True global variables?

[someprogr]

I basically have a couple C files and a couple header files. In one header file, I have the declaration of a struct... Following that is a declaration of a variable of that struct type

Example (myHeader.h):

struct myStruct
{
    int a;
    int b;
    char c;
};

struct myStruct data1;

My question is, how do I make it so that when a .c file "includes" myHeader.h, it shares the same data1 with any other C file that included it?

[laserlight]

Declare data1 as extern in the header file, then define it in one of your source files.

You might want to choose a much better name than data1, of course, given the pitfalls of global variables.

[someprogr]

Declare data1 as extern in the header file, then define it in one of your source files.

You might want to choose a much better name than data1, of course, given the pitfalls of global variables.

Whats the difference between doing that OR in my header doing static instead of extern? If I do static, then I don't have to declare it in my C file... Whats the difference?

[FillYourBrain]

static would make it a different instance for all 'c' files that include it

[FillYourBrain]

extern basically allows a source file access to another source file's global. static doesn't do that, it includes a new instance in each source file.

[someprogr]

Why would I have to declare a variable again in a C file when I already made it extern in a header file? I don't understand that part... if a file includes the header already, shouldn't it be global to whoever includes it? Do I need to keep declaring that variable in every C file to share it? Remember, I only want one instance throughout all C files.

[tabstop]

Extern just means "this variable exists somewhere else". If you don't actually create it at some point, everybody just ends up playing hot potato and the variable can't be found (since it doesn't, you know, exist).

[laserlight]

Why would I have to declare a variable again in a C file when I already made it extern in a header file?

That "extra" declaration is the definition of the variable, without which all you have is a variable name without storage.

Do I need to keep declaring that variable in every C file to share it? Remember, I only want one instance throughout all C files.

Yes, but again extern should be used to get what you want.

[FillYourBrain]

the variable has to exist in actual source code. Which means its compiled into one of your ".o" files. When you put it in a header, you are copying it into whatever source files use the header. You can only do this once or it will mean the same variable is declared again and linking will not work.

so extern is declaring, but not defining the variable. It's just telling others about this variable that exists in some source file.

[FillYourBrain]

My last post was convoluted. Easy description: extern doesn't define the variable. It just tells others about it.