Question

I’m new to C89, and it appears that you must declare a function before calling it. So this is unacceptable:

void foo() {
    bar();
}

void bar() {
    // do stuff
}

Because bar() is defined after foo() in the file. Is there any way to get around this? Do I need a header file?

Answer 1

Add a prototype:

void bar(); // prototype for function bar() which is implemented later

void foo() {
    bar();
}

void bar() {
    // do stuff
}

For projects with multiple source code files, prototypes will typically be placed in header files and included in multiple source files; the implementation need only be specified in a single source file. The compiler just needs the prototype to be able to perform proper type-checking etc.