Working with array of structures in C using pointer to pointer

Question

I am trying to learn better how pointer work in C and pointer to pointer to structure things broke my mind. Here I have 3 questions:

If I have a structure and a pointer to pointer **tests to this structure (so this is something like an array) how can I use p to access to array members (the best way)? tests[i]->id fails (example of source is below)
Having that monstrous line with pointers below is something bad to have in the code but I would like to do work with it as well. I think I've mistaken with its template, the output for every next structure's address looks like jumps 32 bytes ahead, while size of structure is only 4 bytes. So in the line I want to (1) take the initial address of the tests top pointer, (2) add to it size of the structure TestCase multiplied by i so it points to correct array element and (3) add offset of the id field of the structure. And after that I will get address where I can write the id value in memory, right? Am I doing this correct?

Why *tests value was changed? Buffer overflow?

struct TestCase{
    int id;
};
int main()
{
    TestCase ** tests;
    cout << "Size of TestCase: " << sizeof(TestCase) << endl;
    *tests = (TestCase*)malloc(sizeof(TestCase*)*5);
    cout << "*tests = " << *tests << endl;
    for(int i = 0; i < 5; i++)
    {
        //*(int*)(tests+sizeof(TestCase)*i+(int)&(((struct TestCase*)NULL)->id)) = i;

        int addr = tests; // address of structures array in memmory;
        addr += sizeof(TestCase)*i; //address of current structure;
        addr += (int)&(((struct TestCase*)NULL)->id); // Adding id parameter offset in memory to current address
        *(int*)addr = i; // setting id for current structure equal to i
        cout << (int*)(tests+sizeof(TestCase)*i+(int)&(((struct TestCase*)NULL)->id)) << endl;
    }
    cout << "*tests = " << *tests << endl;
    return 0;
}

Output is:

Size of TestCase: 4
*tests = 0x600048600
0x23ab90
0x23abb0
0x23abd0
0x23abf0
0x23ac10
*tests = 0x600000000

P.S.: Updated cycle code from one monstrous line to step by step actions.

Sorry, I can't read that line of code, thus can't understand what the intent is. Thus I can't help you figure out a better way of writing it! Please explain what the intent of the code is. — Oliver Charlesworth
– Oliver Charlesworth, Commented Nov 19, 2014 at 20:51
I asked not about cout, it was used just for debugging, @cacho Or do you think it adds something different to the source logic? — Artur Korobeynyk
– Artur Korobeynyk, Commented Nov 19, 2014 at 20:53
*tests = (TestCase*)malloc(sizeof(TestCase*)); Right here you already are dereferencing an uninitialized pointer. — JS1
– JS1, Commented Nov 19, 2014 at 20:58
Instead of putting everything on two long lines of code, why not break it into smaller steps, storing the intermediate results. — Neil Kirk
– Neil Kirk, Commented Nov 19, 2014 at 21:07

user3386109 · Accepted Answer · 2014-11-19 22:30:19Z

Assuming that you want tests to be an array-of-pointers-to-struct-TestCase with 5 pointers to 5 structs, then you need to

allocate an array of 5 pointers, and
initialize each pointer by allocating memory for the struct

Like this:

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

struct TestCase
{
    int id;
};

int main( void )
{
    int i;
    struct TestCase **tests;

    // allocate memory for an array of 5 pointers
    tests = malloc( 5 * sizeof(struct TestCase *) );
    if ( tests == NULL )
        exit( 1 );

    // allocate memory for 5 structures (and save each pointer in the array)
    for ( i = 0; i < 5; i++ )
    {
        tests[i] = malloc( sizeof(struct TestCase) );
        if ( tests[i] == NULL )
            exit( 1 );
    }

    // initialize the id in each struct
    for ( i = 0; i < 5; i++ )
        tests[i]->id = i;

    // print the id in each struct
    for ( i = 0; i < 5; i++ )
        printf( "%d\n", tests[i]->id );

    // free memory
    for ( i = 0; i < 5; i++ )
        free( tests[i] );
    free( tests );

    return 0;
}

I wasn't able to reproduce the problem described in point 2. Here's the code I tried (which I placed ahead of the code that frees memory)

printf( "%p\n", tests );
for ( i = 0; i < 5; i++ )
{
    intptr_t addr = (intptr_t)tests;
    addr += sizeof(struct TestCase)*i;
    addr += (int)&(((struct TestCase*)NULL)->id);

    printf( "%p\n", (void *)addr );
}

The output from the code is

0xc03950
0xc03950
0xc03954
0xc03958
0xc0395c
0xc03960

The address advances by 4 bytes as expected. Note that I changed the type of addr to intptr_t to guarantee that the pointer can be cast properly, but that doesn't effect the math in the subsequent two lines.

Thanks for solution with tests[i]->id. Now I see my problem. Points 1 and 3 are answered by this, but still I want to do something with 2. This line looks so awesome to me, that it looks like code obfuscation by itself ) Updated that line to 3 lines in step by step instructions as people asked in comments.
@ArturKorobeynyk I wasn't able to duplicate the problem where the address jumps by 32 bytes. I've updated the answer with an explanation.
Well, does not matter already. Thanks dude! Thanks to your explanation I was able to deal with point 2. I posted the solution below. You saved my day )

Rubley Rionders · Accepted Answer · 2014-11-19 21:18:42Z

1

In your code tests is triple *, remove the additional * from the malloc line and test[0]->id assignments will work.
If you need a high level access to an offset, assign values as you initially wanted to in question 1. IE - tests[0]->id = 0x3834 , or memcpy for dynamic assignments. There´s no need for all those references and casts.
Because tests was reassinged in the first line of your iterator.

answered Nov 19, 2014 at 21:18

Rubley Rionders

111 bronze badge

1 Comment

Artur Korobeynyk Over a year ago

Thanks for answer, but as for 2. that was the point, to make that line work ) Just for fun

Artur Korobeynyk · Accepted Answer · 2014-11-19 22:42:30Z

Thanks to @user3386109 I was able finally to do everything I wanted. Well, instead of writing to address using creepy formula I am now reading from address using the same creepy formula:

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

    using namespace std;

    struct TestCase{
        int ida;
        int idb;
    };

    int main()
    {
        TestCase ** tests; //create pointer to pointer
        cout << "Size of TestCase: " << sizeof(TestCase) << endl;
        tests = malloc(sizeof(TestCase*)*5);
        if(tests == NULL)
            exit(1);
        for(int i=0; i< 5; i++)
        {
            tests[i] = malloc(sizeof(struct TestCase));
            if(tests[i] == NULL)
                exit(1);
        }
        cout << "*tests = " << *tests << endl;
        for(int i = 0; i < 5; i++)
        {
            tests[i]->ida = i;
            tests[i]->idb = i+6;
            cout << std::hex << &(tests[i]->idb) << ": " << *(long*)((long)(long*)(tests[i])+(long)&(((TestCase *)NULL)->idb)) << endl;
        }
        for(int i=0; i<5; i++)
        {
            free(tests[i]);
        }
        cout << "*tests = " << *tests << endl;
        free(tests);
        return 0;
    }

Output:

    $ ./a.exe
    Size of TestCase: 8
    *tests = 0x600048630
    0x600048634: 6
    0x600048654: 7
    0x600048674: 8
    0x600048694: 9
    0x6000486b4: a
    *tests = 0x600048630

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