pipe(3p) — Linux manual page


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

PROLOG         top

       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         top

       pipe — create an interprocess channel

SYNOPSIS         top

       #include <unistd.h>

       int pipe(int fildes[2]);

DESCRIPTION         top

       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. The file descriptors shall be allocated as
       described in Section 2.14, File Descriptor Allocation.  The
       O_NONBLOCK and FD_CLOEXEC flags shall be clear on both file
       descriptors. (The fcntl() function can be used to set both these

       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         top

       Upon successful completion, 0 shall be returned; otherwise, -1
       shall be returned and errno set to indicate the error, no file
       descriptors shall be allocated and the contents of fildes shall
       be left unmodified.

ERRORS         top

       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         top

   Using a Pipe to Pass Data Between a Parent Process and a Child
       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 */

           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 */

           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 */



RATIONALE         top

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



SEE ALSO         top

       Section 2.14, File Descriptor Allocation, fcntl(3p), read(3p),

       The Base Definitions volume of POSIX.1‐2017, fcntl.h(0p),

COPYRIGHT         top

       Portions of this text are reprinted and reproduced in electronic
       form from IEEE Std 1003.1-2017, Standard for Information
       Technology -- Portable Operating System Interface (POSIX), The
       Open Group Base Specifications Issue 7, 2018 Edition, Copyright
       (C) 2018 by the Institute of Electrical and Electronics
       Engineers, Inc and The Open Group.  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.opengroup.org/unix/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               2017                          PIPE(3P)

Pages that refer to this page: stropts.h(0p)unistd.h(0p)sh(1p)fstatvfs(3p)ioctl(3p)popen(3p)read(3p)system(3p)write(3p)