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     pipe, pipe2 — create descriptor pair for interprocess communication


     Standard C Library (libc, -lc)


     #include <unistd.h>

     pipe(int fildes[2]);

     pipe2(int fildes[2], int flags);


     The pipe() system call creates a pipe, which is an object allowing bidirectional data flow,
     and allocates a pair of file descriptors.

     The pipe2() system call allows control over the attributes of the file descriptors via the
     flags argument.  Values for flags are constructed by a bitwise-inclusive OR of flags from
     the following list, defined in <fcntl.h>:

     O_CLOEXEC   Set the close-on-exec flag for the new file descriptors.

     O_NONBLOCK  Set the non-blocking flag for the ends of the pipe.

     If the flags argument is 0, the behavior is identical to a call to pipe().

     By convention, the first descriptor is normally used as the read end of the pipe, and the
     second is normally the write end, so that data written to fildes[1] appears on (i.e., can be
     read from) fildes[0].  This allows the output of one program to be sent to another program:
     the source's standard output is set up to be the write end of the pipe, and the sink's
     standard input is set up to be the read end of the pipe.  The pipe itself persists until all
     its associated descriptors are closed.

     A pipe that has had an end closed is considered widowed.  Writing on such a pipe causes the
     writing process to receive a SIGPIPE signal.  Widowing a pipe is the only way to deliver
     end-of-file to a reader: after the reader consumes any buffered data, reading a widowed pipe
     returns a zero count.

     The bidirectional nature of this implementation of pipes is not portable to older systems,
     so it is recommended to use the convention for using the endpoints in the traditional manner
     when using a pipe in one direction.


     The pipe() function returns the value 0 if successful; otherwise the value -1 is returned
     and the global variable errno is set to indicate the error.


     The pipe() and pipe2() system calls will fail if:

     [EMFILE]           Too many descriptors are active.

     [ENFILE]           The system file table is full.

     [ENOMEM]           Not enough kernel memory to establish a pipe.

     The pipe2() system call will also fail if:

     [EINVAL]           The flags argument is invalid.


     sh(1), fork(2), read(2), socketpair(2), write(2)


     The pipe() function appeared in Version 3 AT&T UNIX.

     Bidirectional pipes were first used on AT&T System V Release 4 UNIX.

     The pipe2() function appeared in FreeBSD 10.0.