pipe

PIPE(3P)                   POSIX Programmer's Manual                  PIPE(3P)



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
       pipe — create an interprocess channel

SYNOPSIS
       #include <unistd.h>

       int pipe(int fildes[2]);

DESCRIPTION
       The pipe() function shall create a pipe and place two file descriptors,
       one each into the arguments fildes[0] and fildes[1], that refer to the
       open file descriptions for the read and write ends of the pipe. Their
       integer values shall be the two lowest available at the time of the
       pipe() call. The O_NONBLOCK and FD_CLOEXEC flags shall be clear on both
       file descriptors. (The fcntl() function can be used to set both these
       flags.)

       Data can be written to the file descriptor fildes[1] and read from the
       file descriptor fildes[0].  A read on the file descriptor fildes[0]
       shall access data written to the file descriptor fildes[1] on a first-
       in-first-out basis. It is unspecified whether fildes[0] is also open
       for writing and whether fildes[1] is also open for reading.

       A process has the pipe open for reading (correspondingly writing) if it
       has a file descriptor open that refers to the read end, fildes[0]
       (write end, fildes[1]).

       The pipe's user ID shall be set to the effective user ID of the calling
       process.

       The pipe's group ID shall be set to the effective group ID of the
       calling process.

       Upon successful completion, pipe() shall mark for update the last data
       access, last data modification, and last file status change timestamps
       of the pipe.

RETURN VALUE
       Upon successful completion, 0 shall be returned; otherwise, −1 shall be
       returned and errno set to indicate the error.

ERRORS
       The pipe() function shall fail if:

       EMFILE All, or all but one, of the file descriptors available to the
              process are currently open.

       ENFILE The number of simultaneously open files in the system would
              exceed a system-imposed limit.

       The following sections are informative.

EXAMPLES
   Using a Pipe to Pass Data Between a Parent Process and a Child Process
       The following example demonstrates the use of a pipe to transfer data
       between a parent process and a child process. Error handling is
       excluded, but otherwise this code demonstrates good practice when using
       pipes: after the fork() the two processes close the unused ends of the
       pipe before they commence transferring data.

           #include <stdlib.h>
           #include <unistd.h>
           ...

           int fildes[2];
           const int BSIZE = 100;
           char buf[BSIZE];
           ssize_t nbytes;
           int status;

           status = pipe(fildes);
           if (status == −1 ) {
               /* an error occurred */
               ...
           }

           switch (fork()) {
           case −1: /* Handle error */
               break;

           case 0:  /* Child - reads from pipe */
               close(fildes[1]);                       /* Write end is unused */
               nbytes = read(fildes[0], buf, BSIZE);   /* Get data from pipe */
               /* At this point, a further read would see end of file ... */
               close(fildes[0]);                       /* Finished with pipe */
               exit(EXIT_SUCCESS);

           default:  /* Parent - writes to pipe */
               close(fildes[0]);                       /* Read end is unused */
               write(fildes[1], "Hello world\n", 12);  /* Write data on pipe */
               close(fildes[1]);                       /* Child will see EOF */
               exit(EXIT_SUCCESS);
           }

APPLICATION USAGE
       None.

RATIONALE
       The wording carefully avoids using the verb ``to open'' in order to
       avoid any implication of use of open(); see also write().

FUTURE DIRECTIONS
       None.

SEE ALSO
       fcntl(), read(), write()

       The Base Definitions volume of POSIX.1‐2008, <fcntl.h>, <unistd.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 .



IEEE/The Open Group                  2013                             PIPE(3P)