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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(2) is the presence of flags. With zero flags
parameter, send() is equivalent to write(2). 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 trans‐
mitted.
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-block‐
ing 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.
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 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 proto‐
col 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_NON
BLOCK with the F_SETFL fcntl(2)).
MSG_EOR
Terminates a record (when this notion is supported, as for sock‐
ets 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 trans‐
mitted 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 sock‐
ets 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_con‐
trollen members. The maximum control buffer length the kernel can pro‐
cess is limited per socket by the net.core.optmem_max sysctl; see
socket(7).
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. Addi‐
tional errors may be generated and returned from the underlying proto‐
col 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 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 gener‐
ally 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.
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).
fcntl(2), getsockopt(2), recv(2), select(2), sendfile(2), shutdown(2),
socket(2), write(2), cmsg(3), ip(7), socket(7), tcp(7), udp(7)