You can check the reference manual for the standard C library functions.
There aren't any "interfaces" in C like you'd have in C#, Java, etc, because there are no classes in C. However, the header files that serve as interfaces, as in how to use that module/functionality.
If you're talking about C++, then there are classes. Still not "interface" types, but pure abstract classes would be the equivalent of C#/Java interface types.