Github user jpeach commented on the pull request:
https://github.com/apache/trafficserver/pull/542#issuecomment-203489537
``ptrdiff_t`` is the right type for pointer arithmetic.
That said, I don't really understand the problem here. If ``field`` is not
in the ``MIMEFieldBlockImpl`` you are scanning, then the pointer arithmetic is
not valid in the first place. For the pointer arithmetic to overflow 32bits,
the distance between the pointers would need to be ``2^32 *
sizeof(MIMEFieldBlockImpl)``, which seems unlikely.
A potential real problem here could be the compiler assuming that the
comparison can't happen, since pointer arithmetic across different objects is
not standards compliant, see [here](https://lwn.net/Articles/278137/). To
address this, I'd add ``MIMEFieldBlockImpl::contains(MIMEField*)``,
```C
int
mime_hdr_field_slotnum(MIMEHdrImpl *mh, MIMEField *field)
{
int slots_so_far;
MIMEFieldBlockImpl *fblock;
slots_so_far = 0;
for (fblock = &(mh->m_first_fblock); fblock != NULL; fblock =
fblock->m_next) {
if (block->contains(field)) {
MIMEField *first = &(fblock->m_field_slots[0]);
ptrdiff_t block_slot = field - first; // in units of MIMEField
return (slots_so_far + block_slot);
}
slots_so_far += MIME_FIELD_BLOCK_SLOTS;
}
return -1;
}
```
Can you think of a way to write tests for this?
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