.\"	$OpenBSD: recv.2,v 1.51 2023/08/17 05:45:51 jmc Exp $
.\"	$NetBSD: recv.2,v 1.6 1995/02/27 12:36:08 cgd Exp $
.\"
.\" Copyright (c) 1983, 1990, 1991, 1993
.\"	The Regents of the University of California.  All rights reserved.
.\"
.\" Redistribution and use in source and binary forms, with or without
.\" modification, are permitted provided that the following conditions
.\" are met:
.\" 1. Redistributions of source code must retain the above copyright
.\"    notice, this list of conditions and the following disclaimer.
.\" 2. Redistributions in binary form must reproduce the above copyright
.\"    notice, this list of conditions and the following disclaimer in the
.\"    documentation and/or other materials provided with the distribution.
.\" 3. Neither the name of the University nor the names of its contributors
.\"    may be used to endorse or promote products derived from this software
.\"    without specific prior written permission.
.\"
.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
.\" ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
.\" SUCH DAMAGE.
.\"
.\"     @(#)recv.2	8.3 (Berkeley) 2/21/94
.\"
.Dd $Mdocdate: August 17 2023 $
.Dt RECV 2
.Os
.Sh NAME
.Nm recv ,
.Nm recvfrom ,
.Nm recvmsg ,
.Nm recvmmsg
.Nd receive a message from a socket
.Sh SYNOPSIS
.In sys/socket.h
.Ft ssize_t
.Fn recv "int s" "void *buf" "size_t len" "int flags"
.Ft ssize_t
.Fn recvfrom "int s" "void *buf" "size_t len" "int flags" "struct sockaddr *from" "socklen_t *fromlen"
.Ft ssize_t
.Fn recvmsg "int s" "struct msghdr *msg" "int flags"
.Ft int
.Fn recvmmsg "int s" "struct mmsghdr *mmsg" "unsigned int vlen" "int flags" "struct timespec *timeout"
.Sh DESCRIPTION
.Fn recv ,
.Fn recvfrom ,
.Fn recvmsg ,
and
.Fn recvmmsg
are used to receive messages from a socket,
.Fa s .
.Fn recv
is normally used only on a
.Em connected
socket (see
.Xr connect 2 ) .
.Fn recvfrom ,
.Fn recvmsg ,
and
.Fn recvmmsg
may be used to receive
data on a socket whether or not it is connection-oriented.
.Pp
.Fn recv
is identical to
.Fn recvfrom
with a null
.Fa from
parameter.
.Pp
If
.Fa from
is non-null and the socket is not connection-oriented,
the source address of the message is filled in.
.Fa fromlen
is a value-result parameter, initialized to the size of
the buffer associated with
.Fa from ,
and modified on return to indicate the actual size of the
address stored there.
.Pp
If no messages are available at the socket, the
receive call waits for a message to arrive, unless
the socket is nonblocking (see
.Xr fcntl 2 )
in which case the value
\-1 is returned and the external variable
.Va errno
set to
.Er EAGAIN .
The receive calls normally return any data available,
up to the requested amount,
rather than waiting for receipt of the full amount requested;
this behavior is affected by the socket-level options
.Dv SO_RCVLOWAT
and
.Dv SO_RCVTIMEO
described in
.Xr getsockopt 2 .
.Pp
The
.Xr select 2
or
.Xr poll 2
system calls may be used to determine when more data arrive.
.Pp
The
.Fa flags
argument is the bitwise OR of zero or more of the following values:
.Pp
.Bl -tag -width "MSG_CMSG_CLOEXECXX" -offset indent -compact
.It Dv MSG_OOB
process out-of-band data
.It Dv MSG_PEEK
peek at incoming message
.It Dv MSG_WAITALL
wait for full request or error
.It Dv MSG_DONTWAIT
don't block
.It Dv MSG_CMSG_CLOEXEC
set the close-on-exec flag on received file descriptors
.El
.Pp
The
.Dv MSG_OOB
flag requests receipt of out-of-band data
that would not be received in the normal data stream.
