Using -1 as a flag value for unsigned (size_t) types

by

-1 will always convert to the max unsigned value, this is due to section 4.7 Integral conversions:

If the destination type is unsigned, the resulting value is the least unsigned integer congruent to the source
integer (modulo 2n where n is the number of bits used to represent the unsigned type). [ Note: In a two’s
complement representation, this conversion is conceptual and there is no change in the bit pattern (if there
is no truncation). —end note ]

The same quote for C99 would be from 6.3.1.3:

Otherwise, if the new type is unsigned, the value is converted by repeatedly adding or
subtracting one more than the maximum value that can be represented in the new type
until the value is in the range of the new type.49)

So we end up with:

-1 + (UMAX + 1)

which is:

UMAX

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