Can we define functions in structs in C programming language?
No, You cant define functions inside structures in C programs, However if the extension of your file is .cpp (that is not C), you can have member functions like classes however the default modifier of these functions will be 'public'(unlike class).
Read these links for more information on Structures a good link , another good link, One more good link
As a convention in C++, Classes are used for storing functions and variables both and Structures are used only for storing information (i.e. data).