- Scope of Global variables
in C:
- A global variable is
accessible (= "visible") in
any C program file:
- Starting
from the
location
in the file where the
global variable
is
defined or
declared
- Until the end of the file
- Starting
from the
location
in the file where the
global variable
is
defined or
declared
- A global variable is
accessible (= "visible") in
any C program file:
- Example:
the highlighted area is
the scope (= accessible area)
of the global variable x
in the program files global1.c
and global2.c
Program file global1.c Program file global2.c void f4() { extern int x; /* global var x is declared in scope */ x = 4; // Access x in file global2.c !!! } void f3() { x = 4; // *** Access error ! // x is not defined nor declared } extern int x; /* global var x is declared in scope */ void f2() { x = 4; // Access x in file global2.c !!! }void f1() { x = 4; // *** Access error ! // x is not defined nor declared } int x; /* global var x is defined HERE */ void f2() { x = 4; // OK }
- Example Program:
(Demo above code)
- Prog file: click here
How to run the program:
- Right click on link and
save in a scratch directory
- To compile:
gcc -c global1.c
- Result:
global1.c: In function 'f1': global1.c:(line) 5: error: 'x' undeclared (first use in this function)
- C programming advice:
- Always define/declare the global variables at the top of each C program file !!!
This will make global variables accessible in every function in the program file !!!
- A static global variable is
accessible (= "visible"):
- Starting
from the
location
in the file where the
global variable is
defined or
declared
- Until the
end of the file
- Limited to only the program file where the static (global) variable has been defined
- Starting
from the
location
in the file where the
global variable is
defined or
declared
- Example:
File 1: global4a.c File 2: global4b.c #include <stdio.h> extern double x; extern void func(); int main( int argc, char* argv[] ) { func(); printf( "x = %lf\n", x ); /* x is NOT accessible here !! */ }#include <stdio.h> static double x; // Definition of x void func() { x = 12345; // x accessible here ! printf( "x = %lf\n", x ); }When you compile these programs, you will get an error:
UNIX>> gcc -c global4a.c // No error UNIX>> gcc -c global4b.c // No error UNIX>> gcc global4a.o global4b.o // Error in linking loader phase Undefined first referenced symbol in file x global4a.o ld: fatal: Symbol referencing errors. No output written to a.out collect2: ld returned 1 exit statusExplanation:
- The static global variable
x can only be
access within File 2.
- The main() function is not stored in the same file !
Note: try this
- Remove the keyword
static from
the variable x
- Re-compile --- no error !!!
- The static global variable
x can only be
access within File 2.
- Example Program:
(Demo above code)
- The global4a.c Prog file: click here
- The global4b.c Prog file: click here
How to run the program:
- Right click on link(s) and
save in a scratch directory
- To compile: gcc global4a.c global4b
- A static local variable is
accessible (= "visible"):
- Starting
from the
location
in the function
where the
static local variable is
defined
- Until the
end of the
function
- Starting
from the
location
in the function
where the
static local variable is
defined
- Example:
void func() { static double x; // x is accessible only // WITHIN this function !!! // (Just like a local variable !) x = 12345; // x accessible here ! printf( "x = %lf\n", x ); }
- Note:
- The only difference
between:
- A static local variable
- A (normal) local variable
is:
- A (normal) local variable is
initialized
each time the
function is
invoked
(Because a (normal) local variable is created each time the function is called
- A static local variable is initialized only once (so it will retain its value between function calls)
- The only difference
between: