hex values to string

Hi,

the do***entation of a serial devices explains how to communicate.
I have to send the sequence:

Device adress: 0x01
Function: 0x03
Register adress: 0x0000
....
....
to a serial ****t. I use the  sio_write(int ****t, char *buf, int len) 
API-232 function

but how do i convert the this hex values (0x01, 0x03, 0x000...)
to a char *buf ?

please help