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RECVFROM(3P)		   POSIX Programmer's Manual		  RECVFROM(3P)

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.

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 logically 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-spe‐
			       cific.

		   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 mes‐
			       sage-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.

       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 Either  a  null pointer, if address is a null pointer, or a
		   pointer to a socklen_t object which on input specifies  the
		   length  of  the  supplied sockaddr structure, and on output
		   specifies the length of the stored address.

       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 sock‐
       ets, such as SOCK_STREAM, message boundaries shall be ignored. In  this
       case,  data  shall be returned to the user as soon as it becomes avail‐
       able, 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. Oth‐
       erwise, the function shall return −1 and	 set  errno  to	 indicate  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  blocking  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(),	pselect(),  read(),  recv(),  recvmsg(),  send(),   sendmsg(),
       sendto(), shutdown(), socket(), write()

       The Base Definitions volume of POSIX.1‐2008, <sys_socket.h>

COPYRIGHT
       Portions	 of  this text are reprinted and reproduced in electronic form
       from IEEE Std 1003.1, 2013 Edition, Standard for Information Technology
       --  Portable  Operating	System	Interface (POSIX), The Open Group Base
       Specifications Issue 7, Copyright (C) 2013 by the Institute of Electri‐
       cal  and	 Electronics  Engineers,  Inc  and  The	 Open Group.  (This is
       POSIX.1-2008 with the 2013 Technical Corrigendum	 1  applied.)  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.unix.org/online.html .

       Any typographical or formatting errors that appear  in  this  page  are
       most likely to have been introduced during the conversion of the source
       files to man page format. To report such errors,	 see  https://www.ker‐
       nel.org/doc/man-pages/reporting_bugs.html .

IEEE/The Open Group		     2013			  RECVFROM(3P)
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