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.\"     @(#)recv.2	6.11 (Berkeley) 5/1/91
.\"
.\" Modified Sat Jul 24 00:22:20 1993 by Rik Faith <faith@cs.unc.edu>
.\" Modified Tue Oct 22 17:45:19 1996 by Eric S. Raymond <esr@thyrsus.com>
.\"
.TH RECV 2 "24 July 1993" "BSD Man Page" "Linux Programmer's Manual"
.SH NAME
recv, recvfrom, recvmsg \- receive a message from a socket
.SH SYNOPSIS
.B #include <sys/types.h>
.br
.B #include <sys/socket.h>
.sp 2
.BI "int recv(int " s ", void *" buf ", int " len ", int " flags );
.sp
.BI "int recvfrom(int " s ", void *" buf ", int " len ", unsigned int " flags
.BI "struct sockaddr *" from ", socklen_t *" fromlen );
.sp
.BI "int recvmsg(int " s ", struct msghdr *" msg ", int " flags );
.SH DESCRIPTION
The
.B recvfrom
and
.B recvmsg
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.

If
.I from
is non-nil, and the socket is not connection-oriented, the source address
of the message is filled in.
.I Fromlen
is a value-result parameter, initialized to the size of the buffer
associated with
.IR from ,
and modified on return to indicate the actual size of the address stored
there.

The 
.B recv
call is normally used only on a 
.I connected
socket (see
.BR connect (2))
and is identical to
.B recvfrom
with a nil
.I from
parameter.  As it is redundant, it may not be supported in future
releases.

All three routines return the length of the message on successful
completion.  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
.BR socket (2)).

If no messages are available at the socket, the receive call waits for a
message to arrive, unless the socket is nonblocking (see
.BR fcntl (2))
in which case the value \-1 is returned and the external variable
.I errno
set to
.BR EWOULDBLOCK .
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
.B SO_RCVLOWAT
and
.B SO_RCVTIMEO
described in
.BR getsockopt (2).

The
.BR select (2)
call may be used to determine when more data arrive.

The
.I flags
argument to a recv call is formed by 
.IR or 'ing
one or more of the values:
.TP 0.8i
.TP
MSG_OOB
process out-of-band data
.TP
MSG_PEEK
peek at incoming message
.TP
MSG_WAITALL
wait for full request or error

The
.B 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
.B 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
.B 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
.B recvmsg
call uses a 
.I msghdr
structure to minimize the number of directly supplied parameters.  This
structure has the following form, as defined in
.IR sys/socket.h :

.RS
.nf
struct msghdr {
	caddr_t	msg_name;	/* optional address */
	u_int	msg_namelen;	/* size of address */
	struct	iovec *msg_iov;	/* scatter/gather array */
	u_int	msg_iovlen;	/* # elements in msg_iov */
	caddr_t	msg_control;	/* ancillary data, see below */
	u_int	msg_controllen; /* ancillary data buffer len */
	int	msg_flags;	/* flags on received message */
};
.fi
.RE

Here
.I msg_name
and
.I msg_namelen
specify the destination address if the socket is unconnected;
.I msg_name
may be given as a null pointer if no names are desired or required.
.I Msg_iov
and
.I msg_iovlen
describe scatter gather locations, as discussed in
.BR readv (2).
.IR Msg_control ,
which has length
.IR msg_controllen ,
points to a buffer for other protocol control related messages or other
miscellaneous ancillary data.  The messages are of the form:

.RS
.nf
struct cmsghdr {
	u_int	cmsg_len;	/* data byte count, including hdr */
	int	cmsg_level;	/* originating protocol */
	int	cmsg_type;	/* protocol-specific type */
/* followed by
	u_char	cmsg_data[]; */
};
.fi
.RE

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
.B accept
call.

Open file descriptors are now passed as ancillary data for
.B AF_UNIX
domain sockets, with
.I cmsg_level
set to
.B SOL_SOCKET
and
.I cmsg_type
set to
.BR SCM_RIGHTS .

The
.I msg_flags
field is set on return according to the message received.
.B MSG_EOR
indicates end-of-record; the data returned completed a record (generally
used with sockets of type
.BR SOCK_SEQPACKET ).
.B MSG_TRUNC
indicates that the trailing portion of a datagram was discarded because the
datagram was larger than the buffer supplied.
.B MSG_CTRUNC
indicates that some control data were discarded due to lack of space in the
buffer for ancillary data.
.B MSG_OOB
is returned to indicate that expedited or out-of-band data were received.

.SH "RETURN VALUES"
These calls return the number of bytes received, or \-1
if an error occurred.
.SH ERRORS
.TP 0.8i
.B EBADF
The argument
.I s
is an invalid descriptor.
.TP
.B ENOTCONN
The socket is associated with a connection-oriented protocol
and has not been connected (see
.BR connect (2)
and
.BR accept (2)).
.TP
.B ENOTSOCK
The argument
.I s
does not refer to a socket.
.TP
.B EWOULDBLOCK
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.
.TP
.B EINTR
The receive was interrupted by delivery of a signal before
any data were available.
.TP
.B EFAULT
The receive buffer pointer(s) point outside the process's
address space.
.SH "CONFORMING TO"
4.4BSD (these function calls first appeared in 4.2BSD).
.SH "SEE ALSO"
.BR fcntl "(2), " read "(2), " select "(2), " getsockopt "(2), " socket (2)