/*
 * pager.c -- modified from APUE textbook
 */

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

#define MAXLINE 256

#define DEF_PAGER   "/bin/less"     /* default pager program */

void err_quit(char *msg)
{
   fprintf(stderr, "%s\n", msg);
   exit(-1);
}

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

    if (argc != 2)
        err_quit("usage: <progname> <pathname>");

    if ((fp = fopen(argv[1], "r")) == NULL)
        err_quit("can't open input file.");
    if (pipe(fd) < 0)
        err_quit("pipe error");

    if ((pid = fork()) < 0) {
        err_quit("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_quit("write error to pipe");
        }
        if (ferror(fp))
            err_quit("fgets error");

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

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

        for ( i = 0; i < 3; i++ ) {
           fprintf(stdout, "Parent can continue here ...\n");
           fflush(stdout);
           sleep(1);
        }

        exit(0);
    } else {                                        /* child */
        close(fd[1]);   /* close write end */
        if (fd[0] != STDIN_FILENO) {
            if (dup2(fd[0], STDIN_FILENO) != STDIN_FILENO)
                err_quit("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_quit("execl error for pager");
    }
    exit(0);
}