man-pages/man2/send.2

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.\" Modified 1993-07-24 by Rik Faith <faith@cs.unc.edu>
.\" Modified 1996-10-22 by Eric S. Raymond <esr@thyrsus.com>
.\" Modified Oct 1998 by Andi Kleen
.\" Modified Oct 2003 by aeb
.\" Modified 2004-07-01 by mtk
.\"
.TH SEND 2 2004-07-01 "Linux 2.6.7" "Linux Programmer's Manual"
.SH NAME
send, sendto, sendmsg \- send a message on a socket
.SH SYNOPSIS
.nf
.B #include <sys/types.h>
.B #include <sys/socket.h>
.sp
.BI "ssize_t send(int " s ", const void *" buf ", size_t " len \
", int " flags );
.br
.BI "ssize_t sendto(int " s ", const void *" buf ", size_t " len \
", int " flags ,
.BI "               const struct sockaddr *" to ", socklen_t " tolen );
.br
.BI "ssize_t sendmsg(int " s ", const struct msghdr *" msg \
", int " flags );
.fi
.SH DESCRIPTION
The system calls
.BR send (),
.BR sendto (),
and
.BR sendmsg ()
are used to transmit a message to another socket.
.PP
The
.BR send ()
call may be used only when the socket is in a
.I connected
state (so that the intended recipient is known).
The only difference between
.BR send ()
and
.BR write ()
is the presence of
.IR flags .
With zero
.I flags
parameter,
.BR send ()
is equivalent to
.BR write ().
Also,
.RI send( s , buf , len , flags )
is equivalent to
.RI sendto( s , buf , len , flags ,NULL,0).
.PP
The parameter
.I s
is the file descriptor of the sending socket.
.PP
If
.BR sendto ()
is used on a connection-mode (SOCK_STREAM, SOCK_SEQPACKET) socket,
the parameters
.I to
and
.I 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
.I to
with
.I tolen
specifying its size.
For
.BR sendmsg (),
the address of the target is given by
.IR msg.msg_name ,
with
.I msg.msg_namelen
specifying its size.
.PP
For
.BR send ()
and
.BR sendto (),
the message is found in
.I buf
and has length
.IR len .
For
.BR sendmsg (),
the message is pointed to by the elements of the array
.IR msg.msg_iov .
The
.BR sendmsg ()
call also allows sending ancillary data (also known as control information).
.PP
If the message is too long to pass atomically through the
underlying protocol, the error
.B EMSGSIZE
is returned, and the message is not transmitted.
.PP
No indication of failure to deliver is implicit in a
.BR send ().
Locally detected errors are indicated by a return value of \-1.
.PP
When the message does not fit into the send buffer of the socket,
.BR send ()
normally blocks, unless the socket has been placed in non-blocking I/O
mode.
In non-blocking mode it would return
.B EAGAIN
in this case.
The
.BR select (2)
call may be used to determine when it is possible to send more data.
.PP
The
.I flags
parameter is the bitwise OR
of zero or more of the following flags.
.\" FIXME? document MSG_PROXY (which went away in 2.3.15)
.TP
.BR 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 will regularly reprobe the neighbour (e.g. via a unicast ARP).
Only valid on
.B SOCK_DGRAM
and
.B SOCK_RAW
sockets and currently only implemented for IPv4 and IPv6.
See
.BR arp (7)
for details.
.TP
.B 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 protocol
families that route; packet sockets don't.
.TP
.B MSG_DONTWAIT
Enables non-blocking operation; if the operation would block,
.B EAGAIN
is returned (this can also be enabled using the
.B O_NONBLOCK
with the
.B F_SETFL
.BR fcntl (2)).
.TP
.B MSG_EOR
Terminates a record (when this notion is supported, as for sockets of type
.BR SOCK_SEQPACKET ).
.TP
.BR 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
.BR tcp (7)),
with the difference that this flag can be set on a per-call basis.
.sp
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 only transmitted when a call is performed
that does not specify this flag.
(See also the
.B UDP_CORK
socket option described in
.BR udp (7).)
.TP
.B MSG_NOSIGNAL
Requests not to send
.B SIGPIPE
on errors on stream oriented sockets when the other end breaks the
connection.
The
.B EPIPE
error is still returned.
.TP
.B MSG_OOB
Sends
.I out-of-band
data on sockets that support this notion (e.g. of type
.BR SOCK_STREAM );
the underlying protocol must also support
.I out-of-band
data.
.PP
The definition of the
.I msghdr
structure follows.
See
.BR recv (2)
and below for an exact description of its fields.
.in +0.25i
.nf

