Question

I’m trying to make a generic stack in plain C and I have some problems with pointers and no idea where is the problem.

Here’s the structure and functions, where I have problems:

typedef struct{
    void *elems; 
    int elemSize; 
    int logLength; 
    int allocLength;
} genStack;

void GenStackPush(genStack *s, const void *elemAddr); 
void GenStackPop(genStack *s, void *elemAddr);

That’s the implementation:

void GenStackPush(genStack *s, const void *elemAddr)
{
    s->elems[s->logLength] = elemAddr;
    s->logLength++;
}

void GenStackPop(genStack *s, void *elemAddr)
{
      s->logLength--;
      elemAddr = s->elems[s->logLength];
}

The usage should look like this:

int val; 
genStack IntegerStack;
for (val = 0; val < 6; val++)
    GenStackPush(&IntegerStack, &val);

GenStackPop(&IntegerStack, &val); 
printf("Popped: %d\n",val);

And here are the problems I get:

genstacklib.c: In function ‘GenStackPush’:
genstacklib.c:60:10: warning: dereferencing ‘void *’ pointer [enabled by default]
genstacklib.c:60:2: error: invalid use of void expression
genstacklib.c: In function ‘GenStackPop’:
genstacklib.c:72:23: warning: dereferencing ‘void *’ pointer [enabled by default]
genstacklib.c:72:13: error: void value not ignored as it ought to be

I have tried already several ways to fix the code, but none of them worked.
Thanks.

==========================================================================

So, guys, thanks for help! Now it compiles, but I have changed an API, which was given by our Professor. There was also the problem with ‘const’ qualifier, so I deleted them. Not my code looks like this:

genstacklib.h:

#ifndef GENSTACKLIB_H
#define GENSTACKLIB_H
#define GenStackInitialAlocationSize 4

typedef struct{
    void** elems;
    int elemSize;
    int logLength;
    int allocLength;
}genStack;

void GenStackNew(genStack *s,int elemSize);
void GenStackDispose(genStack *s);
int GenStackEmpty(const genStack *s);
void GenStackPush(genStack *s, void *elemAddr);
void GenStackPop(genStack *s, void *elemAddr);

#endif

genstacklib.c:

#include <stdlib.h>
#include <stdio.h>
#include "genstacklib.h"

void GenStackNew(genStack *s,int elemSize)
{
    void** newElems;

    /* Allocate a new array to hold the contents. */
    newElems = (void**) malloc(elemSize * GenStackInitialAlocationSize);

    if (newElems == NULL)
    {
        fprintf(stderr, "Error with allocating the stack.\n");
        exit(1); /* Exit, returning error code. */
    }
    s->elems = newElems;
    s->allocLength = GenStackInitialAlocationSize;
    s->logLength = 0; /*is empty*/

}

void GenStackDispose(genStack *s)
{
    s->allocLength = 0;
    free(s->elems);
}

int GenStackEmpty(const genStack *s)
{
    return s->logLength == 0;
}

void GenStackPush(genStack *s, void *elemAddr)
{
    s->elems[s->logLength] = elemAddr;
    s->logLength++;
}

void GenStackPop(genStack *s, void *elemAddr)
{
      s->logLength--;
      elemAddr = s->elems[s->logLength];
}

If you have any ideas to improve it or something to say about it, I would hear with pleasure. 😀

Answer 1

you are trying to dereference void pointer without typecasting to some other type which causing the problem.