Thread: Need help with pointer to pointer

Hello Folks,

I have been out of touch with C for a while now. Forgettin some basics. Following code is a simulator of the part of the large code that I am planning to implement. Basically here what it is, i have an array of pointers stored in a structure. I do not know as to how many pointers I will hold this array in the begining. As the code progress I go on adding pointer to the array. I keep a count of how many pointers I added to the array in the structure itself by incrementing it everytime I add a pointer to the array.
Now this is what I am doing (or atleast tryin) in the following code. It's working, but I am not sure if what I am seeing is correct. Can some one help me understand it.

Code:

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

int main()
{
    int **p = NULL;
    int **top = NULL;
    char line[50];
    int arr[10];
    int i,idx;

    idx = 0;

    while (1)
    {
        printf("Enter a number to store. -1 to end: ");
        fgets(line,sizeof(line),stdin);
        sscanf(line,"%d", &i);

        if (i == -1)
          break;
        else
          arr[idx] = i;

        if (p == NULL)
        {
            p = (int **)malloc(sizeof(int *));
            (*p) = (int *)malloc(sizeof(int));

            assert (p != NULL && (*p) != NULL);
            printf("p = %p (*p) = %p\n",p,(*p));
            (*p) = &arr[idx];
            idx++;
            top = p;
        } else {
            p += (sizeof(int **) + sizeof(int *));
            p = (int **)malloc(sizeof(int *));
            (*p) = (int *)malloc(sizeof(int));

            assert (p != NULL && (*p) != NULL);
            printf("p' = %p (*p)' = %p\n",p,(*p));
            (*p) = &arr[idx];
            idx++;
        }
    }

    printf("Entered %d elements\n",idx);

    for (i=0;i< idx; i++)
    {
        printf("p'' = %p   (*p)'' = %p  arr[%d]: %d\n", top, (*top),i,*(*top));
        top = top+ (sizeof(int **) + sizeof(int *));
    }

    return 0;
}

Following is the sample output. My question is why the values of (*p)'' are different than what we are seeing in (*p) and (*p)'.

Enter a number to store. -1 to end: 1
p = 0x8049958 (*p) = 0x8049968
Enter a number to store. -1 to end: 2
p' = 0x8049978 (*p)' = 0x8049988
Enter a number to store. -1 to end: 3
p' = 0x8049998 (*p)' = 0x80499a8
Enter a number to store. -1 to end: 4
p' = 0x80499b8 (*p)' = 0x80499c8
Enter a number to store. -1 to end: 5
p' = 0x80499d8 (*p)' = 0x80499e8
Enter a number to store. -1 to end: -1
Entered 5 elements
p'' = 0x8049958 (*p)'' = 0xbfffe7b0 arr[0]: 1
p'' = 0x8049978 (*p)'' = 0xbfffe7b4 arr[1]: 2
p'' = 0x8049998 (*p)'' = 0xbfffe7b8 arr[2]: 3
p'' = 0x80499b8 (*p)'' = 0xbfffe7bc arr[3]: 4
p'' = 0x80499d8 (*p)'' = 0xbfffe7c0 arr[4]: 5

BTW, this is an intel PC running Linux and my compiler is gcc.

Thanks,

How about this.?

I think this is what I was intending to do.

Code:

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

int main()
{
    int **p = NULL;
    char line[50];
    int arr[220];
    int i,idx;

    idx = 0;

    while (1)
    {
        printf("Enter a number to store. -1 to end: ");
        fgets(line,sizeof(line),stdin);
        sscanf(line,"%d", &i);

        if (i == -1)
          break;
        else
          arr[idx] = i;

        if (p == NULL)
        {
            p = (int **)malloc(sizeof(int *));
            (*p) = &arr[idx];
            assert (p != NULL && (*p) != NULL);
            printf("p = %p (*p) = %p\n",p,(*p));
            idx++;
        } else {
            p = (int **)realloc(p,sizeof(int *));
            p[idx] = &arr[idx];
            assert (&p[idx] != NULL && p[idx] != NULL);
            printf("p' = %p (*p)' = %p\n",(p+idx),*(p+idx));
            idx++;
        }
    }

    printf("Entered %d elements\n",idx);

    for (i=0;i< idx; i++)
    {
        printf("p'' = %p   (*p)'' = %p  arr[%d]: %d\n", (p+i), *(p+i),i,*(*(p+i)));
    }

    return 0;
}

I see some problem with the printf() output, 4th element I add to the array is not displayed correcty. Pointer offsets by one byte memory location. Is this a printf() related problem?

sample Output

Enter a number to store. -1 to end: 1
p = 0x80499d8 (*p) = 0xbfffe670
Enter a number to store. -1 to end: 2
p' = 0x80499dc (*p)' = 0xbfffe674
Enter a number to store. -1 to end: 3
p' = 0x80499e0 (*p)' = 0xbfffe678
Enter a number to store. -1 to end: 4
p' = 0x80499e4 (*p)' = 0xbfffe67c
Enter a number to store. -1 to end: 5
p' = 0x80499e8 (*p)' = 0xbfffe680
Enter a number to store. -1 to end: -1
Entered 5 elements
p'' = 0x80499d8 (*p)'' = 0xbfffe670 arr[0]: 1
p'' = 0x80499dc (*p)'' = 0xbfffe674 arr[1]: 2
p'' = 0x80499e0 (*p)'' = 0xbfffe678 arr[2]: 3
p'' = 0x80499e4 (*p)'' = 0xbfffe67d arr[3]: 83886080 <------
p'' = 0x80499e8 (*p)'' = 0xbfffe680 arr[4]: 5

Thanks for your help

Originally Posted by nkhambal

Code:

            p = (int **)realloc(p,sizeof(int *));

You'll need to change this to

Code:

        idx++;
        p = (int **)realloc(p,sizeof(int *) * idx);

Don't inc idx twice. Use [idx - 1] as the last element of the array.

Also, you _could_ loose you existing array if realloc() returns NULL.

Originally Posted by Kennedy

You'll need to change this to

Code:

        idx++;
        p = (int **)realloc(p,sizeof(int *) * idx);

Don't inc idx twice. Use [idx - 1] as the last element of the array.

Thanks. that fixed it. Should have read the man pages for realloc before.

Originally Posted by Kennedy

Also, you _could_ loose you existing array if realloc() returns NULL.

This what I get from man page "If realloc() fails the original block is left untouched - it is not freed or moved.". I think I should save the pointer to original block before calling realloc()

Originally Posted by nkhambal

This what I get from man page "If realloc() fails the original block is left untouched - it is not freed or moved.". I think I should save the pointer to original block before calling realloc()

Yup, the var is untouched. . . however, you are assigning it the RETURN value of realloc(), which, sometimes is NULL.

Originally Posted by quzah

Hence my example, which you ignored. Also, you don't need to cast any of the *alloc functions. In C you never have to cast assignments from void pointers.


Quzah.

thanks. appreciate it.