send, sendto, sendmsg — send a message on a socket
#include <sys/types.h> #include <sys/socket.h>
ssize_t
send( |
int sockfd, |
| const void *buf, | |
| size_t len, | |
int flags); |
ssize_t
sendto( |
int sockfd, |
| const void *buf, | |
| size_t len, | |
| int flags, | |
| const struct sockaddr *dest_addr, | |
socklen_t addrlen); |
ssize_t
sendmsg( |
int sockfd, |
| 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 a zero
flags argument,
send() is equivalent to
write(2). Also, the
following call
send(sockfd, buf, len, flags);
is equivalent to
sendto(sockfd, buf, len, flags, NULL, 0);
The argument sockfd is the file descriptor
of the sending socket.
If sendto() is used on a
connection-mode (SOCK_STREAM,
SOCK_SEQPACKET) socket, the
arguments dest_addr
and addrlen 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 dest_addr with addrlen 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 nonblocking I/O mode. In
nonblocking mode it would fail with the error EAGAIN or EWOULDBLOCK 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
implemented only for IPv4 and IPv6. See arp(7) for
details.
MSG_DONTROUTEDon't use a gateway to send out the packet, send to hosts only on directly connected networks. This is usually used only by diagnostic or routing programs. This is defined only for protocol families that route; packet sockets don't.
MSG_DONTWAIT (since Linux
2.2)Enables nonblocking operation; if the operation
would block, EAGAIN or
EWOULDBLOCK is
returned. This provides similar behavior to setting
the O_NONBLOCK flag
(via the fcntl(2)
F_SETFL operation), but
differs in that MSG_DONTWAIT is a per-call option,
whereas O_NONBLOCK is a
setting on the open file description (see open(2)), which
will affect all threads in the calling process and as
well as other processes that hold file descriptors
referring to the same open file description.
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 transmitted only 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)Don't generate a SIGPIPE signal if the peer on a
stream-oriented socket has closed the connection. The
EPIPE error is still
returned. This provides similar behavior to using
sigaction(2) to
ignore SIGPIPE, but,
whereas MSG_NOSIGNAL is
a per-call feature, ignoring SIGPIPE sets a process attribute
that affects all threads in the process.
MSG_OOBSends 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 employed by
sendmsg() is as follows:
struct msghdr { void * msg_name;socklen_t msg_namelen;struct iovec * msg_iov;size_t msg_iovlen;void * msg_control;size_t msg_controllen;int msg_flags;};
The msg_name
field is used on an unconnected socket to specify the
target address for a datagram. It points to a buffer
containing the address; the msg_namelen field should be
set to the size of the address. For a connected socket,
these fields should be specified as NULL and 0,
respectively.
The msg_iov and
msg_iovlen fields
specify scatter-gather locations, as for writev(2).
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).
The msg_flags
field is ignored.
On success, these calls return the number of bytes 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(7).)
(For UDP sockets) An attempt was made to send to a network/broadcast address as though it was a unicast address.
The socket is marked nonblocking and the requested operation would block. POSIX.1-2001 allows either error to be returned for this case, and does not require these constants to have the same value, so a portable application should check for both possibilities.
(Internet domain datagram sockets) The socket
referred to by sockfd had not previously
been bound to an address and, upon attempting to bind
it to an ephemeral port, it was determined that all
port numbers in the ephemeral port range are currently
in use. See the discussion of /proc/sys/net/ipv4/ip_local_port_range
in ip(7).
sockfd is
not a valid open file descriptor.
Connection reset by peer.
The socket is not connection-mode, and no peer address is set.
An invalid user space address was specified for an argument.
A signal occurred before any data was transmitted; see signal(7).
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 file descriptor sockfd does not refer to
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 interfaces first appeared in 4.2BSD.
POSIX.1-2001 describes only the MSG_OOB and MSG_EOR flags. POSIX.1-2008 adds a
specification of MSG_NOSIGNAL.
The MSG_CONFIRM flag is a Linux
extension.
According to POSIX.1-2001, the msg_controllen field of the
msghdr structure should be
typed as socklen_t, but glibc
currently types it as size_t.
See sendmmsg(2) for information about a Linux-specific system call that can be used to transmit multiple datagrams in a single call.
fcntl(2), getsockopt(2), recv(2), select(2), sendfile(2), sendmmsg(2), shutdown(2), socket(2), write(2), cmsg(3), ip(7), socket(7), tcp(7), udp(7)
This page is part of release 4.07 of the Linux man-pages project. A
description of the project, information about reporting bugs,
and the latest version of this page, can be found at
https://www.kernel.org/doc/man−pages/.
|
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