about Declaration and Definition

[wind0965]

1. in C, we define a global variable in *.c file, e.g.:
int myval;
and then use "extern" to declare it as a global one in *.h:
extern int myval;

But what if I define it direct in *h as:
int myval;
and use it in my *c file?
What is the difference?


2. What is more, "static" variable in a *c file scope means it is not visible outside this file. e.g.:
static int myval;
But what happens if I declare it in *.h file as "extern":
extern int myval;


3. What is the difference in the following situations(definition in *.c file and never declared in *.h file):
int myval;
and
static int myval;


Thank you in advance!!

[wind0965]

Could someone help me?

[TheCPUWizard]

1) See what happens when you include the .h in two .c files [note there should never be a reason to create a .h file unless it is going to be included in at least two .c files

2) That "should" be a compile error.

3) See what happens if you declare the same variable in another .c file (both with and without the static.