Question

I have a function like this:

uint8_t getULEB128Value(uint8_t *p, uint32_t *to){
    unsigned int result; 
    uint8_t count = 1;
    uint8_t cur;

    result = *p;
    if(result > 0x7F){
        count++;
        cur = *(++p);
        result = (result & 0x7F) | (cur << 7);
        if(cur > 0x7F){
            count++;
            cur = *(++p);
            result |= ((cur & 0x7F) << 14);
            if(cur > 0x7F){
                count++;
                cur = *(++p);
                result |= ((cur & 0x7F) << 21);
                if(cur > 0x7F){
                    count++;
                    cur = *(++p);
                    result |= ((cur & 0x7F) << 28);
                }
            }
        }
    }
    *to = result;
    return count;
}

Now I just want to know the length and I’m not really interested in the value…
Is it possible to have a don’t care pointer? so every value which is wrote there goes to nirvana? like /dev/null?
I mean of course I can create a pointer somewhere and free it afterwards but maybe there is a thing for that already?

Answer 1

The usual pattern for this is to allow passing a null pointer into your function and check this before writing the value:

if (to != NULL) {
  *to = result;
}

Then you can call it like

result = getULEB128Value(p, NULL);

when you do not want to write anything into the “to” pointer.