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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-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 termi-
              nated,  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  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(),   read(),   recv(),  recvmsg(),  select(),  send(),  sendmsg(),
       sendto(), shutdown() , 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 original Standard can be  obtained  online
       at http://www.opengroup.org/unix/online.html .

IEEE/The Open Group                  2003                         RECVFROM(3P)