Trying to Assign Pointer out of a struct to a two-dimensional array

Question

have an assignment where I should use the following struct:

typedef struct natset{
        unsigned long min;
        unsigned int size;
        unsigned char* bits;
}natset_t;

Now I have to assign that pointer to the following two-dimensional array of unsigned chars:

unsigned char bArrayRoll[7][byteNum];

Which I have tried doing like so:

unsigned long min = 20;
unsigned int size = 20;
unsigned char bArrayRoll[7][byteNum];

natset_t newNatset;
newNatset.min = min;
newNatset.size = size;
newNatset.bits = &bArrayRoll;

Variations of that newNatset.bits which I have tried include:

newNatset.bits = bArrayRoll;
newNatset.bits* = &bArrayRoll;

However, the compiler either returns "Assignment from incompatible pointer type" or "Initialization from incompatible pointer type".

How can I assign the bits pointer to this array correctly?

Answer 1

newNatset.bits* = &bArrayRoll;

You are trying to multiply newNatset.bits with the address of bArrayRoll , this is undefined behavior.

newNatset.bits = bArrayRoll;

This one here is trying to assign unsigned char (*)[byteNum] to unsigned char* which is again something about which compiler complained. Types are incompatible clearly.

Correct one would be (correct one as per the compiler)

newNatset.bits = bArrayRoll[0];

wouldn't it have to be: newNatset.bits = bArrayRoll[0][0]?

This is once again a type incompatible assignment. You are assigning unsigned char to unsigned char* . You can do this though,

newNatset.bits = &bArrayRoll[0][0];

Trying to Assign Pointer out of a struct to a two-dimensional array? (The thing you wanted).

Then you need to do this,

  unsigned char (*p)[byteNum] = bArrayRoll;

but in the structure you have declared bits to be of type unsigned char* not of type unsigned char (*)[byteNum] .

Well if you didn't understand what happened when we assigned in the last case and how did the types match - then here is the key idea.

The 2d array decays into the pointer to the first element - which is a single dimensional array of byteNum unsigned characters. Now we are assigning it to p which is again

unsigned char (*)[byteNum]
               ^ ^
               1 2
\-----------/ 
        3

Pointer to (1) an array of bytenum (2) unsigned characters (3).