send() - Unix, Linux System Call
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NAME
send, sendto, sendmsg - send a message on a socket
SYNOPSIS
#include <sys/types.h>
#include <sys/socket.h>
ssize_t send(int s, const void *buf, size_t len, int flags);
ssize_t sendto(int s, const void *buf, size_t len, int flags, const struct sockaddr *to, socklen_t tolen);
ssize_t sendmsg(int s, const struct msghdr *msg, int flags); DESCRIPTION
The system calls
send(), sendto(), and
sendmsg() are used to transmit a message to another socket.
The
send() call may be used only when the socket is in a
connected state (so that the intended recipient is known).
The only difference between
send() and
write() is the presence of
flags. With zero
flags parameter,
send() is equivalent to
write(). Also,
send(s,buf,len,flags) is equivalent to
sendto(s,buf,len,flags,NULL,0).
The parameter
s is the file descriptor of the sending socket.
If
sendto() is used on a connection-mode (SOCK_STREAM, SOCK_SEQPACKET) socket,
the parameters
to and
tolen are ignored (and the error EISCONN may be returned when they are
not NULL and 0), and the error ENOTCONN is returned when the socket was
not actually connected. Otherwise, the address of the target is given by
to with
tolen specifying its size.
For
sendmsg(), the address of the target is given by
msg.msg_name, with
msg.msg_namelen specifying its size.
For
send() and
sendto(), the message is found in
buf and has length
len. For
sendmsg(), the message is pointed to by the elements of the array
msg.msg_iov. The
sendmsg() call also allows sending ancillary data (also known as control information).
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 the bitwise OR
of zero or more of the following flags.
Tag | Description |
MSG_CONFIRM (Linux 2.3+ only) | |
Tell the link layer that forward progress happened: you got a successful
reply from the other side. If the link layer doesnt get this
it will 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.
|
MSG_DONTROUTE | |
Dont 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 dont.
|
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_EOR | |
Terminates a record (when this notion is supported, as for sockets of type
SOCK_SEQPACKET). |
MSG_MORE (Since Linux 2.4.4) | |
The caller has more data to send.
This flag is used with TCP sockets to obtain the same effect
as the TCP_CORK socket option (see
tcp(7)),
with the difference that this flag can be set on a per-call basis.
Since Linux 2.6, this flag is also supported for UDP sockets, and informs
the kernel to package all of the data sent in calls with this flag set
into a single datagram which is only transmitted when a call is performed
that does not specify this flag.
(See also the
UDP_CORK socket option described in
udp(7).)
|
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_OOB | |
Sends
out-of-band data on sockets that support this notion (e.g. of type
SOCK_STREAM); the underlying protocol must also support
out-of-band data.
|
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
On success, these calls return the number of characters sent.
On error, -1 is returned, and
errno is set appropriately.
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.
Tag | Description |
EACCES |
(For Unix domain sockets, which are identified by pathname)
Write permission is denied on the destination socket file,
or search permission is denied for one of the directories
the path prefix. (See
path_resolution(2).)
|
EAGAIN or EWOULDBLOCK | |
The socket is marked non-blocking and the requested operation
would block.
|
EBADF |
An invalid descriptor was specified.
|
ECONNRESET | |
Connection reset by peer.
|
EDESTADDRREQ | |
The socket is not connection-mode, and no peer address is set.
|
EFAULT |
An invalid user space address was specified for a parameter.
|
EINTR |
A signal occurred before any data was transmitted.
|
EINVAL |
Invalid argument passed.
|
EISCONN | |
The connection-mode socket was connected already but a
recipient was specified.
(Now either this error is returned, or the recipient specification
is ignored.)
|
EMSGSIZE | |
The socket type
requires that message be sent atomically, and the size
of the message to be sent made this impossible.
|
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.
(Normally, this does not occur in Linux. Packets are just silently dropped
when a device queue overflows.)
|
ENOMEM |
No memory available.
|
ENOTCONN | |
The socket is not connected, and no target has been given.
|
ENOTSOCK | |
The argument
s is not a socket.
|
EOPNOTSUPP | |
Some bit in the
flags argument is inappropriate for the socket type.
|
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.1-2001.
These function calls appeared in 4.2BSD.
POSIX.1-2001 only describes the
MSG_OOB and
MSG_EOR flags.
The
MSG_CONFIRM flag is a Linux extension.
NOTES
The prototypes given above follow the Single Unix Specification,
as glibc2 also does; the
flags argument was int in 4.x BSD, but unsigned int in libc4 and libc5;
the
len argument was int in 4.x BSD and libc4, but size_t in libc5;
the
tolen argument was int in 4.x BSD and libc4 and libc5.
See also
accept(2).
According to POSIX.1-2001, the
msg_controllen field of the
msghdr structure should be typed as
socklen_t, but glibc currently (2.4) types it as
size_t. BUGS
Linux may return EPIPE instead of ENOTCONN.
SEE ALSO
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