What is `void` in C?

What does void mean in C? I think it’s confusing to think of it having a singular meaning. It is part of a number of unrelated syntactic forms, and should be understood separately in each:

As a return “type”. In the function declaration void foo(int), the void signifies that “foo does not return a value”.
As a parameter type list. In the function declaration char bar(void), the void signifies that “bar has zero parameters”.
As the target “type” of a pointer. In the declaration void* baz, the void signifies that “baz points to a value of unknown type”.

It is confusing to call void a “type”. There are positions where a normal type (int) can be used, but void cannot. For example, int foo; is a valid declaration, but void foo; is not. There are also positions where void can be used, but a normal type cannot. For example, char foo(int) { return 'c'; } is not a valid function definition.

The standard calls void an “incomplete type”, but I find this unenlightening in the case of char bar(void).

Tagged #c, #programming, #void, #semantics.

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