send(2) System Calls Manual send(2)
NAME
send, sendto, sendmsg - send a message on a socket
LIBRARY
Standard C library (libc, -lc)
SYNOPSIS
#include <sys/socket.h>
ssize_t send(int sockfd, const void buf[.len], size_t len, int flags);
ssize_t sendto(int sockfd, const void buf[.len], size_t len, int flags,
const struct sockaddr *dest_addr, socklen_t addrlen);
ssize_t sendmsg(int sockfd, 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 a zero flags argu-
ment, 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. Other-
wise, the address of the target is given by dest_addr with addrlen spec-
ifying 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 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
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 uni-
cast ARP). Valid only on SOCK_DGRAM and SOCK_RAW sockets and
currently implemented only for IPv4 and IPv6. See arp(7) for de-
tails.
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 diag-
nostic 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, EA-
GAIN or EWOULDBLOCK is returned. This provides similar behavior
to setting the O_NONBLOCK flag (via the fcntl(2) F_SETFL opera-
tion), 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 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 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 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 re-
turned. 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.
MSG_FASTOPEN (since Linux 3.7)
Attempts TCP Fast Open (RFC7413) and sends data in the SYN like a
combination of connect(2) and write(2), by performing an implicit
connect(2) operation. It blocks until the data is buffered and
the handshake has completed. For a non-blocking socket, it re-
turns the number of bytes buffered and sent in the SYN packet.
If the cookie is not available locally, it returns EINPROGRESS,
and sends a SYN with a Fast Open cookie request automatically.
The caller needs to write the data again when the socket is con-
nected. On errors, it sets the same errno as connect(2) if the
handshake fails. This flag requires enabling TCP Fast Open
client support on sysctl net.ipv4.tcp_fastopen.
Refer to TCP_FASTOPEN_CONNECT socket option in tcp(7) for an al-
ternative approach.
sendmsg()
The definition of the msghdr structure employed by sendmsg() is as fol-
lows:
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 tar-
get address for a datagram. It points to a buffer containing the ad-
dress; 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 (ancillary data) using the msg_control
and msg_controllen members. The maximum control buffer length the ker-
nel can process is limited per socket by the value in
/proc/sys/net/core/optmem_max; see socket(7). For further information
on the use of ancillary data in various socket domains, see unix(7) and
ip(7).
The msg_flags field is ignored.
RETURN VALUE
On success, these calls return the number of bytes sent. On error, -1
is returned, and errno is set to indicate the error.
ERRORS
These are some standard errors generated by the socket layer. Addi-
tional 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/broad-
cast 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 possibili-
ties.
EAGAIN (Internet domain datagram sockets) The socket referred to by
sockfd had not previously been bound to an address and, upon at-
tempting 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_lo-
cal_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 recip-
ient 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 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.
VERSIONS
According to POSIX.1-2001, the msg_controllen field of the msghdr struc-
ture should be typed as socklen_t, and the msg_iovlen field should be
typed as int, but glibc currently types both as size_t.
STANDARDS
POSIX.1-2008.
MSG_CONFIRM is a Linux extension.
HISTORY
4.4BSD, SVr4, POSIX.1-2001. (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.
NOTES
See sendmmsg(2) for information about a Linux-specific system call that
can be used to transmit multiple datagrams in a single call.
BUGS
Linux may return EPIPE instead of ENOTCONN.
EXAMPLES
An example of the use of sendto() is shown in getaddrinfo(3).
SEE ALSO
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)
Linux man-pages 6.9.1 2024-05-02 send(2)
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