How do I use extern to share variables between source files?

Question

I know that global variables in C sometimes have the `extern` keyword. What is an `extern` variable? What is the declaration like? What is its scope?This is related to sharing variables across source files, but how does that work precisely? Where do I use `extern`?

Answer 1

it is a signal to the compiler that the global var is declared in another file somewhere else.

syntax:

extern type varname;

Here is an example:

suppose we have a pointer to a device handler, like a singleton

printer instance.

in file, printer.h we have

.....

TPrinter * The_Printer; // which is defined, declared, and instantiated via init function or constructor

.....

Now in another header file, summary_report.h, that requires

the use of the printer, we have :

extern TPrinter * The_Printer;

when the source code files and linked, the extern var declarations

in the reporting code are matched against the one in the

printer code.

---------------------------------------

If you go and look inside all of the header files *.h in the include directory,

you will find THOUSANDS of extern statements. I did a grep "extern" *.* in

the include directory and it listed thousands of them, just out of curiosity.

In stdio.h alone there are like 30+

This makes sense, there is only ONE input stream (stdin), there is only

ONE output stream (stdout) , and there is only ONE error log (stderr).

So how can these three I/O streams be shared through so many different

source code files? The answer is the extern keyword.

everytime you #include <stdio.h> , or any header for that matter, your

source code has access to those objects.

That is how to create an API (application programmer interface)

That's how it works