Re: [gpsd-dev] Warnings on 32 bit Fedora 24

[Juha Nurmela]

> >   fld &=  ~(~0ULL << width)

Isn't the ubits() function little bit dangerous?

The prototype and comments look like that it promises to extract 64 bits wide 
fields, but it will quietly fail with unaligned really wide fields. Variable 
fld is asked to hold more than 64 bits in certain cases.

Juha


Not wanting to whine with a sweatless shirt,

{
    int remain = width;
    uint64_t fld = 0;

    assert(width <= sizeof(uint64_t) * CHAR_BIT);

    buf += start / CHAR_BIT;
    start %= CHAR_BIT;

    if (start) {
        int piece = CHAR_BIT - start; /* might be more than remains */

        fld = *buf++ & (0xFF >> start); /* get lsbits */
        remain -= piece;

        if (remain < 0) {
            fld >>= -remain; /* just drop excess bits and all done */
            remain = 0;
        }
    }
    while (remain >= CHAR_BIT) {
        remain -= CHAR_BIT;
        fld <<= CHAR_BIT;
        fld |= *buf++;
    }
    if (remain) {
        fld <<= remain;
        fld |= *buf >> (CHAR_BIT - remain); /* get msbits, rightjusted */
    }

    /* was extraction as a little-endian requested? */