static has a very simple logic to it. If a variable is static, it means that it is a global variable, but it‘s scope is limited to where it is defined (i.e. only visible there). For example:
- Outside a function: global variable but visible only within the file (actually, the compilation unit)
- Inside a function: global variable but visible only within the function
- (C++) Inside a class: global variable but visible only to the class
Now let‘s see what the C11 standard says regarding static and extern (emphasis mine):
6.2.2.3
If the declaration of a file scope identifier for an object or a function contains the storage-class specifier
static, the identifier has internal linkage.6.2.2.4
For an identifier declared with the storage-class specifier
externin a scope in which a prior declaration of that identifier is visible, if the prior declaration specifies internal or external linkage, the linkage of the identifier at the later declaration is the same as the linkage specified at the prior declaration. If no prior declaration is visible, or if the prior declaration specifies no linkage, then the identifier has external linkage.6.2.2.7
If, within a translation unit, the same identifier appears with both internal and external linkage, the behavior is undefined.
So the standard says that first, if you have:
static int m;
extern int m;then the second declaration (with extern) would regard the first one and in the end m would still be static.
However, in any other case, if there are declarations with both internal and external linkage, the behavior is undefined. This actually leaves us with only one option:
extern int m;
static int m;i.e., extern declaration before static declaration. gcc was nice enough to give you error in this case of undefined behavior.