Generic stack don't push or pop value

debugcn Published at Dev

user3868594

I'm trying to implement generic stack in C, but I can't print value that I pushed. Here is the code

#include <stdio.h>
#include <stdlib.h>
#include <assert.h>
#include <memory.h>

typedef struct{
    void *elems;
    int sizeOfElems;
    int allocated;
    int lenght;
}stack;

void allocate_stack(stack *s, int sizeOfElements){
    assert(sizeOfElements > 0);
    s->sizeOfElems = sizeOfElements;
    s->allocated = 2;
    s->lenght = 2;
    s->elems = malloc(sizeOfElements * s->allocated);
}

void deallocate_stack(stack *s){
    free(s->elems);
}

void push_elem(stack *s, void *elem){
    s->lenght += 1;
    if(s->allocated == s->lenght){
        realloc(s, s->lenght * s->sizeOfElems);
        s->lenght *= 2;
    }
     void *target = (char *)s->elems + s->lenght * s->sizeOfElems;
     memcpy(target, elem, s->sizeOfElems);
}
void pop_elem(stack *s, void *elemAddr){
    void *source = (char *)s->elems + (s->lenght-1) * s->sizeOfElems;
    memcpy(elemAddr, source, s->sizeOfElems);
    s->lenght -=1;
}
int main(){
    stack s;
    allocate_stack(&s, 1);
    char a = 'a';
    push_elem(&s, &a);
    char *elem = NULL;
    pop_elem(&s, elem);
    printf("%s", elem);
    deallocate_stack(&s);
    return 0;
}

The problem is that I don't get anything when I run the program, just press return to close the window. I'm programming on Linux using GCC toolkit.

shikamaru

In your push_elem:

    s->lenght += 1;
    if(s->allocated == s->lenght){
        realloc(s, s->lenght * s->sizeOfElems);
        s->lenght *= 2;
    }

It seems like you are trying to re-allocate space if you run out, but your default value of 2 for allocated is going to stay as it is, you should probably do something like:

    if (s->allocated < s->lenght) {
        realloc(s->elems, ....)
        //Increase both allocated and length to represent the current state of the stack
    }

Notice that in the realloc() call you are passing s, which is in the stack, this won't do what you want: http://pubs.opengroup.org/onlinepubs/7908799/xsh/realloc.html As per your structure you should be passing s->elems

Also, before calling pop_elem, you are assigning NULL to elem. Inside your pop_elem, this is where you are trying to do a memcpy() to. To do this you will have to allocate memory:http://pubs.opengroup.org/onlinepubs/009695399/functions/memcpy.html

Collected from the Internet

Please contact [email protected] to delete if infringement.