Type uint8_t is guaranteed to have 8 bit, no padding bits, 2s complement. You are talking about uint_least8_t or int_fast8_t.

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Exact-width integer types

           The  typedef  name  intN_t  designates  a signed integer type with width N, no padding bits, and a two's-complement representation.
           Thus, int8_t denotes a signed integer type with a width of exactly 8 bits.

           The typedef name uintN_t designates an unsigned integer type with width N.  Thus, uint24_t denotes an unsigned integer type with  a
           width of exactly 24 bits.