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recvfrom(2)
NAME
  recvfrom - Receives messages from sockets

SYNOPSIS
  #include <sys/socket.h>
  ssize_t recvfrom(
	  int socket,
	  void *buffer,
	  size_t length,
	  int flags,
	  struct sockaddr *address,
	  size_t *address_len) ;

  [Digital]  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 standards
  as follows:

  recvfrom():  XPG4-UNIX

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

PARAMETERS
  socket    Specifies the socket file descriptor.

  buffer    Specifies a pointer to the buffer to which the message should be
	    written.

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

  flags	    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:

	    MSG_PEEK  Peeks at the incoming message.

	    MSG_OOB   Processes out-of-band data.

	    MSG_WAITALL
		      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, or an error is
		      pending for the socket.

  address   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 or sockaddr_un
	    structure, depending on which of the supported address families
	    is active.

	    [Digital] 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 specifying 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.

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

DESCRIPTION
  The recvfrom() function permits an application program to receive messages
  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].

NOTES
  [Digital]  When compiled in the X/Open UNIX 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 has been defined, use _Erecvfrom to
  refer to the recvfrom() call.	 See standards(5) for further information.

RETURN VALUES
  Upon successful completion, the byte length of the written message is
  returned.  Otherwise, a value of -1 is returned and errno is set to
  indicate the error.


ERRORS
  If the recvfrom() function fails, errno may be set to one of the following
  values:

  [EBADF]   The socket parameter is not a valid file descriptor.

  [ECONNRESET]
	    A connection was forcibly closed by a peer.

  [EFAULT]  A valid message buffer was not specified.  Nonexistent or
	    protected address space is specified for the message buffer.

  [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-oriented socket that is
	    not connected.

  [ENOTSOCK]
	    The socket parameter refers to a file, not a socket.

  [EOPNOTSUPP]
	    The specified flags are not supported this socket type.

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

  [EWOULDBLOCK]
	    The socket is nonblocking; no data is ready to be received.

RELATED INFORMATION
  Functions: recv(2), recvmsg(2), send(2), sendmsg(2), sendto(2), select(2),
  shutdown(2), socket(2), read(2), write(2)

  Standards: standards(5)