Question

I am implementing generic stack in C and I am facing a problem in stackPop method. My struct is as follows:

“Stack.h” file

        typedef struct{
            void *elems;
            int elemSize;
            int allocLength;
            int logLength;
            void (*freefnc)(void *);
        } Stack;

    void stackNew(Stack *s, int elemSize, void (*freefnc)(void *));
    void stackDispose(Stack *s);
    void stackPush(Stack *s, void *elemAddr);
    void stackPop(Stack *s, void *target);

Stack.c

    #inlcude<Stack.h>

    void stackNew(Stack *s, int elemSize, void (*freefnc)(void *)){
        s.allocLength = 4;
        s.logLength = 0;
        s.elemSize = elemSize;
        s.elems = malloc(4*elemSize);
        s.freefnc =  freefnc;
    }

    void stackDispose(Stack *s){
        if(s.freefnc!=NULL){
            for(int i=0; i<s.logLength; i++){
                freefnc((char *)s.elems+i*s->elemSize);
            }
        }
        free(s.elems);
    }

    void Stringfree(void *elem){
        free(*(char**)elem);
    }

    void stackPush(Stack *s, void *elemAddr){
        if(s.alloclength == s.logLength){
            stackGrow(s);
        }
        void *target = (char *)s.elems + s.logLength*s.elemSize;
        memcpy(target,elemAddr,s.elemSize);
        s.logLength++;
    }

    static void stackGrow(Stack *s){
        s.allocLength*=2;
        s.elems = realloc(s.elems, s.alloclength*s.elemSize);
        assert(s.elems!=NULL);
    }

    void stackPop(Stack *s, void *elemAddr){
        void *source = (char *)s.elems + (s.logLength-1)*s.elemSize;
        memcpy(elemAddr,source,s.elemSize);
        s.logLength--;
    }

How do I return/get the value being popped off in stackPop function? I do not want to change the implementation but if there is a good way which keeps the implementation same to the extent as much as possible, then I would appreciate that. Other approaches are also acceptable.

Answer 1

I think the best you could do is to return a pointer to popped data (as a void* since that’s about the best you can do in C for a ‘generic’ function):

void* stackPop(Stack *s, void *elemAddr){
        void *source = (char *)s.elems + (s.logLength-1)*s.elemSize;
        memcpy(elemAddr,source,s.elemSize);
        s.logLength--;
        return elemAddr;
}

Note that the caller still needs to provide the memory and the address to pop the data into; if you want ed you could avoid that by having the function malloc() the memory:

void* stackPop(Stack *s){
        void *source = (char *)s.elems + (s.logLength-1)*s.elemSize;
        void *elemAddr = malloc(s.elemSize);
        // if (!elemAddr) handle_error();
        memcpy(elemAddr,source,s.elemSize);
        s.logLength--;
        return elemAddr;
}

Of course that would require the caller to free() it when it is no longer needed, and adds the minor complication of needing to handle the out of memory situation.