What is and isn't available to linked files in C?

Question

As of late, I've been trying to work through opaque pointers as a programming concept and one of the main things I've had difficulties with is figuring out what is or isn't available to other files. In a previous question, I failed in trying to create an opaque pointer to a struct and even though the answer explained how to fix that, I still don't quite understand where I went wrong.

I think that if a struct is defined in file2.c, file1.c can use it if both files include header.h which includes a declaration of the struct? That doesn't entirely make sense to me. header.h is used by both files, so I can see how they would access the stuff in it, but I don't understand how they would use it to access each other.

When I started programming, I thought it was pretty straight forwards, where you have program files, they can't access anything in each other, and those program files can #include header files with definitions and declarations in them (e.g. file1.c has access to variables/functions/etc. defined in header.h). Turns out I was wrong and things are quite a bit more complicated.

So from what I can tell, func() defined in header.h can be used by file1.c without being declared in file1.c, if file1.c includes header.h. As opposed to var defined in header.h which needs to be declared in file1.c with the extern keyword? And I think if var is defined in file2.c, file1.c can use it if it extern declares it, even if neither file1.c nor file2.c include header.h?

I apologize if the previous paragraphs makes no sense, I'm having quite a bit of difficulty with trying to describe something that confuses me. By all means, please edit this if you are able to fix mistakes or whatnot.

Books and webpages don't seem to help at all. They end up giving me misconceptions because I already don't understand something and draw the wrong conclusions, or they bring up concepts that throw me off even more.

What I'm looking what I'm looking for is an answer that lays this all down in front of me. For example 'this can access this under these circumstances', 'this cannot access this'.

Answer 1

Your question asks about “access” at several points, but I do not think that is what you mean to use. Any object or function can be accessed (for an object: read or written, for a function: called) from anywhere as long as a pointer to it is provided in some way). I think what you mean to ask is what names are available.

Any declaration that is outside of a function is an external declaration. In this use of “external” in the C standard, it simply means outside of a function. (That includes a function declaration or definition; although it is declaring or defining a function, it is not inside itself or any other function declaration, so it is outside of any function.)

Any identifier for an object or function with an external declaration has either internal linkage or external linkage. If it is first declared with static, it has internal linkage (and may be later declared with extern, but that will not change the linkage). Otherwise, it has external linkage.

Any identifier with external linkage will refer to the same object or function in all translation units (provided other rules of the C standard are satisfied—a program can do various things that will result in behavior not defined by the C standard).

Thus your answer is: The name of any object or function that is (a) defined outside of any function and (b) not initially declared with static is available to be linked to from other translation units.

Some technicalities that may be of interested:

• What people think of as a variable is two things: an identifier (the name) and an object (a region of memory that stores the value).
• Identifiers have scope, which is where they are visible in the source code. Identifiers declared outside functions have file scope; they are visible for the rest of the translation unit. Identifiers declared inside functions have various other types of scope: function scope, function prototype scope, and block scope.
• You may sometimes seem people refer to global scope or external scope, but these are misnomers; they are not terms used in the C standard.
• Linkage is related to scope and is sometimes confused with it, but linkage is a different concept: Two identical identifiers declared in different places can be made to refer to the same thing. Those identifiers have different scopes, notably one having file scope in one translation unit and the other having file scope in a different translation unit. Since each translation unit is compiled separately, the compiler generates code regarding each identifier separately. When the object modules are linked, then the code is bounded together, causing the separate identifiers to refer to the same object or function.
• Identifiers can be declared with extern inside functions, but these can only link to objects or functions defined elsewhere; external definitions cannot appear inside functions.

Answer 2

Functions defined in one .c file can use anything defined in another .c file except for those things which are marked as static. Functions and global variables which are marked as static cannot be accessed from other translation units.

Whether something is declared in a header file or not doesn't really matter--you can declare functions locally in the same .c file which calls them if you want.