In computer programming, a global variable is a variable with global scope, meaning that it is visible (hence accessible) throughout the program, unless shadowed. The set of all global variables is known as the global environment or global state. In compiled languages, global variables are generally static variables, whose extent (lifetime) is the entire runtime of the program, though in interpreted languages (including command-line interpreters), global variables are generally dynamically allocated when declared, since they are not known ahead of time.
In some languages, all variables are global, or global by default, while in most modern languages variables have limited scope, generally lexical scope, though global variables are often available by declaring a variable at the top level of the program. In other languages, however, global variables do not exist; these are generally modular programming languages that enforce a module structure, or class-based object-oriented programming languages that enforce a class structure.
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Local and Global Variables | Computer Programming | Khan Academy
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Local vs global variables in C
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Local and Global Variables - C++ Programming Tutorial #25 (PC / Mac 2015)
Transcription
Now that you've mastered the basics of functions, I want to talk about a topic that can get a little tricky: the difference between local and global variables. These terms may not mean anything to you now. So let's start with an example. I made this program here to show you how many inches I grew in my childhood. Since humans grow at different rates, I came up with this function, calcInches, where I can pass in a startAge and an endAge, and an inchesPerYear, and then it would calculate how many inches total were grown during that time. . . . and return it back to whoever called it. So you can see here from 0 to 8 years I call calcInches and I pass 0, 8, and 2.5, because I grew about 2.5 inches per year. And so it does a calculation, and you can see it spits out 20. Then from 8 to 16, I call it from, and I pass it 8 and 16 and then 2 because I didn't quite grow as much, and you can see it spits out 16. So this is neat, but now I want to actually display how many inches I grew, total, in my whole childhood. So how do I do that? Well, I might start by looking at my code and thinking, hmmmm, what values do I have here? Do I have anything that looks like it represents total inches? Well, I do have this totalInches variable inside my calcInches function, so I could just output that, see what it says; start there. So let's say text(totalInches, 10, 200), and put it down at the bottom. All right, let's see, what have we got? Oh, o-oh, we've got the 'Oh noes!' guy. And he says there's a problem. totalInches is not defined. Well, that's weird, because we defined totalInches right here, right? var totalInches =. Well, the problem is that we declared totalInches inside a function. On this line here. And when we declare a variable inside a function, it's considered a local variable. It lives only inside this function here (calcInches). And the code outside the function, all of this, it doesn't see local variables inside functions. It only sees whatever gets returned. It only sees that value, not that variable. So when we try to use totalInches outside the function, it doesn't know what it is, and it says 'Hey, I've never seen this variable before. It's not defined. I can't display it.' So there is a way that we can make it so that the outside code can see this variable. And that's if we turn it from a local variable into a global variable. We can do that by moving the definition outside of the function, into what's called the global scope. And now, inside the function, all we're doing is changing the value of it each time, not defining and declaring it. So you can see that it says 'Total grown over life: 16' So it found the variable because we made it into a global variable. But it's not actually the value that we're looking for. That's just the most recent value. And that's because every time we call this function, it's setting totalInches to whatever it's calculating that time. Right? So what we really want to do is, we want a new variable that we use only for storing the overall total that we add to every time we calculate. . .you know, the total for a range. So let's change this back to being a local variable, and let's make a new global variable called lifeInches. And we'll start it at 0. And then inside the function, we'll add to this global variable by saying lifeInches += totalInches. So we're going to add however much we calculate each time we call this function, we're going to add it to the lifeInches global variable. And then at the bottom, we'll display lifeInches: text(lifeInches, 10, 200). Tada! our total growth over life. Now that's not actually how tall I am. I'm taller than that. But that's because you know, we start off born with more than 0 length. So if I want total, maybe I could start at 20. And there you go, that's how tall I am. All right. So let's review, totalInches is what we call a local variable. And we know that because we see the declaration of it inside this function and not outside the function. And so that means that this outside code here doesn't know about this variable called totalInches. Now lifeInches is what we call a global variable. And we know that because we see its declaration outside of any function, in our global scope. So try to keep this in mind when you're writing your functions and your variables. And think to yourself whether you want a local variable for just your function to use, or global variable for your whole program to use. And don't worry if this is hard to wrap your head around. It's one of the trickiest concepts in programming, and in JavaScript in particular. And it's something you'll get better at as you keep practicing.
