End of integer/structure array?

Question

Consider the following functions
void alloco(int **ppa)
{
    int i;
    printf("inside alloco %d\n",ppa); /*this function allocates and fills 20 * sizeof(int) bytes */
    *ppa = (int *)malloc(20 * sizeof(int));
    /*fill all 20 * sizeof(int) bytes */
}

int main()
{
    int *app = NULL;
    int i;
    printf("inside main\n");
    alloco(&app);
    for(i=0;i<20;i++) /*ISSUE::how will i know to traverse only 20 indexes?*/
    printf("app[%d] = %d \n", i, app[i]);
    return(0);
}

Basically how will main() come to know number of bytes to traverse i.e memory allocated by alloco() function. Is there any delimiter like NULL in character arrays?

Answer 1

That is not possible, you need to keep that value somewhere, for example you could do this,

void alloco(int **ppa, int count)
{
    int i;

    printf("inside alloco %d\n",ppa);
    *ppa = malloc(count * sizeof(int));
    if (*ppa == NULL)
        return;
    for (i = 0 ; i < count ; ++i)
        /* fill it here. */
}

int main()
{
    int *app;
    int  i;
    int  count;

    count = 20;
    app = NULL;
    printf("Inside main\n");
    alloco(&app, count);
    if (app == NULL)
        return -1;    
    for (i = 0 ; i < count ; i++)
        printf("app[%d] = %d \n", i, app[i]);
    /* done with `app' */
    free(app);
    return 0;
}

Many other combinations could work, for example

int alloco(int **ppa)
{
    int i;

    printf("inside alloco %d\n",ppa);
    *ppa = malloc(20 * sizeof(int));
    if (*ppa == NULL)
        return;
    for (i = 0 ; i < count ; ++i)
        /* fill it here. */
    return 20;
}

int main()
{
    int *app;
    int  i;
    int  count;

    printf("Inside main\n");

    app = NULL;
    count = alloco(&app);
    if (app == NULL)
        return -1;    
    for (i = 0 ; i < count ; i++)
        printf("app[%d] = %d \n", i, app[i]);
    /* done with `app' */
    free(app);
    return 0;
}

But I personally don't like this because if there is going to be a fixed number of integers it's not a good idea to use malloc() just,

int main()
{
    int  app[20];
    int  i;

    printf("Inside main\n");    
    for (i = 0 ; i < sizeof(app) / sizeof(app[0]) ; i++)
        printf("app[%d] = %d \n", i, app[i]);
    return 0;
}