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). Valid only on
SOCK_DGRAM and SOCK_RAW sockets and currently implemented
only for IPv4 and IPv6. See arp(7) for details.
- MSG_DONTROUTE
- Don'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_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 employed by
sendmsg() is as
follows:
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 */
size_t msg_controllen; /* Ancillary data buffer len */
int msg_flags; /* Flags (unused) */
};
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.
- 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).)
- (For UDP sockets) An attempt was made to send to a network/broadcast
address as though it was a unicast address.
- EAGAIN or EWOULDBLOCK
- 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.
- EAGAIN
- (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).
- EALREADY
- Another Fast Open is in progress.
- EBADF
- sockfd is not a valid open file descriptor.
- 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 file descriptor sockfd does not refer to 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 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.
Linux may return
EPIPE instead of
ENOTCONN.
An example of the use of
sendto() is shown in
getaddrinfo(3).
fcntl(2),
getsockopt(2),
recv(2),
select(2),
sendfile(2),
sendmmsg(2),
shutdown(2),
socket(2),
write(2),
cmsg(3),
ip(7),
ipv6(7),
socket(7),
tcp(7),
udp(7),
unix(7)