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recvfrom(2)							   recvfrom(2)

NAME
       recvfrom - Receive messages from sockets

SYNOPSIS
       #include <sys/socket.h>

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

       [XNS4.0]	 The  definition  of  the recvfrom() function in XNS4.0 uses a
       size_t data type for the address_len parameter instead of  a  socklen_t
       data type as specified in XNS5.0 (the previous definition).

       [Tru64  UNIX]  The following definition of the recvfrom() function does
       not conform to current standards and is	supported  only	 for  backward
       compatibility (see standards(5)): int recvfrom(
	       int socket,
	       char *buffer,
	       int length,
	       int flags,
	       struct sockaddr *address,
	       int *address_len );

STANDARDS
       Interfaces  documented on this reference page conform to industry stan‐
       dards as follows:

       recvfrom(): XNS5.0

       Refer to the standards(5) reference page	 for  more  information	 about
       industry standards and associated tags.

PARAMETERS
       Specifies  the socket file descriptor.  Specifies a pointer to the buf‐
       fer to which the message should be written.  Specifies the  length  (in
       bytes)  of  the buffer pointed to by the buffer parameter.  Points to a
       value that controls message reception. The parameter to control message
       reception  is  formed by the logical OR of one or more of the following
       values: Peeks at the incoming  message.	 Processes  out-of-band	 data.
       Requests	 that  the  function  block wait until the full amount of data
       requested can be returned. The function may return a smaller amount  of
       data  if a signal is caught, the connection is terminated, MSG_PEEK was
       specified, or an error is pending for the socket.  Points to a sockaddr
       structure,  the	format of which is determined by the domain and by the
       behavior requested for the socket. The sockaddr structure is an overlay
       for  a  sockaddr_in,  sockaddr_un,  sockaddr_in6,  or  sockaddr_storage
       structure, depending on which of	 the  supported	 address  families  is
       active.

	      [Tru64  UNIX]    If  the	compile-time  option  _SOCKADDR_LEN is
	      defined before the sys/socket.h header  file  is	included,  the
	      sockaddr structure takes 4.4BSD behavior, with a field for spec‐
	      ifying the length of the socket address. Otherwise, the  default
	      4.3BSD sockaddr structure is used, with the length of the socket
	      address assumed to be 14 bytes or less.

	      If _SOCKADDR_LEN is defined, the 4.3BSD  sockaddr	 structure  is
	      defined  with  the  name osockaddr.  Specifies the length of the
	      sockaddr structure pointed to by the address parameter.

DESCRIPTION
       The recvfrom() function permits an application program to receive  mes‐
       sages  from  unconnected sockets. It is normally applied to unconnected
       sockets because it includes parameters that permit a calling program to
       retrieve the source endpoint of received data.

       To  obtain  the	source address of the message, specify a nonzero value
       for the address parameter. The recvfrom() function is called  with  the
       address_len  parameter  set  to the size of the buffer specified by the
       address parameter. On return, this function  modifies  the  address_len
       parameter  to  the actual size in bytes of the address specified by the
       address parameter. The recvfrom() function returns the  length  of  the
       message	written to the buffer pointed to by the buffer parameter. When
       a message is too long for the specified buffer,	excess	bytes  may  be
       truncated  depending on the type of socket that issued the message, and
       depending on which flags are set with the flags parameter.

       When no message is available at the  socket  specified  by  the	socket
       parameter,  the	recvfrom()  function  waits  for  a message to arrive,
       unless the socket is nonblocking. When the socket is nonblocking, errno
       is set to [EWOULDBLOCK].

       Use  the	 select()  and	poll()	functions  to determine when more data
       arrives.

NOTES
       [Tru64 UNIX]  When compiled in  the  X/Open  UNIX  environment  or  the
       POSIX.1g	 socket	 environment,  calls  to  the  recvfrom() function are
       internally renamed by prepending _E to the function name. When you  are
       debugging  a module that includes the recvfrom() function and for which
       _XOPEN_SOURCE_EXTENDED  or  _POSIX_PII_SOCKET  has  been	 defined,  use
       _Erecvfrom  to refer to the recvfrom() call.  See standards(5) for fur‐
       ther information.

RETURN VALUES
       Upon successful completion, the byte length of the written  message  is
       returned. If no messages are available and the peer has closed the con‐
       nection, the recv() function returns a value of 0. Otherwise, the func‐
       tion returns a value of -1 and sets errno to indicate the error.

ERRORS
       If  the	recvfrom() function fails, errno may be set to one of the fol‐
       lowing values: The socket parameter is not a valid file descriptor.   A
       connection  was	forcibly  closed  by a peer.  Nonexistent or protected
       address space is specified for the message buffer, sending address,  or
       address length.

	      [Tru64  UNIX]  A valid message buffer was not specified.	A sig‐
	      nal interrupted recvfrom before any  data	 was  available.   The
	      MSG-OOB  flag  is	 set and no out-of-band data is available.  An
	      I/O error occurred while reading from or	writing	 to  the  file
	      system.	Insufficient  resources are available in the system to
	      complete the call.  The system did not have sufficient memory to
	      fulfill  the  request.   The  available  STREAMS	resources were
	      insufficient for	the  operation	to  complete.	A  receive  is
	      attempted on a connection-oriented socket that is not connected.
	      The socket parameter refers to a file, not a socket.  The speci‐
	      fied  flags  are not supported this socket type.	The connection
	      timed out during connection establishment, or due to a transmis‐
	      sion  timeout  on active connection.  The socket is nonblocking;
	      no data is ready to be received.

	      The MSG_OOB flag is set, no out-of-band data is  available,  and
	      either the socket is nonblocking or does not support blocking to
	      await out-of-band data.

SEE ALSO
       Functions:   read(2),   recv(2),	  recvmsg(2),	select(2),    send(2),
       sendmsg(2), sendto(2), shutdown(2), socket(2), write(2).

       Standards: standards(5).

       Network Programmer's Guide

								   recvfrom(2)
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