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 (con-
       sult 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 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
              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 sin-
       gle  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 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  Defi-
       nitions 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,  Copy-
       right (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.open-
       group.org/unix/online.html .



IEEE/The Open Group                                           2003                                                  RECVFROM(3P)