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(2)
is the presence of
flags.
With zero
flags
argument,
send()
is equivalent to
write(2).
Also,
send(s,buf,len,flags)
is equivalent to
sendto(s,buf,len,flags,NULL,0).
The argument
s
is the file descriptor of the sending socket.
If
sendto()
is used on a connection-mode
(SOCK_STREAM,
SOCK_SEQPACKET)
socket, the arguments
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
argument is the bitwise OR
of zero or more of the following flags.
MSG_CONFIRM (Since Linux 2.3.15)
Tell the link layer that forward progress happened: you got a successful
reply from the other side.
If the link layer doesn't get this
it will regularly reprobe the neighbor (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
Don'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 (since Linux 2.2)
Enables non-blocking operation; if the operation would block,
EAGAIN
is returned (this can also be enabled using the
O_NONBLOCK
with the
F_SETFLfcntl(2)).
MSG_EOR (since Linux 2.2)
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 (since Linux 2.2)
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 value in
/proc/sys/net/core/optmem_max;
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.
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(7).)
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 an argument.
EINTR
A signal occurred before any data was transmitted; see
signal(7).
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.
EXAMPLE
An example of the use of
sendto()
is shown in
getaddrinfo(3).
This page is part of release 3.14 of the Linux
man-pages
project.
A description of the project,
and information about reporting bugs,
can be found at
http://www.kernel.org/doc/man-pages/.
