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

#define err_sys(fmt, ...) fprintf(stderr, (fmt "\n"), ##__VA_ARGS__)
#define err_quit(fmt, ...) do { err_sys(fmt, ##__VA_ARGS__); exit(1); } while(0)

#define MAXLINE     4096
#define DEF_PAGER   "/bin/more"     /* default pager program */

int
main(int argc, char *argv[])
{
	int     n;
	int     fd[2];
	pid_t   pid;
	char    *pager, *argv0;
	char    line[MAXLINE];
	FILE    *fp;

	if (argc != 2)
		err_quit("usage: a.out <pathname>");

	if ((fp = fopen(argv[1], "r")) == NULL)
		err_sys("can’t open %s", argv[1]);

	if (pipe(fd) < 0)
		err_sys("pipe error");

	if ((pid = fork()) < 0) {
		err_sys("fork error");
	} else if (pid > 0) {                               /* parent */
		close(fd[0]);       /* close read end */
		/* parent copies argv[1] to pipe */
		while (fgets(line, MAXLINE, fp) != NULL) {
			n = strlen(line);
			if (write(fd[1], line, n) != n)
				err_sys("write error to pipe");
		}
		if (ferror(fp))
			err_sys("fgets error");

		close(fd[1]);   /* close write end of pipe for reader */

		if (waitpid(pid, NULL, 0) < 0)
			err_sys("waitpid error");

		exit(0);
	} else {                                          /* child */
		close(fd[1]); /* close write end */
		if (fd[0] != STDIN_FILENO) {
			if (dup2(fd[0], STDIN_FILENO) != STDIN_FILENO)
				err_sys("dup2 error to stdin");
			close(fd[0]);   /* don’t need this after dup2 */
		}

		/* get arguments for execl() */
		if ((pager = getenv("PAGER")) == NULL)
			pager = DEF_PAGER;
		if ((argv0 = strrchr(pager, '/')) != NULL)
			argv0++;        /* step past rightmost slash */
		else
			argv0 = pager;  /* no slash in pager */
		if (execl(pager, argv0, (char *)0) < 0)
			err_sys("execl error for %s", pager);
	}

	exit(0);
}
