Re: write-only variables in src/sys/ - possible bugs

Christian Peron schrieb:
On Wed, Feb 04, 2009 at 03:54:41PM +0100, Christoph Mallon wrote:
[..]
Yes, function arguments are considered being read. The problem is different here: mtod() should be a macro, but the macro declaration was missing (*cough* hacked build process *cough*). So the parser tried to parse this as function call. Then it hit the "void *", which confused it - it got a type while parsing an expression. I improved the error correction, resolved a few other problems, too, and generated a new list:

http://tron.homeunix.org/unread_variables.log
(The list has a date at the top, if it is missing, you see the old list in your browser cache)

The false positives, which you mentioned, are gone now - thanks for reporting this. The list now contains about 1.000 entries and about 60 concern variables named 'error'.

Also.. one other thing I noticed:

void
bpf_buffer_append_mbuf(struct bpf_d *d, caddr_t buf, u_int offset, void *src,
u_int len)
{ const struct mbuf *m;
u_char *dst;
u_int count;
m = (struct mbuf *)src;
dst = (u_char *)buf + offset;
while (len > 0) {
if (m == NULL)
panic("bpf_mcopy");
count = min(m->m_len, len);
bcopy(mtod(m, void *), dst, count);
m = m->m_next;
dst += count;
len -= count;
}
}

dst += count

In this expression, both dst and count are read since this is the
same thing as:

dst = dst + count;

No, the analysis *explicitly* marks "x" in neither "x += 1" nor "x = x + 1" as read. The value of the variable in these expressions is only read to calculate its own new value. Therefore it will complain about

int x = 23;
x++;
x += 1;
x = x + 1;
return 0;

This is not a bug, it is a feature. (:
The problem here solely was the insufficient error recovery in the bcopy() line, which caused the only "real" user of "dst" to disappear. I corrected this problem and as you can see the false positives are no longer on the list.
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