The primitive for creating a pipe is the pipe
function. This
creates both the reading and writing ends of the pipe. It is not very
useful for a single process to use a pipe to talk to itself. In typical
use, a process creates a pipe just before it forks one or more child
processes (see Creating a Process). The pipe is then used for
communication either between the parent or child processes, or between
two sibling processes.
The pipe
function is declared in the header file
unistd.h.
Preliminary: | MT-Safe | AS-Safe | AC-Safe fd | See POSIX Safety Concepts.
The
pipe
function creates a pipe and puts the file descriptors for the reading and writing ends of the pipe (respectively) into filedes[0]
and filedes[1]
.An easy way to remember that the input end comes first is that file descriptor
0
is standard input, and file descriptor1
is standard output.If successful,
pipe
returns a value of0
. On failure,-1
is returned. The followingerrno
error conditions are defined for this function:
EMFILE
- The process has too many files open.
ENFILE
- There are too many open files in the entire system. See Error Codes, for more information about
ENFILE
. This error never occurs on GNU/Hurd systems.
Here is an example of a simple program that creates a pipe. This program
uses the fork
function (see Creating a Process) to create
a child process. The parent process writes data to the pipe, which is
read by the child process.
#include <sys/types.h>
#include <unistd.h>
#include <stdio.h>
#include <stdlib.h>
/* Read characters from the pipe and echo them to stdout
. */
void
read_from_pipe (int file)
{
FILE *stream;
int c;
stream = fdopen (file, "r");
while ((c = fgetc (stream)) != EOF)
putchar (c);
fclose (stream);
}
/* Write some random text to the pipe. */
void
write_to_pipe (int file)
{
FILE *stream;
stream = fdopen (file, "w");
fprintf (stream, "hello, world!\n");
fprintf (stream, "goodbye, world!\n");
fclose (stream);
}
int
main (void)
{
pid_t pid;
int mypipe[2];
/* Create the pipe. */
if (pipe (mypipe))
{
fprintf (stderr, "Pipe failed.\n");
return EXIT_FAILURE;
}
/* Create the child process. */
pid = fork ();
if (pid == (pid_t) 0)
{
/* This is the child process.
Close other end first. */
close (mypipe[1]);
read_from_pipe (mypipe[0]);
return EXIT_SUCCESS;
}
else if (pid < (pid_t) 0)
{
/* The fork failed. */
fprintf (stderr, "Fork failed.\n");
return EXIT_FAILURE;
}
else
{
/* This is the parent process.
Close other end first. */
close (mypipe[0]);
write_to_pipe (mypipe[1]);
return EXIT_SUCCESS;
}
}