eval() in C

How would I write an eval() function in C? Where do I start?

>>4
Fails for not passing current variables in scope to the code specified in the parameter, and not allowing the specified code to change variables in the current scope.

i.e.
int i = 2; eval("i = 3"); printf("%d", i)
should print 3, not 2.

But this wouldn't work because you didn't even try to output the proper #include's or specify a main function.

Also fails for buffer overflow exploitability.