NAME
pipe, pipe2
— create descriptor pair for
interprocess communication
LIBRARY
library “libc”
SYNOPSIS
#include
<unistd.h>
int
pipe(int
fildes[2]);
int
pipe2(int
fildes[2], int
flags);
DESCRIPTION
Thepipe()
function 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.
IMPLEMENTATION NOTES
The pipe() function calls the
pipe2() system call. As a result, system call traces
such as those captured by
dtrace(1) or
ktrace(1) will show calls to pipe2().
RETURN VALUES
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.
ERRORS
The pipe() and
pipe2() system calls will fail if:
- [
EFAULT] - fildes argument points to an invalid memory location.
- [
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.
SEE ALSO
HISTORY
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.
The pipe() function became a wrapper
around pipe2() in FreeBSD
11.0.