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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

       recv — receive a message from a connected socket

SYNOPSIS

       #include <sys/socket.h>

       ssize_t recv(int socket, void *buffer, size_t length, int flags);

DESCRIPTION

       The  recv() function shall receive a message from a connection-mode or connectionless-mode
       socket. It is normally used  with  connected  sockets  because  it  does  not  permit  the
       application to retrieve the source address of received data.

       The recv() function takes the following arguments:

       socket    Specifies the socket file descriptor.

       buffer    Points to a 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 recv() 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.

       The recv() 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_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.

       If  no messages are available at the socket and O_NONBLOCK is not set on the socket's file
       descriptor, recv() 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, recv() shall fail and
       set errno to [EAGAIN] or [EWOULDBLOCK].

RETURN VALUE

       Upon successful completion, recv() 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,
       recv() shall return 0. Otherwise, −1 shall be returned  and  errno  set  to  indicate  the
       error.

ERRORS

       The recv() 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  The recv() function was interrupted by a signal that was caught,  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 or protocol.

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

       The recv() 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 recv() function is equivalent to recvfrom() with null pointer address and  address_len
       arguments,  and to read() if the socket argument refers to a socket and the flags argument
       is 0.

       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(), recvmsg(), recvfrom(), 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 .