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 */
};
.fi
.in -0.25i
.PP
You may send control information using the
.I msg_control
and
.I msg_controllen
members.
The maximum control buffer length the kernel can process is limited
per socket by the
.B net.core.optmem_max
sysctl; see
.BR socket (7).
.\" Still to be documented:
.\"  Send file descriptors and user credentials using the
.\"  msg_control* fields.
.\"  The flags returned in msg_flags.
.SH "RETURN VALUE"
On success, these calls return the number of characters sent.
On error, \-1 is returned, and
.I errno
is set appropriately.
.SH ERRORS
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.
.TP
.B 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
.BR path_resolution (2).)
.TP
.BR EAGAIN " or " EWOULDBLOCK
The socket is marked non-blocking and the requested operation
would block.
.TP
.B EBADF
An invalid descriptor was specified.
.TP
.B ECONNRESET
Connection reset by peer.
.TP
.B EDESTADDRREQ
The socket is not connection-mode, and no peer address is set.
.TP
.B EFAULT
An invalid user space address was specified for a parameter.
.TP
.B EINTR
A signal occurred before any data was transmitted.
.TP
.B EINVAL
Invalid argument passed.
.TP
.B 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.)
.TP
.B EMSGSIZE
The socket type
.\" (e.g., SOCK_DGRAM )
requires that message be sent atomically, and the size
of the message to be sent made this impossible.
.TP
.B 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.)
.TP
.B ENOMEM
No memory available.
.TP
.B ENOTCONN
The socket is not connected, and no target has been given.
.TP
.B ENOTSOCK
The argument
.I s
is not a socket.
.TP
.B EOPNOTSUPP
Some bit in the
.I flags
argument is inappropriate for the socket type.
.TP
.B EPIPE
The local end has been shut down on a connection oriented socket.
In this case the process
will also receive a
.B SIGPIPE
unless
.B MSG_NOSIGNAL
is set.
.SH "CONFORMING TO"
4.4BSD, SVr4, POSIX.1-2001.
These function calls appeared in 4.2BSD.
.LP
POSIX.1-2001 only describes the
.B MSG_OOB
and
.B MSG_EOR
flags.
The
.B MSG_CONFIRM
flag is a Linux extension.
.SH NOTES
The prototypes given above follow the Single Unix Specification,
as glibc2 also does; the
.I flags
argument was `int' in 4.x BSD, but `unsigned int' in libc4 and libc5;
the
.I len
argument was `int' in 4.x BSD and libc4, but `size_t' in libc5;
the
.I tolen
argument was `int' in 4.x BSD and libc4 and libc5.
See also
.BR accept (2).

According to POSIX.1-2001, the
.I msg_controllen
field of the
.I msghdr
structure should be typed as
.IR socklen_t ,
but glibc currently (2.4) types it as
.IR size_t .
.\" glibc bug raised 12 Mar 2006
.\" http://sourceware.org/bugzilla/show_bug.cgi?id=2448
.\" The problem is an underlying kernel issue: the size of the
.\" __kernel_size_t type used to type this field varies
.\" across architectures, but socklen_t is always 32 bits.
.SH BUGS
Linux may return EPIPE instead of ENOTCONN.
.SH "SEE ALSO"
.BR fcntl (2),
.BR getsockopt (2),
.BR recv (2),
.BR select (2),
.BR sendfile (2),
.BR shutdown (2),
.BR socket (2),
.BR write (2),
.BR cmsg (3),
.BR ip (7),
.BR socket (7),
.BR tcp (7),
.BR udp (7)