4.3BSD-Reno/src/lib/libc/sys/recv.2

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.\"	@(#)recv.2	6.8 (Berkeley) 5/30/90
.\"
.TH RECV 2 "May 30, 1990"
.UC 5
.SH NAME
recv, recvfrom, recvmsg \- receive a message from a socket
.SH SYNOPSIS
.nf
.ft B
#include <sys/types.h>
#include <sys/socket.h>
.PP
.ft B
cc = recv(s, buf, len, flags)
int cc, s;
char *buf;
int len, flags;
.PP
.ft B
cc = recvfrom(s, buf, len, flags, from, fromlen)
int cc, s;
char *buf;
int len, flags;
struct sockaddr *from;
int *fromlen;
.PP
.ft B
cc = recvmsg(s, msg, flags)
int cc, s;
struct msghdr *msg;
int flags;
.ft R
.SH DESCRIPTION
.IR Recvfrom ,
and
.IR recvmsg
are used to receive messages from a socket,
and may be used to receive data on a socket whether
it is in a connected state or not.
.PP
If
.I from
is non-zero, 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.
.PP
The 
.I recv
call is normally used only on a 
.I connected
socket (see
.IR connect (2))
and is identitical to
.I recfrom
with a zero-valued
.I fromlen
parameter.
As it is redundant, it may not be supported in future releases.
.PP
The length of the message is returned in
.IR cc .
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
.IR 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
.IR ioctl (2))
in which case a
.I cc
of \-1 is returned with the external variable errno
set to EWOULDBLOCK.
.PP
The
.IR select (2)
call may be used to determine when more data arrives.
.PP
The
.I flags
argument to a recv call is formed by 
.IR or 'ing
one or more of the values,
.PP
.nf
.RS
.ta \w'#define\ \ 'u +\w'MSG_DONTROUTE\ \ \ 'u +\w'0x\0\0\0\ \ 'u
#define	MSG_OOB	0x1	/* process out-of-band data */
#define	MSG_PEEK	0x2	/* peek at incoming message */
#define	MSG_DONTROUTE	0x4	/* send without using routing tables */
#define	MSG_EOR	0x8	/* data completes record */
#define	MSG_TRUNC	0x10	/* data discarded before delivery */
#define	MSG_CTRUNC	0x20	/* control data lost before delivery */
.RE
.fi
.PP
The
.I 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> :
.PP
.nf
.ta \w'struct  'u +\w'caddr_t   'u +\w'msg_controllen    'u
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
.PP
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.
The
.I msg_iov
and
.I msg_iovlen
describe the scatter gather locations, as described in
.IR read (2).
.IR msg_control ,
which has length
.IR msg_controllen .
This is a buffer for other protocol control related messages
or other miscellaneous ancillary data.
The messages are of the form:
.PP
.nf
.ta \w'struct  'u +\w'u_char   'u +\w'msg_controllen    'u
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 uio provided immediately after an
.IR accept (),
thought of here in the sense of get-next-connection-request without
an implicit connection confirmation.
.PP
Open file descriptors are now passed as ancillary data for AF_UNIX
domain sockets, with cmsg_level being SOL_SOCKET and  cmsg_type being
SCM_RIGHTS.
.PP
.I msg_flags
is set on return in a way that may include some of the same values specified
for the flags parameter to a recv system call.
The returned values MSG_EOR indicates end-of-record, MSG_TRUNC indicates that
some trailing datagram data was discarded, MSG_CTRUNC indicates that some
control data was discarded due to lack of space.
MSG_OOB is returned to indicate that expedited data was received.
.PP
.SH "RETURN VALUE"
These calls return the number of bytes received, or \-1
if an error occurred.
.SH ERRORS
The calls fail if:
.TP 20
[EBADF]
The argument \fIs\fP is an invalid descriptor.
.TP 20
[ENOTSOCK]
The argument \fIs\fP is not a socket.
.TP 20
[EWOULDBLOCK]
The socket is marked non-blocking and the receive operation
would block.
.TP 20
[EINTR]
The receive was interrupted by delivery of a signal before
any data was available for the receive.
.TP 20
[EFAULT]
The data was specified to be received into a non-existent
or protected part of the process address space.
.SH SEE ALSO
fcntl(2), read(2), send(2), select(2), getsockopt(2), socket(2)