Why I this address is being changed after initialization in main function? (C)

Question

I'm new to C and I'm facing some problems with a project I am working on.

In vm.h

#define STACK_MAX 256
typedef double Value;

typedef struct {
   Value values[STACK_MAX];
   Value* top;
}

typedef struct {
   Chunk* chunk;
   uint8_t* ip; // Instruction pointer
   Stack stack;
} VM;

VM initVm();
void resetStack(Stack* stack);

In vm.c

void resetStack(Stack* stack) {
    stack->top = stack->values
}

VM initVM() {
    VM vm;
    resetStack(&vm.stack);
    return VM;
}

In main.c

int main() {
    VM vm = initVM();

    ... 
}

Ok, here's the problem. After I initialize vm, vm.stack.top is pointing to vm.stack.values[0] (which I don't know why is filled with garbage values instead of zeroes), but one instruction later, no matter what, vm.stack.top is changed and starts pointing something else or sometimes the address of the vm.stack.values[0] is changed to another.

With `stack` being a pointer, this cannot compile: `stack.top = stack.values;`. Please show real code that reproduces your problem. Not something made up. — Gerhardh, Nov 18 '21 at 18:38
`Stack *stack; stack.top` is incorrect. You should use `->` instead of `.`. — William Pursell, Nov 18 '21 at 18:39
Assuming your are actually doing something like `stack->top = stack->values` you assign the address of `values` member of local variable `Vm` which does not exist and longer after you return from `initVM` — Gerhardh, Nov 18 '21 at 18:41
Global variables are automatically initialized to zeroes, but local variables have no automatic initialization. So the array won't be zeroed. — Barmar, Nov 18 '21 at 18:41
It is legal to a struct as in `return VM;` but you're putting a lot of load on it, containing an array as it does. — Weather Vane, Nov 18 '21 at 18:41
Either that, or allocate `VM` dynamically, and return a pointer to it. Then the `top` pointer will remain valid. — Barmar, Nov 18 '21 at 18:42
@WeatherVane the struct contains a pointer that is pointing to the array in the local variable. That is a pointer, too ;) — Gerhardh, Nov 18 '21 at 18:45
Does this answer your question? [Return address of local variable in C](https://stackoverflow.com/questions/8743411/return-address-of-local-variable-in-c) — Gerhardh, Nov 18 '21 at 18:48
People should really stop `typedef`ing structs that are not opaque. — Cheatah, Nov 18 '21 at 18:55
@Barmar it’s faster to dereference a pointer than look up an element in an array by index. — Gonzalo, Nov 18 '21 at 19:30
@Gerhardh yep, the problem was I was passing a local variable. Malloquing the vm and the freeing it, resolved the problem. — Gonzalo, Nov 18 '21 at 19:57
@Cheatah, Re "*People should really stop typedefing structs that are not opaque.*", huh? No thanks — ikegami, Nov 18 '21 at 20:21

score 0 · Answer 1 · answered Nov 18 '21 at 21:21

0

The problem was VM was declared in the local scope. Allocating VM in the heap solved it.

answered Nov 18 '21 at 21:21

Gonzalo

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Why I this address is being changed after initialization in main function? (C)

1 Answers1