[lwip-devel] [bug #25544] Warning in DHCP.H

[David Empson]

Follow-up Comment #2, bug #25544 (project lwip):

The warning is valid.

In C, multiplying two values of type int will produce a result of type int.
If the expression is used in a context where a larger type is required, the
typecast to the larger type occurs after the multiplication.

In effect, this is calculating (u32_t)((int)60 * (int)1000).

On a 16-bit platform, the multiplication will overflow (because 60000 is
outside the range of int). The resulting negative value will then be converted
to a signed 32-bit integer, producing an enormous timeout.

I agree with Edward. At a minimum, the 1000 constant should be changed to
1000U, which will result in a 16-bit unsigned multiplication.

This still won't help if DHCP_COARSE_TIMER_SECS was ever set higher than 65,
because then you would get a 16-bit unsigned multiplication overflowing,
producing a result which was too small.

I suggest a better solution would be to use 1000UL, forcing the
multiplication to be calculated using 32-bit unsigned arithmetic.


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