What would occur if a function in C does not have a return type?

In C, every function is expected to have a specified return type to indicate what type of value it will return to the calling code. If a function is defined without a return type, it's effectively treated as returning an "int" by default in some older C standards or by certain compilers. This can lead to undefined behavior because the function is expected to return an int but may not actually do so if there’s no return statement present.

When a function is called and it doesn't specify a return type or lacks a return statement to provide a value, the calling function might receive garbage data when trying to use that return value. This is what is meant by "unpredictable value." The state of the returned data becomes unreliable, leading to potential errors throughout your program, since the calling function might perform operations using this unexpected value.

The design of C emphasizes the importance of clarity and intentionality in function definitions, which is why specifying a return type is critical for proper function behavior and program reliability.