c++ - Is undefined behavior in given code? -

what return value of f(p,p), if value of p initialized 5 before call? note first parameter passed reference, whereas second parameter passed value.

int f (int &x, int c) {        c = c - 1;        if (c==0) return 1;        x = x + 1;        return f(x,c) * x; } 

options are:

1. 3024
2. 6561
3. 55440
4. 161051

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i try explain:

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in code, there 4 recursive calls parameters (6,4), (7,3), (8,2) , (9,1). last call returns 1. due pass reference, x in previous functions 9. hence, value returned f(p,p) 9 * 9 * 9 * 9 * 1 = 6561.

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this question competitive exam gate, (see q.no.-42). answer key given gate "marks all" (means there no option correct.) key set-c, q.no.-42. somewhere explained as:

in gate 2013 marks given same code in c/c++ produces undefined behavior. because * not sequence point in c/c++. correct code must replace

return f(x,c) * x; 

with

 res = f(x,c);  return res * x; 

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but given code works fine. gate's key wrong? or mistake question?

   return f(x,c) * x; 

the result of operation depends on order in 2 things evaluated. since cannot predict order they'll evaluated, cannot predict result of operation.