How do you solve undefined references error in c?

Linker errors are many and varied. Without knowing the error it is impossible to say how you solve it.

Most linker errors relate to undefined symbols. For instance, if you declare a function but do not implement it, the function is undefined. This can also happen when you include a function library header but do not link to the function library containing the definitions. The compiler uses the header declarations to ensure all calls to the functions are valid even if the definition hasn't yet been compiled. It is the linker's job to ensure all declarations are defined. The only exception is templates which must be defined before they are used. This is why all template definitions are placed in the header alongside the declarations.