Hi, Thanks to all those who answered, especially to Didi for valgrind. I installed it and run it immediately: I had tried debug but its output was not informative. I guess that this is more relevant than looking into good C book: I tried to look into Kernigha and Ritchie's bible. But this is a poor way to find a mistake. I should probably reread it from cover to cover, not try to get to the specific point of "calling by reference" and relying on some old programming experience, the details of which is already forgotten. And thanks a lot to Oleg Goldsmidt for pointing to Comp.lang. Now I'll check and learn the other pieces of advice... And thanks for your indulgence, Avraham
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