io day 4

 两个子进程 copy 文件，每个进程copy一半   

#include "head.h"

int main(int argc, const char *argv[])
{
  int flag = 0;
  off_t size = 0;
  char buf;

  int fd_r = open("./a.txt", O_RDONLY);
  if (fd_r < 0)
  {
    perror("fd_r error\n");
    return -1;
  }
  int fd_w = open("./b.txt", O_RDWR | O_TRUNC | O_CREAT, 0776);
  if (fd_w < 0)
  {
    perror("fd_w error\n");
    return -1;
  }
  size = lseek(fd_r, 0, SEEK_END);

  //创建父子进程
  int pid = fork();
  if (pid < 0)
  {
    perror("fork error\n");
    return -1;
  }
  else if (pid > 0)
  {
    //父进程
    int pid_f = fork();
    if (pid_f > 0)
    {
      waitpid(pid, NULL, 0);
      waitpid(pid_f, NULL, 0);
    }
    else if (pid_f == 0)
    {
      //父:子进程
      lseek(fd_r, 0, SEEK_SET);
      lseek(fd_w, 0, SEEK_SET);

      for (int i = 0; i < size / 2; i++)
      {
        int ret_r = read(fd_r, &buf, 1);
        if (ret_r < 0)
        {
          perror("read error\n");
          return -1;
        }
        int ret_w = write(fd_w, &buf, 1);
        if (ret_w < 0)
        {
          perror("write error\n");
          return -1;
        }
      }
      printf("前半段拷贝完成\n");
    }
    else if (pid_f < 0)
    {
      perror("fork_f error\n");
      return -1;
    }
  }
  else if (pid == 0)
  {
    //子进程
    sleep(2);
    lseek(fd_r, size / 2, SEEK_SET);
    lseek(fd_w, size / 2, SEEK_SET);
    for (int i = size / 2; i < size; i++)
    {
      int ret_r = read(fd_r, &buf, 1);
      if (ret_r < 0)
      {
        perror("read error\n");
        return -1;
      }
      int ret_w = write(fd_w, &buf, 1);
      if (ret_w < 0)
      {
        perror("write error\n");
        return -1;
      }
    }
    printf("后半段拷贝完成\n");
  }

  close(fd_r);
  close(fd_w);
  return 0;
}