15.2.c

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

#define DEF_PAGER "/bin/more"
#define MAXLINE 1024

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

    if (2 != argc) {
        printf("usage: a.out <pathname>\n");
        exit(-1);
    }

    if ((fp = fopen(argv[1], "r")) == NULL) {
        printf("can't open %s\n", argv[1]);
        exit(-1);
    }

    if (pipe(fd) < 0) {
        printf("pipe error\n");
        exit(-1);
    }

    if ((pid = fork()) < 0) {
        printf("fork error\n");
        exit(-1);
    } else if (pid > 0) {
        close(fd[0]);

        while (fgets(line, MAXLINE, fp) != NULL) {
            n = strlen(line);
            if (write(fd[1], line, n) != n) {
                printf("write error to pipe\n");
                exit(-1);
            }
        }

        if (ferror(fp)) {
            printf("fgets error\n");
            exit(-1);
        }

        close(fd[1]);

        if (waitpid(pid, NULL, 0) < 0) {
            printf("waitpid error\n");
            exit(-1);
        }

        return 0;
    } else {
        close(fd[1]);

        if (fd[0] != STDIN_FILENO) {
            if (dup2(fd[0], STDIN_FILENO) != STDIN_FILENO) {
                printf("dup2 error to stdin\n");
                close(fd[0]);
                exit(-1);
            }
        }

        if ((pager = getenv("PAGER")) == NULL) {
            pager = DEF_PAGER;
        }

        if ((argv0 = strrchr(pager, "/")) != NULL) {
            argv0++;
        } else {
            argv0 = pager;
        }

        if (execl(pager, argv0, (char *)0) < 0) {
            printf("execl error for %s\n", pager);
            exit(-1);
        }
    }

    return 0;
}