Dead simple stack implemented as a linked list in C.

stack.c

struct stack {
    struct stack* next;
    void* data;
};

void push(struct stack** stack, void* data) {
    struct stack* new = malloc(sizeof *new);
    new->next = *stack;
    new->data = data;
    *stack = new;
}

void pop(struct stack** stack) {
    struct stack* old = *stack;
    *stack = old->next;
    free(old);
}