尽管文件已被另一个程序更改，但fread不会读取更新的值 [英] fread isnt reading an updated value despite the file has been changed by another program

问题描述

有两个程序同时在处理我的特殊文件.他们有一个共同的开始:

  #define TASK_POSITION 0x0100#include< stdio.h>#include< stdlib.h>#include< inttypes.h>int main(){FILE * control_file;unsigned int task_code;fpos_t task_position;control_file = fopen("/home/anonymous/.control"，"r +");fseek(control_file，TASK_POSITION，SEEK_SET);fgetpos(control_file，& task_position); 

但是后来他们有了完全不同的代码

• 第一个程序可能会在此文件的TASK_POSITION偏移处更改一个值:

   /* ...那里有很多代码... */task_code = 0xFEEDFACE;fsetpos(control_file，& task_position);fwrite(& task_code，4，1，control_file);fflush(control_file);睡眠(1);fclose(control_file);返回0;} 

• 第二个程序反复读取与此文件相同偏移量的值:

  <;> <代码>代表(;;){fsetpos(control_file，& task_position);fread(& task_code，4，1，control_file);如果(task_code == 0xFEEDFACE){/* ...做点很棒的事！... */}else {//调试后删除fprintf(stdout，尚未饿:P 0x％08x值已被读取... \ n")；fflush(stdout);}睡眠(60);}//以防万一fclose(control_file);返回0;}

默认情况下，在TASK_POSITION偏移处存储了0x12345678值.

这是个问题:

在第一个程序启动并完成其工作之后，我可以在十六进制编辑器中看到一个特殊文件已成功更改.但是:由于某些我不知道的原因，第二程序的fread会继续读取相同的旧值-0x12345678！

尽管我找到了一个临时的解决方法，方法是在第二个程序的无限循环的每次迭代期间(在fread检查之间)关闭/打开此文件，但这似乎不是最好的解决方案！请告诉:如何强制fread实际从文件中重新读取新值，而不是仅仅返回先前读取的值(从其缓存"中?)?

解决方案

C的标准I/O函数通常被缓冲.要禁用缓冲，您可以执行以下操作:

  setvbuf(control_file，NULL，_IONBUF，0); 

立即打开文件后，对文件执行任何I/O操作.

There are two programs that are working with my special file at the same time. They have a common beginning:

    #define TASK_POSITION 0x0100

    #include <stdio.h>
    #include <stdlib.h>
    #include <inttypes.h>

    int main() {

        FILE * control_file;
        unsigned int task_code;
        fpos_t task_position;

        control_file = fopen("/home/anonymous/.control", "r+");
        fseek(control_file, TASK_POSITION, SEEK_SET);
        fgetpos(control_file, &task_position);

But later they have a very different code

• 1st program could change a value at TASK_POSITION offset of this file:

      /* ... lots of code there ... */
  
      task_code = 0xFEEDFACE;
  
      fsetpos(control_file, &task_position);
      fwrite(&task_code, 4, 1, control_file);
        fflush(control_file);
          sleep(1);
  
      fclose(control_file);
      return 0;
  }
  

• 2nd program is repeatedly freading a value at the same offset of this file:

      for (;;) {
          fsetpos(control_file, &task_position);
          fread(&task_code, 4, 1, control_file);
          if (task_code == 0xFEEDFACE) {
              /* ... Do something awesome! ... */
          }
          else { // remove after debugging
            fprintf(stdout, "not hungry yet :P 0x%08x value has been read... \n"); 
            fflush(stdout);
          }
          sleep(60);
      }
  
      // just in case
      fclose(control_file);
      return 0; 
  }
  

By default, there is 0x12345678 value stored at TASK_POSITION offset.

Here is a problem:

after the 1st program has been launched and completed its' work, I could see in a hex editor that a special file has been changed successfully. However: for some unknown to me reason, fread of 2nd program keeps reading the same old value - 0x12345678 !

Although I found a temporary workaround by fclosing/fopening this file during the each iteration of 2nd program's infinite cycle (between the fread checks) it does not look like the best possible solution! Please tell: how to force fread to actually re-read a new value from a file, instead of just returning a previously read value (from its' cache??) ?

解决方案

C's standard I/O functions are typically buffered. To disable buffering, you can do:

setvbuf(control_file, NULL, _IONBUF, 0);

immediately after you open the file, before you perform any I/O on it.

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