Table of structures (realloc in C)答案

【问题标题】：Table of structures (realloc in C)Table of structures (realloc in C)
【发布时间】：2022-12-01 21:52:24
【问题描述】：

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;

}

【问题讨论】：

s->tablica=malloc(0);... Why malloc(0) here? Why not just s->tablica=NULL;?
element_t **tablica; Why is that ptr-to-ptr? If you do it that way, you also need to malloc each item. Try changing to element_t *tablica;
s->tablica[s->rozmiar]->nazwa = name; will not copy the string, it will copy thepointer. If you reuse the same underlying array then all pointers will be the same, and all structures will have the same pointer to the same underlying array.

标签： c structure realloc

【解决方案1】：

A big part of your problem is that tablica is an array ofpointers, 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 toundefined 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 structureobjectsinstead of pointers:

element_t *tablica;  // tablica is an array of objects, not an array of pointers

I recommend solution 2.

【讨论】：

【解决方案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 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 within the function init it will be much better ro write

void init(stos_t *s)
{
    s->rozmiar=0;
    s->tablica = NULL;
}

【讨论】：