.\" Copyright (c) 2001-2003 The Open Group, All Rights Reserved 
.TH "RECVFROM" 3P 2003 "IEEE/The Open Group" "POSIX Programmer's Manual"
.\" recvfrom 
.SH PROLOG
This manual page is part of the POSIX Programmer's Manual.
The Linux implementation of this interface may differ (consult
the corresponding Linux manual page for details of Linux behavior),
or the interface may not be implemented on Linux.
.SH NAME
recvfrom \- receive a message from a socket
.SH SYNOPSIS
.LP
\fB#include <sys/socket.h>
.br
.sp
ssize_t recvfrom(int\fP \fIsocket\fP\fB, void *restrict\fP \fIbuffer\fP\fB,
size_t\fP \fIlength\fP\fB,
.br
\ \ \ \ \ \  int\fP \fIflags\fP\fB, struct sockaddr *restrict\fP \fIaddress\fP\fB,
.br
\ \ \ \ \ \  socklen_t *restrict\fP \fIaddress_len\fP\fB);
.br
\fP
.SH DESCRIPTION
.LP
The \fIrecvfrom\fP() function shall receive a message from a connection-mode
or connectionless-mode socket. It is normally used
with connectionless-mode sockets because it permits the application
to retrieve the source address of received data.
.LP
The \fIrecvfrom\fP() function takes the following arguments:
.TP 7
\fIsocket\fP
Specifies the socket file descriptor.
.TP 7
\fIbuffer\fP
Points to the buffer where the message should be stored.
.TP 7
\fIlength\fP
Specifies the length in bytes of the buffer pointed to by the \fIbuffer\fP
argument.
.TP 7
\fIflags\fP
Specifies the type of message reception. Values of this argument are
formed by logically OR'ing zero or more of the following
values: 
.TP 7
MSG_PEEK
.RS
Peeks at an incoming message. The data is treated as unread and the
next \fIrecvfrom\fP() or similar function shall still
return this data.
.RE
.TP 7
MSG_OOB
.RS
Requests out-of-band data. The significance and semantics of out-of-band
data are protocol-specific.
.RE
.TP 7
MSG_WAITALL
.RS
On SOCK_STREAM sockets this requests that the function block until
the full amount of data can be returned. The function may
return the smaller amount of data if the socket is a message-based
socket, if a signal is caught, if the connection is terminated,
if MSG_PEEK was specified, or if an error is pending for the socket.
.RE
.sp
.TP 7
\fIaddress\fP
A null pointer, or points to a \fBsockaddr\fP structure in which the
sending address is to be stored. The length and format of
the address depend on the address family of the socket.
.TP 7
\fIaddress_len\fP
Specifies the length of the \fBsockaddr\fP structure pointed to by
the \fIaddress\fP argument.
.sp
.LP
The \fIrecvfrom\fP() function shall return the length of the message
written to the buffer pointed to by the \fIbuffer\fP
argument. For message-based sockets, such as  SOCK_RAW,  SOCK_DGRAM,
and SOCK_SEQPACKET, the entire message shall be read in a single operation.
If a message is too long to fit
in the supplied buffer, and MSG_PEEK is not set in the \fIflags\fP
argument, the excess bytes shall be discarded. For stream-based
sockets, such as SOCK_STREAM, message boundaries shall be ignored.
In this case, data shall be returned to the user as soon as it
becomes available, and no data shall be discarded.
.LP
If the MSG_WAITALL flag is not set, data shall be returned only up
to the end of the first message.
.LP
Not all protocols provide the source address for messages. If the
\fIaddress\fP argument is not a null pointer and the protocol
provides the source address of messages, the source address of the
received message shall be stored in the \fBsockaddr\fP
structure pointed to by the \fIaddress\fP argument, and the length
of this address shall be stored in the object pointed to by the
\fIaddress_len\fP argument.
.LP
If the actual length of the address is greater than the length of
the supplied \fBsockaddr\fP structure, the stored address
shall be truncated.
.LP
If the \fIaddress\fP argument is not a null pointer and the protocol
does not provide the source address of messages, the value
stored in the object pointed to by \fIaddress\fP is unspecified.
.LP
If no messages are available at the socket and O_NONBLOCK is not set
on the socket's file descriptor, \fIrecvfrom\fP() shall
block until a message arrives. If no messages are available at the
socket and O_NONBLOCK is set on the socket's file descriptor,
\fIrecvfrom\fP() shall fail and set \fIerrno\fP to [EAGAIN] or [EWOULDBLOCK].
.SH RETURN VALUE
.LP
Upon successful completion, \fIrecvfrom\fP() shall return the length
of the message in bytes. If no messages are available to
be received and the peer has performed an orderly shutdown, \fIrecvfrom\fP()
shall return 0. Otherwise, the function shall return
-1 and set \fIerrno\fP to indicate the error.
.SH ERRORS
.LP
The \fIrecvfrom\fP() function shall fail if:
.TP 7
.B EAGAIN \fRor\fP EWOULDBLOCK
.sp
The socket's file descriptor is marked O_NONBLOCK and no data is waiting
to be received; or MSG_OOB is set and no out-of-band data
is available and either the socket's file descriptor is marked O_NONBLOCK
or the socket does not support blocking to await
out-of-band data.
.TP 7
.B EBADF
The \fIsocket\fP argument is not a valid file descriptor.
.TP 7
.B ECONNRESET
A connection was forcibly closed by a peer.
.TP 7
.B EINTR
A signal interrupted \fIrecvfrom\fP() before any data was available.
.TP 7
.B EINVAL
The MSG_OOB flag is set and no out-of-band data is available.
.TP 7
.B ENOTCONN
A receive is attempted on a connection-mode socket that is not connected.
.TP 7
.B ENOTSOCK
The \fIsocket\fP argument does not refer to a socket.
.TP 7
.B EOPNOTSUPP
The specified flags are not supported for this socket type.
.TP 7
.B ETIMEDOUT
The connection timed out during connection establishment, or due to
a transmission timeout on active connection.
.sp
.LP
The \fIrecvfrom\fP() function may fail if:
.TP 7
.B EIO
An I/O error occurred while reading from or writing to the file system.
.TP 7
.B ENOBUFS
Insufficient resources were available in the system to perform the
operation.
.TP 7
.B ENOMEM
Insufficient memory was available to fulfill the request.
.sp
.LP
\fIThe following sections are informative.\fP
.SH EXAMPLES
.LP
None.
.SH APPLICATION USAGE
.LP
The \fIselect\fP() and \fIpoll\fP() functions can
be used to determine when data is available to be received.
.SH RATIONALE
.LP
None.
.SH FUTURE DIRECTIONS
.LP
None.
.SH SEE ALSO
.LP
\fIpoll\fP(), \fIread\fP(), \fIrecv\fP(), \fIrecvmsg\fP(), \fIselect\fP(),
\fIsend\fP(), \fIsendmsg\fP(), \fIsendto\fP(), \fIshutdown\fP()
,
\fIsocket\fP(), \fIwrite\fP(), the Base Definitions volume of
IEEE\ Std\ 1003.1-2001, \fI<sys/socket.h>\fP
.SH COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 6, Copyright (C) 2001-2003 by the Institute of
Electrical and Electronics Engineers, Inc and The Open Group. In the
event of any discrepancy between this version and the original IEEE and
The Open Group Standard, the original IEEE and The Open Group Standard
is the referee document. The original Standard can be obtained online at
http://www.opengroup.org/unix/online.html .