sendto
SEND(2) Linux Programmer’s Manual SEND(2)
NAME
send, sendto, sendmsg - send a message from a socket
SYNOPSIS
#include <sys/types.h>
#include <sys/socket.h>
ssize_t send(int s, const void *buf, size_t len, int flags);
ssize_t sendto(int s, const void *buf, size_t len, int flags, const
struct sockaddr *to, socklen_t tolen);
ssize_t sendmsg(int s, const struct msghdr *msg, int flags);
DESCRIPTION
The system calls send, sendto, and sendmsg are used to transmit a mes-
sage 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 is the presence of flags. With zero flags
parameter, send is equivalent to write. Also, send(s,buf,len) is
equivalent to sendto(s,buf,len,NULL,0).
The parameter s is the file descriptor of the sending socket.
If sendto is used on a connection-mode (SOCK_STREAM, SOCK_SEQPACKET)
socket, the parameters to and tolen 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 to with tolen 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 non-blocking I/O
mode. In non-blocking mode it would return EAGAIN in this case. The
select(2) call may be used to determine when it is possible to send
more data.
The flags parameter is the bitwise OR of zero or more of the following
flags.
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_EOR
Terminates a record (when this notion is supported, as for
sockets of type SOCK_SEQPACKET).
MSG_DONTROUTE
Don’t use a gateway to send out the packet, only send to hosts
on directly connected networks. This is usually used only by
diagnostic or routing programs. This is only defined for proto-
col families that route; packet sockets don’t.
MSG_DONTWAIT
Enables non-blocking operation; if the operation would block,
EAGAIN is returned (this can also be enabled using the O_NON-
BLOCK with the F_SETFL fcntl(2)).
MSG_NOSIGNAL
Requests not to send SIGPIPE on errors on stream oriented sock-
ets when the other end breaks the connection. The EPIPE error
is still returned.
MSG_CONFIRM (Linux 2.3+ only)
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’ll regularly reprobe the neighbour (e.g. via a uni-
cast ARP). Only valid on SOCK_DGRAM and SOCK_RAW sockets and
currently only implemented for IPv4 and IPv6. See arp(7) for
details.
The definition of the msghdr structure follows. See recv(2) and below
for an exact description of its fields.
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 */
socklen_t msg_controllen; /* ancillary data buffer len */
int msg_flags; /* flags on received message */
};
You may send control information using the msg_control and msg_con-
trollen members. The maximum control buffer length the kernel can pro-
cess is limited per socket by the net.core.optmem_max sysctl; see
socket(7).
RETURN VALUE
The calls return the number of characters sent, or -1 if an error
occurred.
ERRORS
These are some standard errors generated by the socket layer. Addi-
tional errors may be generated and returned from the underlying proto-
col modules; see their respective manual pages.
EAGAIN or EWOULDBLOCK
The socket is marked non-blocking and the requested operation
would block.
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 a parameter.
EINTR A signal occurred before any data was transmitted.
EINVAL Invalid argument passed.
EISCONN
The connection-mode socket was connected already but a recipi-
ent 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 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 argument s 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.
CONFORMING TO
4.4BSD, SVr4, POSIX 1003.1-2001. These function calls appeared in
4.2BSD.
POSIX only describes the MSG_OOB and MSG_EOR flags. The MSG_CONFIRM
flag is a Linux extension.
NOTE
The prototypes given above follow the Single Unix Specification, as
glibc2 also does; the flags argument was ‘int’ in BSD 4.*, but
‘unsigned int’ in libc4 and libc5; the len argument was ‘int’ in BSD
4.* and libc4, but ‘size_t’ in libc5; the tolen argument was ‘int’ in
BSD 4.* and libc4 and libc5. See also accept(2).
BUGS
Linux may return EPIPE instead of ENOTCONN.
SEE ALSO
fcntl(2), recv(2), select(2), getsockopt(2), sendfile(2), socket(2),
write(2), socket(7), ip(7), tcp(7), udp(7)
Linux Man Page 2003-10-25 SEND(2)
