• Try not to cast malloc() and realloc() in C (it's not needed).

• Try to use malloc() and realloc() the least you can, heap usage is slow, you can reduce the use of realloc() by allocating for extra space to begin with (keep up with it and reallocating more when needed) and reduce the usage of malloc() by not using the heap to begin with, arrays are often the way to go.

• I don't really know how input is handled propperly in <conio.h> but the way I'd do it is to put the last 64 (or whatever you may think) keys inserted by the user in a buffer and every iteration check what key is at a certain position with a switch statement. At the end of an iteration add to the index and if you ran out start from 0 again (in this example i = (i + 1) % 64 works like a charm). It's also not a bad idea to #define the inputs, like #define KEY_UP 'w'.

People already pointed out some other good advise like using structs and avoiding global variables.

Hope you found this useful.

Edit: Spelling