When to use a pointer to a pointer in C?

Question

I am reading the following function declaration.

static uint8_t prv_read(uint16_t instanceId,
                        int * numDataP, 
                        lwm2m_data_t ** dataArrayP,
                        lwm2m_object_t * objectP)

The 3rd argument lwm2m_data_t ** dataArrayP is a pointer to a pointer to the following struct.

typedef struct _lwm2m_data_t lwm2m_data_t;

struct _lwm2m_data_t
{
    lwm2m_data_type_t type;
    uint16_t    id;
    union
    {
        bool        asBoolean;
        int64_t     asInteger;
        double      asFloat;
        struct
        {
            size_t    length;
            uint8_t * buffer;
        } asBuffer;
        struct
        {
            size_t         count;
            lwm2m_data_t * array;
        } asChildren;
        struct
        {
            uint16_t objectId;
            uint16_t objectInstanceId;
        } asObjLink;
    } value;
};

I don't have much experience with pointers to pointers and my question is - is there an obvious reason why the author used a ** here? Or why would you use one?

because the function is going to allocate the struct and that `**` will receive the address of it — pm100, Nov 09 '17 at 16:27
if we could see prv_read code then we could tell you without guessing — pm100, Nov 09 '17 at 16:28

score 1 · Accepted Answer · answered Nov 09 '17 at 16:28

You use a pointer to type T when you need to add a level of indirection to an object of type T. When type T happens to be a pointer, you make a pointer to a pointer.

In your particular situation it appears that the function needs to return several things at once. A common idiom in C for functions providing several results is to pass pointers to results into the function, and let the function set them. Your function provides three results:

The number of items in dataArrayP, through numDataP,
An array of data, through dataArrayP, and
An lwm2m object, through objectP

All three results are returned by setting pointers inside the function. The second result happens to be a pointer, i.e. lwm2m_data_t *. In order to return a pointer by setting a pointer, a double-pointer must be passed:

int numData;
lwm2m_data_t *dataArray;
lwm2m_object_t object;
uint8_t status = prv_read(instanceId, &numDataP, &dataArray, &object);

Note: Another common use for double-pointers is defining jagged arrays.

When to use a pointer to a pointer in C?

1 Answers1