C: why the pointer type not compatible?

I have compilation error of "incompatible pointer type" with the following code.

f(unsigned char** arr)
{
   unsigned char* p = *arr;
}

int main()
{
   unsigned char a[31];
   f(&a);
}

I would like to understand why and how to fix.

Edit

Sorry, I omit one point.

When I compile with gcc, it is only a warning, but g++ raises it as error.

cannot convert ‘unsigned char (*)[31]’ to ‘unsigned char**’`

I want to fix the error when using g++.

>Solution :

a is an array of 31 unsigned char (unsigned char[31]).

&a is a pointer to such an array (unsigned char (*)[31]).

You’re trying to "assign" this to arr, which is a pointer to a pointer to an unsigned char.

&a does not point to a pointer. &a is not compatible with unsigned char **.

Usual approach: (Pointer to first element)

void f( unsigned char *arr ) {
   unsigned char *p = arr;
}

int main( void ) {
   unsigned char a[31];
   f( a );  // Short for `f( &(a[0]) )`.
}

Alternate approach: (Pointer to the array)

void f( unsigned char (*arr)[31] ) {
   unsigned char *p = *arr;    // Short for `p = &((*arr)[0])`.
}

int main( void ) {
   unsigned char a[31];
   f( &a );
}