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RECVFROM(P)			    POSIX Programmer's Manual			      RECVFROM(P)

NAME
       recvfrom - receive a message from a socket

SYNOPSIS
       #include <sys/socket.h>

       ssize_t recvfrom(int socket, void *restrict buffer, size_t length,
	      int flags, struct sockaddr *restrict address,
	      socklen_t *restrict address_len);

DESCRIPTION
       The  recvfrom() 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.

       The recvfrom() function takes the following arguments:

       socket Specifies the socket file descriptor.

       buffer Points to the buffer where the message should be stored.

       length Specifies the length in bytes of the buffer pointed to by the buffer argument.

       flags  Specifies the type of message reception. Values of this argument are formed by log-
	      ically OR'ing zero or more of the following values:

       MSG_PEEK
	      Peeks at an incoming message. The data is treated as unread and the next recvfrom()
	      or similar function shall still return this data.

       MSG_OOB
	      Requests	out-of-band  data. The significance and semantics of out-of-band data are
	      protocol-specific.

       MSG_WAITALL
	      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  ter-
	      minated, if MSG_PEEK was specified, or if an error is pending for the socket.

       address
	      A  null  pointer, or points to a sockaddr 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.

       address_len
	      Specifies the length of the sockaddr structure pointed to by the address argument.

       The  recvfrom()	function  shall  return  the  length of the message written to the buffer
       pointed to by the  buffer  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
       flags  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.

       If the MSG_WAITALL flag is not set, data shall be returned only up to the end of the first
       message.

       Not all protocols provide the source address for messages. If the address 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 sockaddr structure  pointed  to  by
       the address argument, and the length of this address shall be stored in the object pointed
       to by the address_len argument.

       If the actual length of the address is greater than the length of  the  supplied  sockaddr
       structure, the stored address shall be truncated.

       If the address 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 address is unspecified.

       If no messages are available at the socket and O_NONBLOCK is not set on the socket's  file
       descriptor,  recvfrom()	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, recvfrom() shall fail
       and set errno to [EAGAIN] or [EWOULDBLOCK].

RETURN VALUE
       Upon successful completion, recvfrom() 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,
       recvfrom()  shall return 0. Otherwise, the function shall return -1 and set errno to indi-
       cate the error.

ERRORS
       The recvfrom() function shall fail if:

       EAGAIN or EWOULDBLOCK

	      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 block-
	      ing to await out-of-band data.

       EBADF  The socket argument is not a valid file descriptor.

       ECONNRESET
	      A connection was forcibly closed by a peer.

       EINTR  A signal interrupted recvfrom() before any data was available.

       EINVAL The MSG_OOB flag is set and no out-of-band data is available.

       ENOTCONN
	      A receive is attempted on a connection-mode socket that is not connected.

       ENOTSOCK
	      The socket argument does not refer to a socket.

       EOPNOTSUPP
	      The specified flags are not supported for this socket type.

       ETIMEDOUT
	      The  connection timed out during connection establishment, or due to a transmission
	      timeout on active connection.

       The recvfrom() function may fail if:

       EIO    An I/O error occurred while reading from or writing to the file system.

       ENOBUFS
	      Insufficient resources were available in the system to perform the operation.

       ENOMEM Insufficient memory was available to fulfill the request.

       The following sections are informative.

EXAMPLES
       None.

APPLICATION USAGE
       The select() and poll() functions can be used to determine when data is	available  to  be
       received.

RATIONALE
       None.

FUTURE DIRECTIONS
       None.

SEE ALSO
       poll()  ,  read()  , recv() , recvmsg() , select() , send() , sendmsg() , sendto() , shut-
       down() , socket() ,  write()  ,	the  Base  Definitions	volume	of  IEEE Std 1003.1-2001,
       <sys/socket.h>

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 orig-
       inal Standard can be obtained online at http://www.opengroup.org/unix/online.html .

IEEE/The Open Group			       2003				      RECVFROM(P)
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