Each C function in a program is on equal footing with the others. Each can call any other function or be called by any other function. This makes the C function somewhat different from Pascal and Modula-2 procedures because those procedures can be nested within other procedures. Procedures in one nest are ignorant of procedures in another nest.
Isn't the function main() special? Yes, it is a little special in that when a program of several functions is put together, execution starts with the first statement in main(), but that is the limit of its preference. Even main() can be called by itself recursively or by other functions, although this is rarely done.