The parameters are specified here:
Code:
void srtheap(void *, size_t, size_t, int (*)(const void *, const void *));
You use the same parameters for all sort functions, as far as I can tell.

A void pointer is just a "anything pointer", so you can sort any type of object, be it a simple list of characters in an array, or a large struct of personal details [where you perhaps use the lastname + firstname or some identity number (social security/national insurance or such) as the "sort key"].

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Mats