shared variable mex file and workspace

i have 2 questions about passing parameters between workspace and mex function

1 - how i can share a same variable between them

2 - i have a mex function y = myfunction(a, b, c) within a for loop, and i want to evaluate at each iteration the value of y, but c is a constant and i want to passe him only in the first iteration, how i can do that (passing parameter at one time).

thanks.