sqrt() / pow() / fabs() / ceil() / floor()

double sqrt(double x);          // Returns the positive square root of x (x >= 0 required).
double pow(double x, double y); // Returns x raised to the power of y.
double fabs(double x);          // Returns the absolute value of a double.
double ceil(double x);          // Returns the smallest integer value >= x (ceiling).
double floor(double x);         // Returns the largest integer value <= x (floor).
double round(double x);         // Rounds to the nearest integer (C99 and later).
double fmax(double x, double y);// Returns the larger of x and y (C99 and later).
double fmin(double x, double y);// Returns the smaller of x and y (C99 and later).

#include <stdio.h>
#include <math.h>

int main(void) {
    // Compute a square root with sqrt.
    printf("sqrt(2.0) = %.6f\n", sqrt(2.0));   // Outputs "1.414214".

    // Compute an exponent with pow.
    printf("2^10 = %.0f\n", pow(2.0, 10.0));   // Outputs "1024".

    // Compute the absolute value of a floating-point number with fabs.
    printf("fabs(-3.7) = %.1f\n", fabs(-3.7)); // Outputs "3.7".

    // Round toward an integer with ceil and floor.
    double val = 2.3;
    printf("ceil(%.1f) = %.1f\n", val, ceil(val));   // Outputs "3.0".
    printf("floor(%.1f) = %.1f\n", val, floor(val)); // Outputs "2.0".

    double neg = -2.3;
    printf("ceil(%.1f) = %.1f\n", neg, ceil(neg));   // Outputs "-2.0".
    printf("floor(%.1f) = %.1f\n", neg, floor(neg)); // Outputs "-3.0".

    // Example: computing Euclidean distance.
    double dx = 3.0, dy = 4.0;
    printf("distance = %.1f\n", sqrt(dx*dx + dy*dy)); // Outputs "distance = 5.0".

    return 0;
}

Some environments (e.g., GCC on Linux) will produce a linker error if you omit the -lm flag from the compile command.

Passing a negative value to sqrt() returns NaN (Not a Number). Always verify that the argument is non-negative. Error conditions can be detected via errno or fpclassify().

For trigonometric functions, see sin() / cos() / tan(). For logarithms and exponentiation, see log() / exp().