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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. 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 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, no file descriptors shall be allocated and the contents of fildes shall be left
unmodified.
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
Section 2.14, File Descriptor Allocation, fcntl(), read(), write()
The Base Definitions volume of POSIX.1‐2017, <fcntl.h>, <unistd.h>
COPYRIGHT
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 .