malloc and two dimensional array ( char * * )

I want to allocate memory which I can use as a two dimensional array. But the following code is not working

unsigned char xdata ** memo;
unsigned char i = 0;
// n is number of rows
// size is number of columns
memo = (unsigned char xdata **) malloc( n*2 );
for ( ; i < size; i++)
	memo[i] = (unsigned char xdata *)malloc( size );

this code was inspired from
C++ new operator

memo = new char * [n];
for (int i=0; i < n; i++)
	memo[i*2] = new char [size];

Can you help me resolving this problem? Is there any problem with the logic?

PS: Fairly large memory has been allocated using init_mempool().