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.\"     @(#)recv.2	8.3 (Berkeley) 2/21/94
.\"
.Dd March 19, 2018
.Dt RECV 2
.Os
.Sh NAME
.Nm recv ,
.Nm recvfrom ,
.Nm recvmsg ,
.Nm recvmmsg
.Nd receive a message from a socket
.Sh LIBRARY
.Lb libc
.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 * restrict buf" "size_t len" "int flags" "struct sockaddr * restrict from" "socklen_t * restrict 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" "unsigned int flags" "struct timespec *timeout"
.Sh DESCRIPTION
.Fn recvfrom ,
.Fn recvmsg
and
.Fn recvmmsg
are used to receive messages from a socket,
and may be used to receive data on a socket whether or not
it is connection-oriented.
.Pp
If
.Fa from
is non-nil, 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
The
.Fn recvmmsg
call can be used to receive multiple messages in the same call using an array of
.Fa mmsghdr
elements with the following form, as defined in
.Ao Pa sys/socket.h Ac :
.Bd -literal
struct mmsghdr {
	struct msghdr	msg_hdr;	/* the message to be sent */
	unsigned int	msg_len;	/* number of bytes received */
};
.Ed
.Pp
The
.Fa msg_len
member contains the number of bytes received for each
.Fa msg_hdr
member.
The array has
.Fa vlen
elements, which is limited to
.Dv 1024 .
If there is an error, a number fewer than
.Fa vlen
may be returned, and the error may be retrieved using
.Xr getsockopt 2
with
.Dv SO_ERROR .
If the flag
.Dv MSG_WAITFORONE
is set in
.Fa flags
then the
.Fn recvmmsg
call will wait for one message, and set
.Dv MSG_DONTWAIT
for the rest.
If the
.Fa timeout
parameter is not
.Dv NULL ,
then
.Fn recvmmsg
will return if that time is exceeded.
.Pp
The
.Fn recv
call is normally used only on a
.Em connected
socket (see
.Xr connect 2 )
and is identical to
.Fn recvfrom
with a nil
.Fa from
parameter.
As it is redundant, it may not be supported in future releases.
.Pp
.Fn recv ,
.Fn recvfrom
and
.Fn recvmsg
routines return the length of the message on successful
completion.
.Fn recvmmsg
returns the number of messages received.
If a message is too long to fit in the supplied buffer,
excess bytes may be discarded depending on the type of socket
the message is received from (see
.Xr socket 2 ) .
.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 .
If no data is available and the remote peer was shut down,
0 is returned.
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
call may be used to determine when more data arrives.
.Pp
The
.Fa flags
argument to a recv call is formed by
.Em or Ap ing
one or more of the values:
.Bl -column MSG_CMSG_CLOEXEC -offset indent
.It Dv MSG_CMSG_CLOEXEC Ta set the close on exec property for passed file descriptors
.It Dv MSG_OOB Ta process out-of-band data
.It Dv MSG_PEEK Ta peek at incoming message
.It Dv MSG_WAITALL Ta wait for full request or error
.El
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.
.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
.Ao Pa sys/socket.h Ac :
.Bd -literal
struct msghdr {
	void		*msg_name;	/* optional address */
	socklen_t	msg_namelen;	/* size of address */
	struct iovec	*msg_iov;	/* scatter/gather array */
	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.
If the socket is connected,
.Fa msg_name
and
.Fa msg_namelen
are ignored.
.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
As an example, one could use this to learn of changes in the data-stream
in XNS/SPP, or in ISO, to obtain user-connection-request data by requesting
a recvmsg with no data buffer provided immediately after an
.Fn accept
call.
.Pp
Open file descriptors are now passed as ancillary data for
.Dv AF_LOCAL
domain 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.
.Dv MSG_EOR
indicates end-of-record;
the data returned completed a record (generally used with sockets of type
.Dv SOCK_SEQPACKET ) .
.Dv MSG_TRUNC
indicates that
the trailing portion of a datagram was discarded because the datagram
was larger than the buffer supplied.
.Dv MSG_CTRUNC
indicates that some
control data were discarded due to lack of space in the buffer
for ancillary data.
.Dv MSG_OOB
is returned to indicate that expedited or out-of-band data were received.
.Sh RETURN VALUES
The
.Fn recv ,
.Fn recvfrom
and
.Fn recvmsg
calls return the number of bytes received, or \-1
if an error occurred.
For connected sockets whose remote peer was shut down,
0 is returned when no more data is available.
The
.Fn recvmmsg
call returns the number of messages received, or \-1
if an error occurred.
.Sh ERRORS
The calls fail if:
.Bl -tag -width Er
.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 EBADF
The argument
.Fa s
is an invalid descriptor.
.It Bq Er EFAULT
The receive buffer pointer(s) point outside the process's
address space.
.It Bq Er EINTR
The receive was interrupted by delivery of a signal before
any data were available.
.It Bq Er EINVAL
The total length of the I/O is more than can be expressed by the ssize_t
return value.
.It Bq Er ENOBUFS
A message was not delivered because it would have overflowed the buffer.
.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.
.El
.Pp
.Fn recvmsg
will also fail if:
.Bl -tag -width Er
.It Bq Er EMSGSIZE
The
.Fa msg_iovlen
member of the
.Fa msg
structure is less than or equal to 0,
or is greater than
.Brq Dv IOV_MAX .
.El
.Sh SEE ALSO
.Xr fcntl 2 ,
.Xr getsockopt 2 ,
.Xr poll 2 ,
.Xr read 2 ,
.Xr select 2 ,
.Xr socket 2
.Sh HISTORY
The
.Fn recv
function call appeared in
.Bx 4.2 .
The
.Fn recvmmsg
function call appeared in
.Tn Linux 2.6.32
and
.Nx 7.0 .