#include "apue.h"#include <sys/wait.h>#define	DEF_PAGER	"/bin/more"		/* default pager program */intmain(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);}