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c++clinuxraspberry-pibinaryfiles

How to rewrite an register in binary file only if different?

I need write a byte in binary file, but only if this is diferent (something looked like EEPROM.update(a) from Arduino IDE). I think there is some function that does this, but now I can't find it/I'm not sure that exists.

So far I have do this:

FILE *fp = fopen("file.dat", "r+");
fseek(fp, (long) address, SEEK_SET);
fread(&value, sizeof(uint8_t), 1, fp);

if (value == val) {
    fclose(fp);
    exit();
}

fseek(fp, (long) address, SEEK_SET);
fwrite(v, sizeof(uint8_t), 1, fp);
fclose(fp);

This code is an example, not the real code.

Thanks for your time.

Solution

  • The fread advances the file pointer, so your fwrite doesn't overwrite the byte at offset address, but rather the following byte. You need to fseek back to offset address again before writing.

    Also there is a v in your sample code which isn't defined. Probably it should be &val or something like that.