abs() / labs() / div()

// Returns the absolute value of an int.
int abs(int n);

// Returns the absolute value of a long.
long labs(long n);

// Returns the absolute value of a long long (C99 and later).
long long llabs(long long n);

// Computes the quotient and remainder of an int division at once, returned as a div_t struct.
div_t div(int numer, int denom);

// Computes the quotient and remainder of a long division at once, returned as an ldiv_t struct.
ldiv_t ldiv(long numer, long denom);

#include <stdio.h>
#include <stdlib.h>

int main(void) {
    // Use abs to get the absolute value of an int.
    printf("abs(-10) = %d\n", abs(-10)); // Prints: abs(-10) = 10
    printf("abs(7)   = %d\n", abs(7));   // Prints: abs(7) = 7

    // Use labs to get the absolute value of a long.
    long big = -123456789L;
    printf("labs(%ld) = %ld\n", big, labs(big)); // Prints the positive value.

    // Use div to get the quotient and remainder at once (no need to write / and % separately).
    div_t result = div(17, 5);
    printf("17 / 5 = quotient %d remainder %d\n", result.quot, result.rem);
    // Prints: 17 / 5 = quotient 3 remainder 2

    // A typical use of abs: computing the distance between two coordinates.
    int x1 = 3, x2 = -4;
    int distance = abs(x2 - x1);
    printf("Distance between x1 and x2: %d\n", distance); // Prints: Distance between x1 and x2: 7

    // Use div to convert minutes into hours and minutes.
    int total_minutes = 137;
    div_t time = div(total_minutes, 60);
    printf("%d minutes = %d hour(s) %d minute(s)\n", total_minutes, time.quot, time.rem);
    // Prints: 137 minutes = 2 hour(s) 17 minute(s)

    return 0;
}

The absolute value of INT_MIN (the minimum value of int) cannot be represented as an int, so the result of abs(INT_MIN) is undefined behavior. If this value may appear in your input, use long or long long, or add a check beforehand.

div() computes both the quotient and remainder in a single operation. Depending on compiler optimizations, it may generate the same code as separate / and % operators, but it is also useful for making the intent of the code explicit.

For the absolute value of a floating-point number, use fabs().