4.2BSD/usr/man/man2/recv.2

.TH RECV 2 "7 July 1983"
.UC 4
.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 Recv ,
.IR recvfrom ,
and
.IR recvmsg
are used to receive messages from a socket.
.PP
The 
.I recv
call may be used only on a 
.I connected
socket (see
.IR connect (2)),
while 
.I recvfrom
and 
.I recvmsg
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.
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 send call is formed by 
.IR or 'ing
one or more of the values,
.PP
.nf
.RS
.DT
#define	MSG_PEEK	0x1	/* peek at incoming message */
#define	MSG_OOB	0x2	/* process out-of-band data */
.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
.RS
.DT
struct msghdr {
	caddr_t	msg_name;		/* optional address */
	int	msg_namelen;		/* size of address */
	struct	iov *msg_iov;		/* scatter/gather array */
	int	msg_iovlen;		/* # elements in msg_iov */
	caddr_t	msg_accrights;		/* access rights sent/received */
	int	msg_accrightslen;
};
.RE
.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).
Access rights to be sent along with the message are specified
in 
.IR msg_accrights ,
which has length
.IR msg_accrightslen .
.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
read(2), send(2), socket(2)