mypipe.c

#include <signal.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/wait.h>
#include <unistd.h>

#define SHELL	"/bin/sh"

extern int	main(int, char*[]);
static int	readingend(int, int, const char*);
static int	writingend(int, int, const char*);
static int	usr1handler();

extern int
main(int argc, char *argv[])
{
	int pipefd[2], status;
	pid_t rpid, wpid;

	if (argc != 3) {
		fprintf(stderr, "Invalid argument count\n");
		return (EXIT_FAILURE);
	}

	if (pipe(pipefd)) {
		perror("Cannot make a pipe");
		return (EXIT_FAILURE);
	}

	signal(SIGUSR1, (void(*)(int))usr1handler);

	if ((rpid = fork()) < 0) {
		perror("Cannot fork");
		return (EXIT_FAILURE);
	}

	if (rpid == 0)
		return (readingend(pipefd[0], pipefd[1], argv[2]));


	if ((wpid = fork()) < 0) {
		perror("Cannot fork");
		kill(rpid, SIGKILL);
		return (EXIT_FAILURE);
	}

	if (wpid == 0)
		return (writingend(pipefd[0], pipefd[1], argv[1]));

	close(pipefd[0]);
	close(pipefd[1]);

	waitpid(rpid, &status, 0);
	return (status);
}

static int
readingend(int rfd, int wfd, const char *cmd)
{
	close(wfd);
	dup2(rfd, STDIN_FILENO);

	/* execl() only returns on error */
	execl(SHELL, SHELL, "-c", cmd, NULL);
	fprintf(stderr, "Cannot execute %s: ", cmd);
	perror(NULL);

	/* clean up */
	kill(getppid(), SIGUSR1);
	return (EXIT_FAILURE);
}

static int
writingend(int rfd, int wfd, const char *cmd)
{
	close(rfd);
	dup2(wfd, STDOUT_FILENO);

	execl(SHELL, SHELL, "-c", cmd, NULL);
	fprintf(stderr, "Cannot execute %s: ", cmd);
	perror(NULL);

	/* clean up */
	kill(getppid(), SIGUSR1);
	return (EXIT_FAILURE);
}

static int
usr1handler()
{
	/* child already printed diagnostic */
	exit(EXIT_FAILURE);
}