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`if (fpm_shm_size - size > 0)` will be rewritten by the compiler as this: `if (fpm_shm_size != size)`, which is undesirable. The reason this happens is that both variables are size_t, so subtracting them cannot be negative. The only way it can be not > 0, is if they're equal because the result will then be 0. This means that the else branch won't work properly. E.g. if `fpm_shm_size == 50` and `size == 51`, then `fpm_shm_size` will wraparound instead of becoming zero. To showcase that the compiler actually does this, take a look at this isolated case: https://godbolt.org/z/azobdWcrY. Here we can see the usage of the compare instruction + cmove, so the "then" branch is only done if the variables are equal.