C从文件中读取字符串内容并比较相等性

Question

我正在从文件中读取内容，并且希望查找特定值，例如“伦敦”。为此，我正在阅读内容，用“\n”对其进行标记，然后使用“strcmp”将每个值与“伦敦”进行比较。

但我认为我仍然不明白 C 如何存储数据并进行比较，因此下面的代码无法按我的预期工作。我想我在这里缺少一些 C 的基础知识，请帮助:

测试命令行:

./myprogram datafile.txt "london"

输入数据文件.txt:

london
manchester
britain
...

代码myprogram.c:

int main(int argc, char **argv) {
  FILE* fl;
  char st1[2000];
  char * buffer = 0;
  long length;

  fl = fopen (argv[1], "r"); //the data file content is shown above
  if (fl){
    fseek (fl, 0, SEEK_END);
    length = ftell (fl);
    fseek (fl, 0, SEEK_SET);
    buffer = malloc (length+1);
    if (buffer){
        //fread (buffer, 1, length, fl);
        fread(buffer, sizeof(char), length, fl);
        buffer[length] = '\0';
    }
  fclose (fl);    
  }else{
    printf("data file not found");
    return -1;
  }  


  //firstly let's compare the value passed by command line with "london"
  strcpy(st1, argv[2]); 
  if(strcmp(st1,"london")==0)
    printf("equals\n");  //as expected, I get "equals" printed
  else
    printf("unequal\n");


  //now let's compare the values extracted from the data file, 
  char* entity = strtok(buffer, "\n"); 
  while (entity != NULL) {
    strcpy(st1, entity); //copy the value from the char pointer entity to the char array st1 so we can compare with other strings

    printf("%s\n", st1);  //this prints london, ....

    if(strcmp(st1,"london")==0)
        printf("equals\n");  //I was expecting this..
    else
        printf("unequal\n"); //but i got this...
    entity = strtok(NULL, "\n");
  }
  return 0;

}

我期望上述程序的输出为:

equals
london
equals
manchester
unequal
britain
unequal
...

但我不明白为什么我会这样

equals
london
unequal  <=============== why and how to fix?
manchester
unequal
britain
unequal
...

我应该如何更改它，以便从文件中读取的值“london”等于“london”的实际“字符串”？

非常感谢

Answer 1

我尝试了以下操作，并将 strcpy 的调用替换为 strncpy。您最初的问题是 st1 变量末尾的“\0”。所以你可以这样做:

  while (entity != NULL) {
    strncpy(st1, entity,strlen(entity)-1); //copy the value from the char pointer entity to the char array st1 so we can compare with other strings

    printf("%s\n", st1);  //this prints london, ....

    if(!strcmp(st1,"london"))
        printf("equals\n");  //I was expecting this..
    else
        printf("unequal\n"); //but i got this...
    entity = strtok(NULL, "\n");
  }

或者另一种解决方案是将“\0”自己放在st1的末尾，如下所示:

strcpy(st1, entity);
st1[strlen(entity)-1] = '\0';

我测试了两者并且它有效。