As it is written in the title this very simple C program is calculating circumference of circle by given radius of the circle.
Interface is in serbian so I will translate it:
first program will ask you to enter circle radius (Unesite poluprecnik kruga) and then you will get result : Circle circumference with radius r=something is O=something else (Obim kruga sa poluprecnikom r=?? je O??).

Entered radius can not be zero or negative.

/* opis: program za racunanje obima autor: Milko Leporis E11011 datum: 25.10.2005. */ #include <stdio.h> #define Pi 3.14156 double obim(double r); int main(){ double r; //poluprecnik char c; //odgovor do{ printf("\n\nUnesite poluprecnik kruga:"); scanf("%lf",&r); c = getchar(); if(r>=0) printf("\nObim kruga sa poluprecnikom r=%lf je O=%lf\n\n",r,obim(r)); else printf("\nObim ne moze biti negativan ili nula!\n Mora biti pozitivan realan broj!\n\n"); printf("Da li zelite da ponovite unos? (d) "); c = getchar(); } while(c == ´d´); return 0; } double obim(double r){ return 2*r*Pi; }