How access assembler bit variable from C program?

I have in assembler code define one bit variable:

BSEG       AT      21h
BUS_GRESKA: DBIT 1

how I can access this bit in C code ? I try with

extern bit BUS_GRESKA

and then

BUS_GRESKA=1;

or

BUS_GRESKA=0;

but not work?Why?

Thank you