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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, errno is set  appropriately,  and
       sv is left unchanged

       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 domains for this call are AF_UNIX (or synonymously, AF_LOCAL)
       and AF_TIPC (since Linux 4.12).

       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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