The Open Group Base Specifications Issue 6
IEEE Std 1003.1, 2004 Edition
Copyright © 2001-2004 The IEEE and The Open Group, All Rights reserved.
A newer edition of this document exists here


pipe - create an interprocess channel


#include <unistd.h>

int pipe(int


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]).

Upon successful completion, pipe() shall mark for update the st_atime, st_ctime, and st_mtime fields of the pipe.


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


The pipe() function shall fail if:

More than {OPEN_MAX} minus two file descriptors are already in use by this process.
The number of simultaneously open files in the system would exceed a system-imposed limit.

The following sections are informative.


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); }




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




fcntl(), read(), write(), the Base Definitions volume of IEEE Std 1003.1-2001, <fcntl.h>, <unistd.h>


First released in Issue 1. Derived from Issue 1 of the SVID.

Issue 6

The following new requirements on POSIX implementations derive from alignment with the Single UNIX Specification:

IEEE Std 1003.1-2001/Cor 2-2004, item XSH/TC2/D6/65 is applied, adding the example to the EXAMPLES section.

End of informative text.

UNIX ® is a registered Trademark of The Open Group.
POSIX ® is a registered Trademark of The IEEE.
[ Main Index | XBD | XCU | XSH | XRAT ]