Why do some C programs work in debug but not in release?
c, codeblocks, crash, debugging, release
Solution
The basic answer to the title question ("Why do some C programs work in debug but not in release?") is "when they invoke undefined behaviour".
Here, in
memmove(temp,*array,(position+1)*sizeof(int)); // copy entire array before position
memmove(temp+position,(*array)+(position+1),(arraySize - position)*sizeof(int)); // copy entire array after postion
you copy too much. To see why, observe that the first `memmove` copies to `temp[0]`, `temp[1]`, ..., `temp[position]`, and the second copies to `temp[position]`, `temp[position+1]`, ..., `temp[position+arraySize-position-1] = temp[arraySize-1]` (note the overlap at `temp[position]`). But `temp` only has space for `arraySize-1` elements -- you copied one more than it was allowed to hold, so you get undefined behaviour.
It probably works in debug but not release mode because the heap is laid out differently (debug-mode allocators may pad the allocations with extra space to catch bugs like this when running under a debugger or profiler).
Problem
Ok, So I am stuck here. I have code for a program that systematically executes people standing in a circle based off an algorithm, but I am having a problem with it crashing in release mode. My code runs fine if I run it using the debugger (codeblocks), but if I don't it crashes. I looked around online, and the only thing I am finding is unintialized variables, but I tried immediately setting values for my variables at declaration and it didn't fix the problem. If anyone can see what my problem is, I would greatly appreciate help. ``` #include<stdio.h> #include<stdlib.h> #include<string.h> // if the program does not work, please run in debugger mode. It will work. void remove_person(int** array, int arraySize, int position) { int i; for (i = 0; i < arraySize; ++i) printf("%d ", (*array)[i]); printf("\n"); int* temp = malloc((arraySize - 1) * sizeof(int)); // create temporary array smaller by one element memmove(temp,*array,(position+1)*sizeof(int)); // copy entire array before position memmove(temp+position,(*array)+(position+1),(arraySize - position)*sizeof(int)); // copy entire array after postion for (i = 0; i < arraySize - 1; ++i) printf("%d ", (temp)[i]); printf("\n"); free (*array); *array = temp; } int kill(int** a, int n) { int pos = 0; int round = 1; while(n > 1) { pos = pos + 2 - (round % 2); while(pos >= n) pos = pos - n; remove_person(a,n,pos); n--; while(pos >= n) pos = pos - n; round++; } return *a[0]; } void main() { int n, survivor, i; int* people; printf("Enter number of people for Russian Roulette: \n"); scanf("%d", &n); people = (int*) malloc(n*sizeof(int)); for(i=0; i < n; i++) { people[i] = i; } survivor = kill(&people, n); printf("The survivor is person #%d\n", survivor); } ```