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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | VERSIONS | STANDARDS | HISTORY | NOTES | BUGS | EXAMPLES | SEE ALSO | COLOPHON |
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send(2) System Calls Manual send(2)
send, sendto, sendmsg - send a message on a socket
Standard C library (libc, -lc)
#include <sys/socket.h>
ssize_t send(size_t size;
int sockfd, const void buf[size], size_t size, int flags);
ssize_t sendto(size_t size;
int sockfd, const void buf[size], size_t size, 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, size, flags);
is equivalent to
sendto(sockfd, buf, size, 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 size
size. 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
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 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.
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 returns 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 connected.
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 alternative approach.
sendmsg()
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 size */
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 (ancillary data) using the
msg_control and msg_controllen members. The maximum control
buffer size the kernel 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.
On success, these calls return the number of bytes sent. On
error, -1 is returned, and errno is set to indicate the error.
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.
According to POSIX.1-2001, the msg_controllen field of the msghdr
structure should be typed as socklen_t, and the msg_iovlen field
should be typed as int, but glibc currently types both as size_t.
POSIX.1-2008.
MSG_CONFIRM is a Linux extension.
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.
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)
This page is part of the man-pages (Linux kernel and C library
user-space interface documentation) project. Information about
the project can be found at
⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report
for this manual page, see
⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
This page was obtained from the tarball man-pages-6.15.tar.gz
fetched from
⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on
2025-08-11. If you discover any rendering problems in this HTML
version of the page, or you believe there is a better or more up-
to-date source for the page, or you have corrections or
improvements to the information in this COLOPHON (which is not
part of the original manual page), send a mail to
man-pages@man7.org
Linux man-pages 6.15 2025-06-28 send(2)
Pages that refer to this page: getrlimit(2), io_uring_enter2(2), io_uring_enter(2), recvmmsg(2), select(2), select_tut(2), sendmmsg(2), socket(2), socketcall(2), splice(2), syscalls(2), audit_request_features(3), cmsg(3), getaddrinfo(3), getifaddrs(3), if_nameindex(3), io_uring_prep_send(3), io_uring_prep_send_bundle(3), io_uring_prep_sendmsg(3), io_uring_prep_sendmsg_zc(3), io_uring_prep_send_set_addr(3), io_uring_prep_sendto(3), io_uring_prep_send_zc(3), io_uring_prep_send_zc_fixed(3), rtime(3), size_t(3type), sockaddr(3type), sockatmark(3), arp(7), bpf-helpers(7), ddp(7), ip(7), ipv6(7), landlock(7), mctp(7), netlink(7), packet(7), raw(7), sctp(7), signal(7), signal-safety(7), socket(7), tcp(7), udp(7), unix(7), vsock(7), trafgen(8)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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