In order for that to work, you should define the C method like this:
void cMethod(id param);
and when you call it, call it like this:
cMethod(self);
then, you would be able to write:
[param objcMethod];
In your cMethod.
This is because the self variable is a special parameter passed to Objective-C methods automatically. Since C methods don’t enjoy this privilege, if you want to use self you have to send it yourself.
See more in the Method Implementation section of the programming guide.