Some protocols place expedited data at the head of the normal
data queue, and thus this flag cannot be used with such protocols.
The
.Dv MSG_PEEK
flag causes the receive operation to return data
from the beginning of the receive queue without removing that
data from the queue.
Thus, a subsequent receive call will return the same data.
The
.Dv MSG_WAITALL
flag requests that the operation block until
the full request is satisfied.
However, the call may still return less data than requested
if a signal is caught, an error or disconnect occurs,
or the next data to be received is of a different type than that returned.
The
.Dv MSG_DONTWAIT
flag requests the call to return when it would block otherwise.
If no data is available,
.Va errno
is set to
.Er EAGAIN .
This flag is not available in strict ANSI or C99 compilation mode.
The
.Dv MSG_CMSG_CLOEXEC
requests that any file descriptors received as ancillary data with
.Fn recvmsg
and
.Fn recvmmsg
(see below)
have their close-on-exec flag set.
.Pp
The
.Fn recvmsg
call uses a
.Fa msghdr
structure to minimize the number of directly supplied parameters.
This structure has the following form, as defined in
.In sys/socket.h :
.Bd -literal
struct msghdr {
	void		*msg_name;	 /* optional address */
	socklen_t	 msg_namelen;	 /* size of address */
	struct iovec	*msg_iov;	 /* scatter/gather array */
	unsigned int	 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 */
};
.Ed
.Pp
Here
.Fa msg_name
and
.Fa msg_namelen
specify the source address if the socket is unconnected;
.Fa msg_name
may be given as a null pointer if no names are desired or required.
.Fa msg_iov
and
.Fa msg_iovlen
describe scatter gather locations, as discussed in
.Xr read 2 .
.Fa msg_control ,
which has length
.Fa msg_controllen ,
points to a buffer for other protocol control related messages
or other miscellaneous ancillary data.
The messages are of the form:
.Bd -literal
struct cmsghdr {
	socklen_t	cmsg_len;   /* data byte count, including hdr */
	int		cmsg_level; /* originating protocol */
	int		cmsg_type;  /* protocol-specific type */
/* followed by u_char	cmsg_data[]; */
};
.Ed
.Pp
See
.Xr CMSG_DATA 3
for how these messages are constructed and decomposed.
.Pp
Open file descriptors are now passed as ancillary data for
.Dv AF_UNIX
domain and
.Xr socketpair 2
sockets, with
.Fa cmsg_level
set to
.Dv SOL_SOCKET
and
.Fa cmsg_type
set to
.Dv SCM_RIGHTS .
.Pp
The
.Fa msg_flags
field is set on return according to the message received.
It will contain zero or more of the following values:
.Pp
.Bl -tag -width MSG_CTRUNC -offset indent -compact
.It Dv MSG_OOB
Returned to indicate that expedited or out-of-band data was received.
.It Dv MSG_EOR
Indicates end-of-record;
the data returned completed a record (generally used with sockets of type
.Dv SOCK_SEQPACKET ) .
.It Dv MSG_TRUNC
Indicates that
the trailing portion of a datagram was discarded because the datagram
was larger than the buffer supplied.
.It Dv MSG_CTRUNC
Indicates that some
control data were discarded due to lack of space in the buffer
for ancillary data.
.It Dv MSG_BCAST
Indicates that the packet was received as broadcast.
.It Dv MSG_MCAST
Indicates that the packet was received as multicast.
.El
.Pp
The
.Fn recvmmsg
call uses an array of the
.Fa mmsghdr
structure of length
.Fa vlen
to group multiple
.Fa msghdr
structures into a single system call.
.Fa vlen
is capped at maximum
.Dv 1024
messages that are received in a single call.
The
.Fa flags
field allows setting
.Dv MSG_WAITFORONE
to wait for one
.Fa msghdr ,
and set
.Dv MSG_DONTWAIT
for all subsequent messages.
A provided
.Fa timeout
limits the time spent in the function but it does not limit the
time spent in lower parts of the kernel.
