使以下代码合法的'extern'关键字是什么？

Question

Why does the following code compile? when is the 'num' variable getting its value?

#include <stdio.h>

extern int num;

void main()
{
    printf("%d", num); //prints 3
}

int num = 3;

Example: Live Code

Answer 1

In your code,

 extern int num;

is called a declaration . OTOH,

int num = 3;

is the definition with explicit initialization. This statement sets the value. This value is decided at the compile time and set just before main() starts it's execution.

That said, void main() should be int main(void) , at least to conform to the standard.

Answer 2

It works because you've declared num so it can be named in the function's code, and because you've defined it at namespace scope so that it's initialised at static-initialisation time, which is just before main is executed.

In that sense, your program as currently written is largely indistinguishable from the following:

#include <stdio.h>

int num = 3;

void main()
{
    printf("%d", num); //prints 3
}

"Where" you initialise the variable isn't particularly relevant, as long as that occurs in time before you try to use it.

By the way, you must make main return int , not void .

Answer 3

extern marks a declaration of a variable that's defined later. The definition, the int num = 3; part, is what actually allocates the memory for the variable and sets it's value (it also doubles as a declaration). C/C++ are declare-before-use, if you didn't do that extern int num; then num wouldn't be declared at the point you use it. You could also of course drop the extern line and move the definition of num up to above main() .

There's two things that come into play here. During compile, num has to be declared at a point in the source file before any use. As for when it gets it's value assigned, that comes during program loading. All variables at file scope (outside any functions) like num have their memory allocated and their values if any initialized before main() is called. If they aren't given a value in their definition they may contain anything, so don't make any assumptions about the value of uninitialized variables. You also can't make any assumptions about the order they're initialized in, so don't refer to other file-scope variables when initializing a variable like this.

Answer 4

extern is used with the declaration. It tells the compiler that the thing will be defined elsewhere. It's useful when you have multiple files using the same variable or function. You would put an extern declaration in a header file, and define the shared variable/function in a source file.

If you remove extern from your code you will be defining num twice, which isn't allowed. To get rid of the extern declaration, just put your int num = 3; at the top of your file.