The Standard Library: An Overview

A few headers worth knowing exist, beyond stdio.h and string.h already used so far:

• <stdlib.h> — malloc/free, rand/srand, atoi/atof (string-to-number conversion), exit.
• <math.h> — sqrt, pow, floor, ceil (link with -lm on some systems: gcc file.c -o file -lm).
• <stdbool.h> — a real bool type (true/false) as a readable alias over C's underlying "0 is false" rule.
• <time.h> — time(), used earlier to seed rand().
• <ctype.h> — isdigit, isalpha, toupper, tolower.

#include <math.h>
#include <stdbool.h>

bool isPositive = true;
double root = sqrt(16.0); // 4.0