Memory allocation for string array with malloc()

Question

This code below asks for a num for the amount of strings I want to store in vetor which in this case is the pointer char *strings[num].

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

int main()
{
    int num = 0;
    char *strings[num];
    void string_alunos(char *strings[], int num);

    printf("Quantas strings voce deseja armazenar? ");
    scanf("%d", &num);
    printf("\n");

    printf("Digite as palavras:\n");
    for (int i = 0; i < num; i++)
    {
        strings[i] = (char *)malloc(sizeof(char) * 30);
        if (strings[i] != NULL)
        {
            fflush(stdin);
            scanf("%30[^\n]", strings[i]);
        }
        else
        {
            printf("*** Não foi possível alocar memoria! ***");
            exit(1);
        }
    }
    string_alunos(strings, num);
    free(strings);

    return 0;
}
void string_alunos(char *strings[], int num)
{
    printf("\n::: PALAVRAS :::\n");
    for (int i = 0; i < num; i++)
    {
        printf("%s\n", strings[i]);
    }
}

When using the malloc() i want a maximum string of 30 characters. With malloc() in theory it should reserve 30 bytes in memory. right? Or not? Anyway, I circled the debug mode, and the positions between strings give a total of 56 bytes of difference between one and the other, why does this happen? There should be a difference of 30 bytes?

Answer 1

The main problem is that you are initiating the array 0 then will only corrupt the memory by putting things where nothing has been reserved. Sending it to boot after you know how big it should be.

There is a problem that is not serious in such an exercise, but since it was to make the release then release something correct. strings was never allocated with malloc() so there’s no point in giving free() but all elements of it were initialized with malloc() then it must give free() in all. I would not even have created the function to write on the screen, because then had to make 2 loops without need.

Still had an error in data entry formatter which I fixed.

And I’ve added an extra character to the terminator of string, otherwise would give problem in some cases.

I took some things that do not make sense or it is innocuous or even wrong to do. You must have learned to do so in bad sources.

I didn’t optimize to avoid the malloc(). Since the size of string is known could allocate everything in the same stack (although it would be good to validate the maximum), since it started doing this, and avoid the malloc() which would simplify the code, but I’m not sure what the goal is. Anyway it would make more sense everything in automatic memory or everything in dynamic memory.

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

void string_alunos(char *strings[], int num) {
    printf("\n::: PALAVRAS :::\n");
    for (int i = 0; i < num; i++) printf("%s\n", strings[i]);
}

int main() {
    int num;
    printf("Quantas strings voce deseja armazenar? ");
    scanf("%d", &num);
    char *strings[num];
    printf("\n");
    printf("Digite as palavras:\n");
    for (int i = 0; i < num; i++) {
        strings[i] = malloc(31);
        if (strings[i] != NULL) scanf("%30s[^\n]", strings[i]);
        else {
            printf("*** Não foi possível alocar memoria! ***");
            exit(1);
        }
    }
    string_alunos(strings, num);
    for (int i = 0; i < num; i++) free(strings[i]);
}

Behold working in the ideone. And in the repl it.. Also put on the Github for future reference.