SEND(2) manual page
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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 lenint " flags );
ssize_t sendto(int sockfd, const void *buf, size_t lenint " 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_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 can also be enabled using the O_NONBLOCK flag with the F_SETFL fcntl(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 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)
- 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 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)
.
- 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 sockfd 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 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)
,
socket(7)
, tcp(7)
, udp(7)
This page is part of release 3.78 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 http://www.kernel.org/doc/man-pages/.
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