send, sendto, sendmsg — send a message on a socket
#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) ; |
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 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.
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 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
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
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).
On success, these calls return the number of characters
sent. On error, −1 is returned, and errno
is set appropriately.
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.
(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).)
The socket is marked non-blocking and the requested operation would block.
An invalid descriptor was specified.
Connection reset by peer.
The socket is not connection-mode, and no peer address is set.
An invalid user space address was specified for a parameter.
A signal occurred before any data was transmitted.
Invalid argument passed.
The connection-mode socket was connected already but a recipient was specified. (Now either this error is returned, or the recipient specification is ignored.)
The socket type requires that message be sent atomically, and the size of the message to be sent made this impossible.
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.)
No memory available.
The socket is not connected, and no target has been given.
The argument s
is not a socket.
Some bit in the flags
argument is
inappropriate for the socket type.
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.
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
.
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)
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