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       socketpair - create a pair of connected sockets


       #include <sys/types.h>          /* See NOTES */
       #include <sys/socket.h>

       int socketpair(int domain, int type, int protocol, int sv[2]);


       The  socketpair()  call  creates  an  unnamed  pair  of connected sockets in the specified
       domain, of the specified type, and using the optionally specified protocol.   For  further
       details of these arguments, see socket(2).

       The  file descriptors used in referencing the new sockets are returned in sv[0] and sv[1].
       The two sockets are indistinguishable.


       On success, zero is returned.  On error, -1 is returned, and errno is set appropriately.

       On Linux (and other systems), socketpair() does not modify sv on failure.   A  requirement
       standardizing this behavior was added in POSIX.1-2016.


              The specified address family is not supported on this machine.

       EFAULT The address sv does not specify a valid part of the process address space.

       EMFILE The per-process limit on the number of open file descriptors has been reached.

       ENFILE The system-wide limit on the total number of open files has been reached.

              The specified protocol does not support creation of socket pairs.

              The specified protocol is not supported on this machine.


       POSIX.1-2001,  POSIX.1-2008,  4.4BSD.   socketpair()  first  appeared  in  4.2BSD.   It is
       generally portable to/from non-BSD systems supporting  clones  of  the  BSD  socket  layer
       (including System V variants).


       On  Linux, the only supported domain for this call is AF_UNIX (or synonymously, AF_LOCAL).
       (Most implementations have the same restriction.)

       Since Linux 2.6.27, socketpair() supports the SOCK_NONBLOCK and SOCK_CLOEXEC flags in  the
       type argument, as described in socket(2).

       POSIX.1  does  not  require  the  inclusion  of <sys/types.h>, and this header file is not
       required on Linux.  However, some historical (BSD) implementations  required  this  header
       file, and portable applications are probably wise to include it.


       pipe(2), read(2), socket(2), write(2), socket(7), unix(7)


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