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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 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  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(),  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  Electrical  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.kernel.org/doc/man-pages/reporting_bugs.html .