Table of structures (realloc in C) - c

Question

i am trying to write a code in C but i am having some problems with realloc. I had to write a code that will create a stack, and will add to it (dodaj_do_stosu), reamove from it (usun_ze_stosu) and will look at the top thing that is on this stack. I have problem with compiling(it does work for first two words but then it returns (0xC0000374)).
I think i am usining the realloc wrong and the sizeof my structure. If someone could look at my code (especially at the function (dodaj_do_stosu) and tell me what am i doing wrong thx. My code look like this:
#include <stdio.h>
#include <stdlib.h>
#include <time.h>
typedef struct {
int n;
char *nazwa;
}element_t;
typedef struct {
int rozmiar;
element_t **tablica;
}stos_t;
void top_of_stack(stos_t *s){
printf("ostatni element stosu:rozmiar = %d nazwa=%s, n=%d\n", s->rozmiar, s->tablica[s->rozmiar]->nazwa, s->tablica[s->rozmiar]->n);
}
void init(stos_t *s)
{
s->rozmiar=0;
s->tablica=malloc(0);
}
void dodaj_do_stosu(stos_t *s, int n, char *name)
{
s->tablica = realloc(s->tablica, (s->rozmiar + 1) * sizeof(s->tablica));
s->tablica[s->rozmiar]->nazwa = name;
s->tablica[s->rozmiar]->n = n;
printf("rozmiar=%d, n=%d , nazwa=%s\n",s->rozmiar, s->tablica[s->rozmiar]->n, s->tablica[s->rozmiar]->nazwa);
s->rozmiar++;
}
void usun_ze_stosu(stos_t *s)
{
s->tablica = realloc(s->tablica, (s->rozmiar - 1) * sizeof(s->tablica[0]));
s->rozmiar--;
}
void rm(stos_t s)
{
free(s.tablica);
}
int main(int argc, char **argv)
{
stos_t s;
init(&s);
int i;
srand(time(0));
if (argc>1)
for(i=1;i<argc;i++){
printf("%s\n", argv[i]);
dodaj_do_stosu(&s, rand() % 10, argv[i]);
}
for(i=0;i<argc-1;i++){
//printf("i=%d, n=%d, nazwa=%s\n",i, s.tablica[i].n, s.tablica[i].nazwa);
}
//top_of_stack(&s);
//usun_ze_stosu(&s);
//top_of_stack(&s);
rm(s);
return 0;
}

Answer 1

A big part of your problem is that tablica is an array of pointers, but you never initialize the pointers themselves.
The dodaj_do_stosu function reallocates the array, but doesn't create the element_t objects. Therefore any dereference of e.g. s->tablica[s->rozmiar] will lead to undefined behavior.
There are two possible solutions:
Allocate a new element_t structure:
s->tablica[s->rozmiar] = malloc(sizeof(element_t));
before you initialize the element_t structure members.
Make tablica an array of structure objects instead of pointers:
element_t *tablica; // tablica is an array of objects, not an array of pointers
I recommend solution 2.

Answer 2

At least the function dodaj_do_stosu is wrong. The data member tablica is declared like
element_t **tablica;
So the expression s->tablica[s->rozmiar] has the type element_t * and an indeterminate value. Thus dereferencing the pointer expression for example like
s->tablica[s->rozmiar]->nazwa
invokes undefined behavior.
You have to allocate memory for objects of the structure type element_t not for pointers of the type element_t *.
So you need to declare the data member like
element_t *tablica;
and within the function to write
s->tablica = realloc(s->tablica, (s->rozmiar + 1) * sizeof( *s->tablica));
Also it is safer to use an intermediate pointer for calls of realloc.
The function can look the following way
int dodaj_do_stosu( stos_t *s, int n, char *name )
{
element_t *tmp = realloc( s->tablica, ( s->rozmiar + 1 ) * sizeof( *s->tablica ) );
int success = tmp != NULL;
if ( success )
{
s->tablica = tmp;
s->tablica[s->rozmiar]->nazwa = name;
s->tablica[s->rozmiar]->n = n;
printf("rozmiar=%d, n=%d , nazwa=%s\n", s->rozmiar, s->tablica[s->rozmiar]->n, s->tablica[s->rozmiar]->nazwa );
++s->rozmiar;
}
return success;
}
Consequently the function should be redefined at least the following way. As is it can for example invoke undefined behavior when s->rozmiar is equal to 0.
int usun_ze_stosu( stos_t *s )
{
int success = s->rozmiar != 0;
if ( success )
{
element_t *tmp = realloc( s->tablica, ( s->rozmiar - 1 ) * sizeof( *s->tablica ) );
success = tmp != NULL;
if ( success )
{
s->tablica = tmp;
--s->rozmiar;
}
}
return success;
}
Also within the function init it will be much better ro write
void init(stos_t *s)
{
s->rozmiar=0;
s->tablica = NULL;
}
Another problem is the function rm
void rm(stos_t s)
{
free(s.tablica);
}
You should pass the original object through a pointer to it and within the function to write
void rm(stos_t *s)
{
free( s->tablica );
s->tablica = NULL;
s->rozmiar = 0;
}