.Pp
The
.Fa mmsghdr
structure has the following form, as defined in
.In sys/socket.h :
.Bd -literal
struct mmsghdr {
	struct msghdr msg_hdr;
	unsigned int msg_len;
};
.Ed
.Pp
Here
.Fa msg_len
indicated the number of bytes received for each
.Fa msg_hdr
member.
.Sh RETURN VALUES
The
.Fn recv ,
.Fn recvfrom ,
and
.Fn recvmsg
calls return the number of bytes received, or \-1 if an error occurred.
The
.Fn recvmmsg
call returns the number of messages received, or \-1
if an error occurred before the first message has been received.
.Sh ERRORS
.Fn recv ,
.Fn recvfrom ,
.Fn recvmsg ,
and
.Fn recvmmsg
fail if:
.Bl -tag -width "[EHOSTUNREACH]"
.It Bq Er EBADF
The argument
.Fa s
is an invalid descriptor.
.It Bq Er ENOTCONN
The socket is associated with a connection-oriented protocol
and has not been connected (see
.Xr connect 2
and
.Xr accept 2 ) .
.It Bq Er ENOTSOCK
The argument
.Fa s
does not refer to a socket.
.It Bq Er EAGAIN
The socket is marked non-blocking, and the receive operation
would block, or
a receive timeout had been set,
and the timeout expired before data were received.
.It Bq Er EINTR
The receive was interrupted by delivery of a signal before
any data were available.
.It Bq Er EFAULT
The receive buffer pointer(s) point outside the process's
address space.
.It Bq Er EHOSTUNREACH
A socket operation was attempted to an unreachable host.
.It Bq Er EHOSTDOWN
A socket operation failed
because the destination host was down.
.It Bq Er ENETDOWN
A socket operation encountered a dead network.
.It Bq Er ECONNREFUSED
The socket is associated with a connection-oriented protocol
and the connection was forcefully rejected (see
.Xr connect 2 ) .
.El
.Pp
In addition,
.Fn recv
and
.Fn recvfrom
may return the following error:
.Bl -tag -width Er
.It Bq Er EINVAL
.Fa len
was larger than
.Dv SSIZE_MAX .
.El
.Pp
And
.Fn recvmsg
and
.Fn recvmmsg
may return one of the following errors:
.Bl -tag -width Er
.It Bq Er EINVAL
The sum of the
.Fa iov_len
values in the
.Fa msg_iov
array overflowed an
.Em ssize_t .
.It Bq Er EMSGSIZE
The
.Fa msg_iovlen
member of
.Fa msg
was less than 0 or larger than
.Dv IOV_MAX .
.It Bq Er EMSGSIZE
The receiving program did not have sufficient
free file descriptor slots.
The descriptors are closed
and any pending data can be returned
by another call to
.Fn recvmsg .
.El
.Sh SEE ALSO
.Xr connect 2 ,
.Xr fcntl 2 ,
.Xr getsockopt 2 ,
.Xr poll 2 ,
.Xr read 2 ,
.Xr select 2 ,
.Xr socket 2 ,
.Xr socketpair 2 ,
.Xr CMSG_DATA 3 ,
.Xr sockatmark 3
.Sh STANDARDS
The
.Fn recv ,
.Fn recvfrom ,
and
.Fn recvmsg
functions conform to
.St -p1003.1-2008 .
The
.Dv MSG_DONTWAIT ,
.Dv MSG_BCAST ,
and
.Dv MSG_MCAST
flags are extensions to that specification.
.Sh HISTORY
The
.Fn recv
function call appeared in
.Bx 4.1c .
The
.Fn recvmmsg
syscall first appeared in Linux 2.6.33 and was added to
.Ox 7.2 .
.Sh CAVEATS
Calling
.Fn recvmsg
with a control message having no or an empty scatter/gather array
exposes variations in implementations.
To avoid these, always use an
.Fa iovec
with at least a one-byte buffer and set
.Fa msg_iov
and an
.Fa msg_iovlen
to use this vector.