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.\"     $Id: recv.2,v 1.3 1999/05/13 11:33:38 freitag Exp $
.\"
.\" 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>
.\" Modified 1998,1999 by Andi Kleen
.\"
.TH RECV 2 "Apr 1999" "Linux 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
.BI "int recv(int " s ", void *" buf ", size_t " len ", int " flags );
.sp
.BI "int recvfrom(int " s ", void *" buf ", size_t " len ,
.BI "int " flags ", 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
calls 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
.I from
is not NULL, and the socket is not connection-oriented, the source address
of the message is filled in.
The argument
.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 
.B recv
call is normally used only on a 
.I connected
socket (see
.BR connect (2))
and is identical to
.B recvfrom
with a NULL
.I from
parameter. 
.PP
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)).
.PP
If no messages are available at the socket, the receive calls wait 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 EAGAIN .
The receive calls normally return any data available, up to the requested
amount, rather than waiting for receipt of the full amount requested.
.PP
The
.BR select (2) 
or
.BR poll (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 following values:
.TP
.B MSG_OOB
This 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.
.TP
.B MSG_PEEK
This 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.
.TP
.B MSG_WAITALL
This 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.
.TP
.B MSG_NOSIGNAL 
This flag turns off raising of 
.B SIGPIPE
on stream sockets when the other end disappears.
.TP
.B MSG_ERRQUEUE
This flag
specifies that queued errors should be received from the socket error queue.
The error is passed in
an ancillary message with a type dependent on the protocol (for IPv4
.BR IP_RECVERR 
). The user should supply a buffer of sufficient size.
See 
.BR cmsg (3)
for more information on ancillary messages.

The error is supplied in a 
.B sock_extended_err
structure:
.IP
.RS
.ne 18
.nf
.ta 4n 20n 32n
#define SO_EE_ORIGIN_NONE	0
#define SO_EE_ORIGIN_LOCAL	1
#define SO_EE_ORIGIN_ICMP	2
#define SO_EE_ORIGIN_ICMP6	3

struct sock_extended_err
{
	u_int32_t	ee_errno;	/* error number */
	u_int8_t	ee_origin;	/* where the error originated */ 
	u_int8_t	ee_type;	/* type */
	u_int8_t	ee_code;	/* code */
	u_int8_t	ee_pad;
	u_int32_t	ee_info;	/* additional information */
	u_int32_t	ee_data;	/* other data */  
	/* More data may follow */ 
};

struct sockaddr *SOCK_EE_OFFENDER(struct sock_extended_err *);
.ta
.fi
.RE
.IP
.B ee_errno 
contains the errno number of the queued error. 
.B ee_origin
is the origin code of where the error originated. 
The other fields are protocol specific. The macro
.B SOCK_EE_OFFENDER 
returns a pointer to the address of the network object
where the error originated from given a pointer to the ancillary message.
If this address is not known, the
.I sa_family 
member of the 
.B sockaddr 
contains 
.B AF_UNSPEC
and the other fields of the 
.B sockaddr 
are undefined. The payload of the packet
that caused the error is passed as normal data. 
.IP
For local errors, no address is passed (this
can be checked with the 
.I cmsg_len 
member of the 
.BR cmsghdr ). 
For error receives,
the 
.B MSG_ERRQUEUE 
is set in the 
.BR msghdr . 
After an error has been passed, the pending socket error
is regenerated based on the next queued error and will be passed
on the next socket operation.
.PP
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> :
.IP
.RS
.nf
.ta 4n 17n 33n
struct msghdr {
	void	* msg_name;	/* optional address */
	socklen_t	msg_namelen;	/* size of address */
	struct iovec	* msg_iov;	/* scatter/gather array */
	size_t	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 */
};
.ta
.fi
.RE
.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 fields
.I msg_iov
and
.I msg_iovlen
describe scatter-gather locations, as discussed in
.BR readv (2).
The field
.IR msg_control ,
which has length
.IR msg_controllen ,
points to a buffer for other protocol control related messages or 
miscellaneous ancillary data. When 
.B recvmsg 
is called, 
.I msg_controllen
should contain the length of the available buffer in 
.IR msg_control ; 
upon return from a successful call it will contain the length
of the control message sequence.
.PP
The messages are of the form:
.PP
.RS
.nf
.ta 4n 16n 28n
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[]; */
};
.ta
.fi
.RE
.PP
Ancillary data should only be accessed by the macros defined in 
.BR cmsg (3).
.PP
As an example, Linux uses this auxiliary data mechanism to pass extended
errors, IP options or file descriptors over Unix sockets. 
.PP
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.
.B MSG_ERRQUEUE
indicates that no data was received but an extended error from the socket
error queue.  
.SH "RETURN VALUES"
These calls return the number of bytes received, or \-1
if an error occurred.
.SH ERRORS
These are some standard errors generated by the socket layer. Additional errors
may be generated and returned from the underlying protocol modules; see their
manual pages.
.TP
.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 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 was 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.
.TP
.B EINVAL
Invalid argument passed. 
.SH "CONFORMING TO"
4.4BSD (these function calls first appeared in 4.2BSD).
.SH NOTE
The prototypes given above follow glibc2.
The Single Unix Specification agrees, except that it has return values
of type `ssize_t' (while BSD 4.* and libc4 and libc5 all have `int').
The
.I flags
argument is `int' in BSD 4.*, but `unsigned int' in libc4 and libc5.
The
.I len
argument is `int' in BSD 4.*, but `size_t' in libc4 and libc5.
The
.I fromlen
argument is `int *' in BSD 4.*, libc4 and libc5.
The present  `socklen_t *' was invented by POSIX.
See also
.BR accept (2).
.SH "SEE ALSO"
.BR fcntl (2),
.BR read (2),
.BR select (2),
.BR getsockopt (2), 
.BR socket (2), 
.BR cmsg (3)