C program goes into infinite loop with scanf [duplicate]

Check return value of scanf and clear input buffer in case of illegal input.

Like this:

#include <stdio.h>
#include <stdbool.h>

int main(void) {
    float sales, commission, earnings;
    int state;

    while(true) {
        printf( "Enter sales in dollars ( -1 to end ): " );
        if((state = scanf("%f", &sales )) != 1){
            if(state == EOF)
                return 0;
            printf("invalid input.\n");
            while(getchar() != '\n');//clear input

            continue;
        }

        if ( sales == -1 ) {
            return 0;
        } else if(sales < 0){
            printf("invalid input.\nA negative value was entered.\n");
            continue;
        }

        commission = sales / 100 * 9;
        earnings = commission + 200;

        printf( "Salary is %.2f\n", earnings );
    }

    return 0;
}