It will be plain I am not a competent writer of C, but was wondering, in connection with this question Ackermann very inefficient with Haskell/GHC (which irritating me perhaps irrationally) why the author's program to calculate Wilhelm Ackermann's famous function:
int ack(int m, int n) { if (m == 0) return n+1; if (n == 0) return ack(m-1, 1); return ack(m-1, ack(m, n-1)); } int main() { printf("%d\n", ack(4,1)); return 0; } works fine, but when given an obvious optimization (which helps elsewhere) and given the argument (4,2) -- which is of course algorithmically cruel -- ends with Segmentation fault: 11 :
int ack(int m, int n) { if (m == 0) return n+1; if (m == 1) return n+2; if (n == 0) return ack(m-1, 1); return ack(m-1, ack(m, n-1)); } int main() { printf("%d\n", ack(4,2)); return 0; } If I comment out the 'optimizing' line if (m == 1) return n+2;, the program runs on and on as it does in other languages, but doesn't have the same effect -- at least not after say 5 minutes of operation. [Correction, it seems it does -- after 8m41s] (I should note that I am using gcc that comes with os x, but the same seems to be happening with e.g. the gcc-4.7.2 on ideone.com.)
I agree the program doesn't deserve even to be compiled, but wonder why a segmentation fault, which is generally viewed with horror and as a language flaw or compiler fault in other languages I am familiar with, is here the appropriate response for the gcc .
