How do I cast a char* into a structure?

I mean that I have for example the following structure

struct myStruct
{
char sFirstParam[6];
char sSecondParam[8];
char sThirdParam[4];
char sForthParam[2];
};

and I need a char* holding the string "12345612345678123412"
I need sFirstParam to hold the first 6 chars 123456
I need sSecondParam to hold the first 6 chars 12345678
I need sThirdParam to hold the first 6 chars 1234
I need sForthParam to hold the first 6 chars 12

How do I do it without copying (memcpy for example)
I need to do it by casting

any idea how?

really thanks