p1.c

#include <sys/wait.h>
#include "apue.h"

#define DEF_PAGER "/bin/more"

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) {
    close(fd[0]);

    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]);

    if (waitpid(pid, NULL, 0) < 0) {
      err_sys("waitpid error");
    }
    exit(0);
  } else {
    close(fd[1]);
    if (fd[0] != STDIN_FILENO) {
      if (dup2(fd[0], STDIN_FILENO) != STDIN_FILENO) {
        err_sys("dup2 error to stdin");
      }
      close(fd[0]);
    }

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

    if (execl(pager, argv0, (char *)0) < 0) {
      err_sys("excel error for %s", pager);
    }
  }
  exit(0);
}