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162 | // !> A pipe between two programs [fv]
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <unistd.h>
#include <errno.h>
#include <wait.h>
#include <stdio.h>
#include <assert.h>
int main(int argc, char **argv) {
/* Open files after program starts:
* 0: stdin
* 1: stdout
* 2: stderr
*/
int pipe_fd[2];
// create a pipe:
// pipe_fd[0] is the _read_ end
// pipe_fd[1] is the _write_ end
assert(pipe(pipe_fd) != -1);
/* Open files after pipe call:
* 0: stdin
* 1: stdout
* 2: stderr
* 3: pipe read end (pipe_fd[0])
* 4: pipe write end (pipe_fd[1])
*/
int write_fd = pipe_fd[1];
int read_fd = pipe_fd[0];
printf("read: %d, write: %d\n", read_fd, write_fd);
pid_t reading_child = fork();
assert(reading_child != -1);
if (reading_child == 0) {
/* Open files in reading_child:
* 0: stdin
* 1: stdout
* 2: stderr
* 3: pipe read end (pipe_fd[0] / read_fd)
* 4: pipe write end (pipe_fd[1] / write_fd)
*/
// both parent and child share both ends, so we'll just close the end we
// don't use
close(pipe_fd[1]);
/* Open files after closing write end:
* 0: stdin
* 1: stdout
* 2: stderr
* 3: pipe read end (pipe_fd[0] / read_fd)
* 4: -
*/
// close stdin
close(0);
/* Open files after closing stdin:
* 0: -
* 1: stdout
* 2: stderr
* 3: pipe read end (pipe_fd[0] / read_fd)
* 4: -
*/
// replace fd 0 with pipe_fd[0] / read_fd
assert(dup(read_fd) == 0);
/* Open files after dup:
* 0: pipe read end (pipe_fd[0] / read_fd)
* 1: stdout
* 2: stderr
* 3: pipe read end (pipe_fd[0] / read_fd)
* 4: -
*/
printf("About to run sort...\n");
assert(-1 != execlp("sort", "sort", NULL));
// should never get here
return 1;
}
int writing_child = fork();
assert(writing_child != -1);
if (writing_child == 0) {
/* Open files in writing_child:
* 0: stdin
* 1: stdout
* 2: stderr
* 3: pipe read end (pipe_fd[0] / read_fd)
* 4: pipe write end (pipe_fd[1] / write_fd)
*/
close(read_fd);
/* Open files after closing read end:
* 0: stdin
* 1: stdout
* 2: stderr
* 3: -
* 4: pipe write end (pipe_fd[1] / write_fd)
*/
// close stdout
close(1);
/* Open files after closing stdout:
* 0: stdin
* 1: -
* 2: stderr
* 3: -
* 4: pipe write end (pipe_fd[1] / write_fd)
*/
// replace stsdout with write end of pipe
assert(dup(write_fd) == 1);
/* Open files after dup:
* 0: stdin
* 1: pipe write end (pipe_fd[1] / write_fd)
* 2: stderr
* 3: -
* 4: pipe write end (pipe_fd[1] / write_fd)
*/
// run: shuf -i 1-9
assert(-1 != execlp("shuf", "shuf", "-i", "1-9", NULL));
// should never get here
close(pipe_fd[1]);
close(1);
return 1;
}
/* Open files in parent:
* 0: stdin
* 1: stdout
* 2: stderr
* 3: pipe read end (pipe_fd[0] / read_fd)
* 4: pipe write end (pipe_fd[1] / write_fd)
*/
close(write_fd);
close(read_fd);
/* Open files in parent after closing both ends of the pipe:
* 0: stdin
* 1: stdout
* 2: stderr
* 3: -
* 4: -
*/
// Wait for both children to finish
wait(NULL);
wait(NULL);
return 0;
}
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