C: Casting minimum 32-bit integer (-2147483648) to float gives positive number (2147483648.0)

by

Replace

#define INT32_MIN (-2147483648L)

with

#define INT32_MIN (-2147483647 - 1)

-2147483648 is interpreted by the compiler to be the negation of 2147483648, which causes overflow on an int. So you should write (-2147483647 - 1) instead.
This is all C89 standard though. See Steve Jessop’s answer for C99.
Also long is typically 32 bits on 32-bit machines, and 64 bits on 64-bit machines. int here gets the things done.

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