problems with c and extern

[alaios]

Hi i have some questions in c... If u can help me :)

a)I am trying to understand the extern.. Today i have faced the following problem
int main(){
int isvisible;
creates-1-thread();
creates-2-thread();
creates-3-thread();
}
I have written the code for each function in a separate file
creates-1thread.h etc
In each function i use the extern int isvisible; but still the compiler says that the variable is not declared :(
When i have changed the position of isvisible and put it below int main() everything got smoothly
int isvisible
int main(){
...
}

Why??? :(

[aman]

Any variables you declare inside a function are local to that function. Variables declared outside of that function are global for that file, and can be accessed by other files by using the 'extern' keyword.

[alaios]

Thx but threads usually share the same environments and the same variables... Thats why i cant understand why i should declare this variabla outside of the main fuinction and not inside

[aman]

Well it's the same idea as calling another function from within your function. Function 2 has no idea what variables there are inside of function 1, just like thread a,b, or c has no idea what variables are inside of a function. Any variables you want to access in another function (or thread) need to either be passed to that function, or declared in global scope.

void functiona()
{
int a;

functionb();

pthread( callback );
}

void functionb()
{
a = 10; <-- functionb has no clue what variable 'a' is
}

int callback()
{
a = 10; <-- this thread's callback has no clue what variable 'a' is

return a;
}