Question

我想使用fgets而不是fscanf来获取stdin并通过管道将其发送到子进程。下面的代码用于对文件中的行进行排序，但替换

fscanf(stdin, "%s", word)

与

fgets(word, 5000, stdin)

给我警告

warning: comparison between pointer and integer [enabled by default]

否则该程序似乎有效。我有什么想法得到警告吗？

int main(int argc, char *argv[])
{
  pid_t sortPid;
  int status;
  FILE *writeToChild;
  char word[5000];
  int count = 1;

  int sortFds[2];
  pipe(sortFds);

  switch (sortPid = fork()) {
    case 0: //this is the child process
      close(sortFds[1]); //close the write end of the pipe
      dup(sortFds[0]);
      close(sortFds[0]);
      execl("/usr/bin/sort", "sort", (char *) 0);
      perror("execl of sort failed");
      exit(EXIT_FAILURE);
    case -1: //failure to fork case
      perror("Could not create child");
      exit(EXIT_FAILURE);
    default: //this is the parent process
      close(sortFds[0]); //close the read end of the pipe
      writeToChild = fdopen(sortFds[1], "w");
      break;
  }

  if (writeToChild != 0) { //do this if you are the parent
    while (fscanf(stdin, "%s", word) != EOF) {
      fprintf(writeToChild, "%s %d\n",  word, count);
    }   
  }  

  fclose(writeToChild);

  wait(&status);

  return 0;
}

Answer 1

fscanf返回int，fgets为char *。由于EOF为char *，因此与EOF的比较会导致int发出警告。

fgets在EOF或错误时返回NULL，因此请检查。

Answer 2

fgets的原型是：

char * fgets（char * str，int num，FILE * stream）;

fgets会将换行符读入你的字符串，所以如果你使用它，你的部分代码可能写成：

if (writeToChild != 0){
    while (fgets(word, sizeof(word), stdin) != NULL){
        count = strlen(word);
        word[--count] = '\0'; //discard the newline character 
        fprintf(writeToChild, "%s %d\n",  word, count);
    }
}

如何在C中的stdin上使用fgets（）而不是fscanf（）？

2 个答案: