Help me understand this code...

I want to make sure i understand what the following code is doing:

Code:

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#include <math.h>
    float expdev(long *idum) {
              float ran0(long *idum);
              float dummy;
              do
                  dummy=ran0(idum);
              while (dummy == 0.0);
            return -log(dummy);
}

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So when someone calls on expdev, passing in idum, the value of idum is changed via ran0 correct? so if idum is 3 to start out with and runs once, the 2nd run of this function will have a different idum value right?

ran0 can change the value pointed to by idum, yes; whether it does or not, who knows.