05-27-2010
IPC between processes, pipes, etc
I need help with understanding this in C-programming style(the systemcalls only):
Three processes communicates via two pipes. (when the processes creates all stdin is the keyboard and all stdout is the screen)
This is how the communication goes:
Process 2 stdin (keyboard) and stdout goes via pipe2, this then becomes process 3:s stdin, and process 3:s stdout goes through pipe1, and then becomes Process 1:s stdin, Process 1:s stdout goes to the screen.
What is the system calls from each process(1, 2, 3)?
for example:
close(0);
dup(pipe2[0]);
etc,
(Donīt need to make a close where the need comes from the processes relationships)
Please help out! I just want the correct systemcalls, with the correct pipe and read or write end, so I get this finally :-)
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PIPE(2) Linux Programmer's Manual PIPE(2)
NAME
pipe, pipe2 - create pipe
SYNOPSIS
#include <unistd.h>
int pipe(int pipefd[2]);
#define _GNU_SOURCE
#include <unistd.h>
int pipe2(int pipefd[2], int flags);
DESCRIPTION
pipe() creates a pipe, a unidirectional data channel that can be used for interprocess communication. The array pipefd is used to return
two file descriptors referring to the ends of the pipe. pipefd[0] refers to the read end of the pipe. pipefd[1] refers to the write end
of the pipe. Data written to the write end of the pipe is buffered by the kernel until it is read from the read end of the pipe. For fur-
ther details, see pipe(7).
If flags is 0, then pipe2() is the same as pipe(). The following values can be bitwise ORed in flags to obtain different behavior:
O_NONBLOCK Set the O_NONBLOCK file status flag on the two new open file descriptions. Using this flag saves extra calls to fcntl(2) to
achieve the same result.
O_CLOEXEC Set the close-on-exec (FD_CLOEXEC) flag on the two new file descriptors. See the description of the same flag in open(2) for
reasons why this may be useful.
RETURN VALUE
On success, zero is returned. On error, -1 is returned, and errno is set appropriately.
ERRORS
EFAULT pipefd is not valid.
EINVAL (pipe2()) Invalid value in flags.
EMFILE Too many file descriptors are in use by the process.
ENFILE The system limit on the total number of open files has been reached.
VERSIONS
pipe2() was added to Linux in version 2.6.27; glibc support is available starting with version 2.9.
CONFORMING TO
pipe(): POSIX.1-2001.
pipe2() is Linux-specific.
EXAMPLE
The following program creates a pipe, and then fork(2)s to create a child process; the child inherits a duplicate set of file descriptors
that refer to the same pipe. After the fork(2), each process closes the descriptors that it doesn't need for the pipe (see pipe(7)). The
parent then writes the string contained in the program's command-line argument to the pipe, and the child reads this string a byte at a
time from the pipe and echoes it on standard output.
#include <sys/wait.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
int
main(int argc, char *argv[])
{
int pipefd[2];
pid_t cpid;
char buf;
if (argc != 2) {
fprintf(stderr, "Usage: %s <string>
", argv[0]);
exit(EXIT_FAILURE);
}
if (pipe(pipefd) == -1) {
perror("pipe");
exit(EXIT_FAILURE);
}
cpid = fork();
if (cpid == -1) {
perror("fork");
exit(EXIT_FAILURE);
}
if (cpid == 0) { /* Child reads from pipe */
close(pipefd[1]); /* Close unused write end */
while (read(pipefd[0], &buf, 1) > 0)
write(STDOUT_FILENO, &buf, 1);
write(STDOUT_FILENO, "
", 1);
close(pipefd[0]);
_exit(EXIT_SUCCESS);
} else { /* Parent writes argv[1] to pipe */
close(pipefd[0]); /* Close unused read end */
write(pipefd[1], argv[1], strlen(argv[1]));
close(pipefd[1]); /* Reader will see EOF */
wait(NULL); /* Wait for child */
exit(EXIT_SUCCESS);
}
}
SEE ALSO
fork(2), read(2), socketpair(2), write(2), popen(3), pipe(7)
COLOPHON
This page is part of release 3.25 of the Linux man-pages project. A description of the project, and information about reporting bugs, can
be found at http://www.kernel.org/doc/man-pages/.
Linux 2009-09-15 PIPE(2)