fread isnt reading an updated value despite the file has been changed by another program

Question

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??) ?

jamesdlin · Accepted Answer · 2016-09-06T12:29:06.260

3

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.

edited Sep 06 '16 at 12:29

answered Sep 06 '16 at 11:36

jamesdlin

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Your answer is really helpful! Because when I tried googling this problem I didn't find any answers (and thought of cache instead of buffer) Thank you very much! Thanks to you, now I can feed that face in an elegant way ;-) – Philip Brown Sep 06 '16 at 12:18
What if you simply set the buffer to the size of the previous message that went through it or the size of the next message, whichever is larger? A buffer being available usually speeds up processing, so I'm a bit hesitant with the idea of just getting rid of it, but also am not a fan of hard-coded limits unless they have a good reason. – kayleeFrye_onDeck Oct 01 '18 at 20:15
1

@kayleeFrye_onDeck I doubt that would help; you could still read a stale cached value. Buffering *is* useful; it's a fairly rare situation like this where you would want to disable it. If you have a different case where you want buffering but might read stale data, then perhaps you should ask a separate question for your own situation. – jamesdlin Oct 02 '18 at 04:22
@jamesdlin That makes sense. What if you disabled it, and then immediately re-enabled it? – kayleeFrye_onDeck Oct 02 '18 at 18:42

fread isnt reading an updated value despite the file has been changed by another program

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