gno/usr.man/man2/recv.2

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.\"     @(#)recv.2	8.3 (Berkeley) 2/21/94
.\"
.TH RECV 2 "16 January 1997" GNO "System Calls"
.SH NAME
.BR recv ,
.BR recvfrom ,
.BR recvmsg
\- receive a message from a socket
.SH SYNOPSIS
.br
#include <sys/types.h>
.br
#include <sys/socket.h>
.sp 1
int
\fBrecv\fR
(int \fIs\fR, void *\fIbuf\fR, size_t \fIlen\fR, unsigned int \fIflags\fR);
.sp 1
int
\fBrecvfrom\fR 
(int \fIs\fR, void *\fIbuf\fR, size_t \fIlen\fR, unsigned int \fIflags\fR,
struct sockaddr *\fIfrom\fR, int *\fIfromlen\fR);
.sp 1
int
\fBrecvmsg\fR
(int \fIs\fR, struct msghdr *\fImsg\fR, unsigned int \fIflags\fR);
.SH DESCRIPTION
.BR Recvfrom 
and
.BR 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.
.LP
If
.I from
is non-NULL, 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.
.LP
The 
.BR recv 
call is normally used only on a 
.IR connected 
socket (see
.BR connect (2))
and is identical to
.BR recvfrom 
with a NULL
.I from
parameter.
As it is redundant, it may not be supported in future releases.
.LP
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)).
.LP
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
.IR errno
set to 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
.BR SO_RCVLOWAT
and
.BR SO_RCVTIMEO
described in
.BR getsockopt (2).
.LP
The
.BR select (2)
call may be used to determine when more data arrive.
.LP
The
.I flags
argument to a recv call is formed by 
.IR or 'ing
one or more of the values:
.RS
.TP
MSG_OOB
process out-of-band data
.TP
MSG_PEEK
peek at incoming message
.TP
MSG_WAITALL
wait for full request or error
.RE
.LP
The
.BR 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 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 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.
.LP
The
.BR recvmsg 
call uses a 
.I msghdr
structure to minimize the number of directly supplied parameters.
This structure has the following form, as defined in <sys/socket.h>:
.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
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 read (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:
.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
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
.BR accept 
call.
.LP
Open file descriptors are now passed as ancillary data for
.BR AF_UNIX
domain sockets, with
.I cmsg_level
set to
.BR SOL_SOCKET
and
.I cmsg_type
set to
.BR SCM_RIGHTS .
.LP
The
.I msg_flags
field is set on return according to the message received.
.BR MSG_EOR
indicates end-of-record;
the data returned completed a record (generally used with sockets of type
.BR SOCK_SEQPACKET ).
.BR MSG_TRUNC
indicates that
the trailing portion of a datagram was discarded because the datagram
was larger than the buffer supplied.
.BR MSG_CTRUNC
indicates that some
control data were discarded due to lack of space in the buffer
for ancillary data.
.BR MSG_OOB
is returned to indicate that expedited or out-of-band data were received.
.LP
.SH RETURN VALUES
These calls return the number of bytes received, or -1
if an error occurred.
.SH ERRORS
The calls fail if:
.RS
.IP \fBEBADF\fR
The argument
.I s
is an invalid descriptor.
.IP \fBENOTCONN\fR
The socket is associated with a connection-oriented protocol
and has not been connected (see
.BR connect (2)
and
.BR accept (2)).
.IP \fBENOTSOCK\fR
The argument
.I s
does not refer to a socket.
.IP \fBEAGAIN\fR
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.
.IP \fBEINTR\fR
The receive was interrupted by delivery of a signal before
any data were available.
.IP \fBEFAULT\fR
The receive buffer pointer(s) point outside the process's
address space.
.RE
.SH CONVORMANCE
The GNO prototypes of these routines differ slightly from that of
4.4BSD.
.SH SEE ALSO
.BR fcntl (2),
.BR read (2),
.BR select (2),
.BR getsockopt (2),
.BR socket (2)
.SH HISTORY
The
.BR recvmsg
function call appeared in 4.2BSD.
The first appearance in GNO was in v2.0.5.