it means you pass the pointer to be initialized inside the function
mm_alloc is called from inside of mm_read.
and inside the mm_read
mm_read (&xysize, &x_matrix, &y_matrix);
/*here should be an error check but strangly enough mm_read does not return error code */
where the *xysize was previously initialized - probably from the file or user input
*x_matrix = mm_alloc( *xysize);
*y_matrix = mm_alloc( *xysize);
Show the code - it should be very straightforward
But I'm kinda screwed for mm_multiply.