSIX 1003.1g draft (these function calls appeared in 4.2BSD).
MSG_CONFIRM
is a Linux extension.
NOTE
The prototypes given above follow the Single Unix Specification,
as glibc2 also does; the
flags
argument was `int' in BSD 4.*, but `unsigned int' in libc4 and libc5;
the
len
argument was `int' in BSD 4.* and libc4, but `size_t' in libc5;
the
tolen
argument was `int' in BSD 4.* and libc4 and libc5.
See also
accept(2).
SEE ALSO
fcntl(2),
recv(2),
select(2),
getsockopt(2),
sendfile(2),
socket(2),
write(2),
socket(7),
ip(7),
tcp(7),
udp(7)
Index
- NAME
-
- SYNOPSIS
-
- DESCRIPTION
-
- RETURN VALUE
-
- ERRORS
-
- CONFORMING TO
-
- NOTE
-
- SEE ALSO
-
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Time: 04:21:45 GMT, December 04, 2001