Accessing struct array inside another struct

Question

I have a struct named id_item which has 2 elements (id and tipo). I need it this way so I can get back the information of each item at once. Then I have another struct named id_list which has an array of id_item, the total number of elements inserted on it (count) and the a number representing the out position (position).

There is a function named id_init to initialize the struct, id_insert to insert an element and id_getNext to retrieve next element.

I think I'm having problem accessing the id_item array elements inside id_list. The program compiles just fine but when I run it I don't get what I want. Code and the output is presented bellow.

I have the code bellow inside a library ".h".

#define ID_SIZE 30

typedef struct {
    uint8_t id;
    uint8_t tipo;
} id_item;

typedef struct {
    id_item modulo[ID_SIZE];
    uint8_t position;
    uint8_t count;
} id_list;

void id_init(id_list *list) {
    uint8_t i;

    for (i = 0; i < ID_SIZE; i++) {
        list->modulo[i].id = 0x00;
        list->modulo[i].tipo = 0x00;
    }
    list->position = 0;
    list->count = 0;
}

uint8_t id_insert(id_list *list, uint8_t id, uint8_t tipo) {
    if (list->count < ID_SIZE) {
        list->modulo[list->count].id        = id;
        list->modulo[list->count].tipo      = tipo;
        list->count++;
        return 0x06; /*ACK*/
    }
    else {
        // Lista cheia
        return 0x15; /*NAK*/
    }
}

void id_getNext(id_list *list, id_item *modulo) {
    uint8_t pos;

    pos = list->position++;

    if (list->position >= list->count)
        list->position = 0x00;

    modulo = &(list->modulo[pos]);
}

In the main file I do have included the previous library. After initialization I inserted 30 elements on IDs where the element id is assumed to be the value o "i" and tipo is always the integer "1" and the main code is:

int main()
{
    uint8_t i;
    id_list IDs;
    id_item item;

    id_init(&IDs);

    printf("count: %02d, Position: %02d\n", IDs.count, IDs.position);

    for (i = 0; i < ID_SIZE; i++) {
        printf("%d..\n", i);
        printf("insert: %02X\n",id_insert(&IDs, i, 0x01));
    }
    printf("\n");

    for (i = 0; i < ID_SIZE; i++) {
        id_getNext(&IDs, &item);

        printf("Position: %02d, ID: %d, Tipo: %02d\n", IDs.position, item.id, item.tipo);
    }
    return 0;
}

The program output is showed bellow:

0..
insert: 06
1..
insert: 06
2..
insert: 06
3..
insert: 06
4..
insert: 06
5..
insert: 06
6..
insert: 06
7..
insert: 06
8..
insert: 06
9..
insert: 06
10..
insert: 06
11..
insert: 06
12..
insert: 06
13..
insert: 06
14..
insert: 06
15..
insert: 06
16..
insert: 06
17..
insert: 06
18..
insert: 06
19..
insert: 06
20..
insert: 06
21..
insert: 06
22..
insert: 06
23..
insert: 06
24..
insert: 06
25..
insert: 06
26..
insert: 06
27..
insert: 06
28..
insert: 06
29..
insert: 06

count: 30, Position: 00
Position: 01, ID: 0, Tipo: 64    
Position: 02, ID: 0, Tipo: 64
Position: 03, ID: 0, Tipo: 64
Position: 04, ID: 0, Tipo: 64
Position: 05, ID: 0, Tipo: 64
Position: 06, ID: 0, Tipo: 64
Position: 07, ID: 0, Tipo: 64
Position: 08, ID: 0, Tipo: 64
Position: 09, ID: 0, Tipo: 64
Position: 10, ID: 0, Tipo: 64
Position: 11, ID: 0, Tipo: 64
Position: 12, ID: 0, Tipo: 64
Position: 13, ID: 0, Tipo: 64
Position: 14, ID: 0, Tipo: 64
Position: 15, ID: 0, Tipo: 64
Position: 16, ID: 0, Tipo: 64
Position: 17, ID: 0, Tipo: 64
Position: 18, ID: 0, Tipo: 64
Position: 19, ID: 0, Tipo: 64
Position: 20, ID: 0, Tipo: 64
Position: 21, ID: 0, Tipo: 64
Position: 22, ID: 0, Tipo: 64
Position: 23, ID: 0, Tipo: 64
Position: 24, ID: 0, Tipo: 64
Position: 25, ID: 0, Tipo: 64
Position: 26, ID: 0, Tipo: 64
Position: 27, ID: 0, Tipo: 64
Position: 28, ID: 0, Tipo: 64
Position: 29, ID: 0, Tipo: 64
Position: 00, ID: 0, Tipo: 64

Process returned 0 (0x0)   execution time : 0.201 s
Press any key to continue.

The expected output would be each id starting from 1 and going to 29 and all tipo being 1, but all id were 0 and all tipo 64 (I don't even know where this 64 came from).

It doesn't explain your problem, but do not put function definitions in header files. Prototypes, yes. Definitions (bodies), no. If you put function definitions in a header file then you cannot include the header in more than one other file without generating duplicate function definitions, which defeats the whole purpose. — John Bollinger
– John Bollinger, Commented Apr 12, 2015 at 17:01
My problem was presented on the last line of the post, an execution problem, not a compile time one. About the header file, yes.. this is not a good practice. I just did it to make my code cleaner. Thanks for the explanation anyway. I'll try not to do it rsrs — trebew
– trebew, Commented Apr 12, 2015 at 20:22

Iharob Al Asimi · Accepted Answer · 2015-04-12 17:36:46Z

4

Here

modulo = &(list->modulo[pos]);

you are making the local pointer point to the address of list->modulo[pos], when the function returns, that doesn't affect the passed variable, in fact, after that line you can't alter the passed variable anymore because you overwrite it's address in this statement.

It should be

*modulo = list->modulo[pos];

or maybe you meant

void id_getNext(id_list *list, id_item **modulo) {
    uint8_t pos;

    pos = list->position++;

    if (list->position >= list->count)
        list->position = 0x00;

    *modulo = &(list->modulo[pos]);
}

and

int main()
{
    uint8_t  i;
    id_list  IDs;
    id_item *item;

    id_init(&IDs);

    printf("count: %02d, Position: %02d\n", IDs.count, IDs.position);

    for (i = 0; i < ID_SIZE; i++) {
        printf("%d..\n", i);
        printf("insert: %02X\n",id_insert(&IDs, i, 0x01));
    }
    printf("\n");

    for (i = 0; i < ID_SIZE; i++) {
        id_getNext(&IDs, &item);

        printf("Position: %02d, ID: %d, Tipo: %02d\n", IDs.position, item->id, item->tipo);
    }
    return 0;
}

which will change the pointer, without copying the whole structure.

edited Apr 12, 2015 at 17:36

answered Apr 12, 2015 at 17:01

Iharob Al Asimi

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4 Comments

Matthew Carlson Over a year ago

That will update the whole structure, I'm pretty sure he wants to change the pointer, to loop.

trebew Over a year ago

Oh man... yes yes. I saw my mistake now. But let me see if I understood well the two solutions you presented. In the first solution, *modulo = list->modulo[pos];, I do copy the values of an array's element, an id_item from IDs.modulo[n], into variable item, from function main().

trebew Over a year ago

The second one, I pass the address of a pointer, *item, and inside the function id_getNext() I just pass the address of the id_item I want to the pointer *item from function main(). Did I explained it right? Is that it? If yes, indeed, I wanted the second solution.

Iharob Al Asimi Over a year ago

Yes that is it, I still think both explanations must be present in the anser, I am glad it worked.

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