Calling system calls and library functions in C runtime

I know there is a way to call winapi in runtime in windows. I want to ask how can I call a system call and library functions in C in runtime (when I don't know what it can be)?
 
I didn't understand the question. What do you mean by "when I don't know what it can be"? If you call a function you should know return type and parameter types as minimum.
 
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