Use
Interaction mechanisms with global variables are called global environment (see also global state) mechanisms. The global environment paradigm is contrasted with the local environment paradigm, where all variables are local with no shared memory (and therefore all interactions can be reconducted to message passing).
Global variables are used extensively to pass information between sections of code that do not share a caller/callee relation like concurrent threads and signal handlers. Languages (including C) where each file defines an implicit namespace eliminate most of the problems seen with languages with a global namespace though some problems may persist without proper encapsulation. Without proper locking (such as with a mutex), code using global variables will not be thread-safe except for read only values in protected memory.
Environment variables
Environment variables are a facility provided by some operating systems. Within the OS's shell (ksh in Unix, bash in Linux, COMMAND.COM in DOS and CMD.EXE in Windows) they are a kind of variable: for instance, in unix and related systems an ordinary variable becomes an environment variable when the export
keyword is used. Program code other than shells has to access them by API calls, such as
getenv()
and setenv()
.
They are local to the process in which they were set. That means if we open two terminal windows (Two different processes running shell) and change value of environment variable in one window, that change will not be seen by other window.
When a child process is created, it inherits all the environment variables and their values from the parent process. Usually, when a program calls another program, it first creates a child process by forking, then the child adjusts the environment as needed and lastly the child replaces itself with the program to be called. Child processes therefore cannot use environment variables to communicate with their peers, avoiding the action at a distance problem.
Global-only and global-by-default
A number of non-structured languages, such as (early versions of) BASIC, COBOL and Fortran I (1956) only provide global variables. Fortran II (1958) introduced subroutines with local variables, and the COMMON keyword for accessing global variables. Usage of COMMON in FORTRAN continued in FORTRAN 77,[1] and influenced later languages such as PL/SQL. Named COMMON groups for globals behave somewhat like structured namespaces.[2] Variables are also global by default in Forth, Lua, Perl, and most shells.
By language
C and C++
The C language does not have a global
keyword. However, variables declared outside a function have "file scope," meaning they are visible within the file. Variables declared with file scope are visible between their declaration and the end of the compilation unit (.c
file) (unless shadowed by a like-named object in a nearer scope, such as a local variable); and they implicitly have external linkage and are thus visible to not only the .c
file or compilation unit containing their declarations but also to every other compilation unit that is linked to form the complete program. External linkage, however, is not sufficient for such a variable's use in other files: for a compilation unit to correctly access such a global variable, it will need to know its type. This is accomplished by declaring the variable in each file using the extern
keyword. (It will be declared in each file but may be defined in only one.) Such extern
declarations are often placed in a shared header file, since it is common practice for all .c
files in a project to include at least one .h
file: the standard header file errno.h
is an example, making the errno
variable accessible to all modules in a project. Where this global access mechanism is judged problematic, it can be disabled using the static
keyword which restricts a variable to file scope, and will cause attempts to import it with extern
to raise a compilation (or linking) error.[3]
An example of a "global" variable in C:
#include <stdio.h>
// This is the file-scope variable (with internal linkage), visible only in
// this compilation unit.
static int shared = 3;
// This one has external linkage (not limited to this compilation unit).
extern int over_shared;
// Also internal linkage.
int over_shared_too = 2;
static void ChangeShared() {
// Reference to the file-scope variable in a function.
shared = 5;
}
static void LocalShadow() {
// Local variable that will hide the global of the same name.
int shared;
// This will affect only the local variable and will have no effect on the
// file-scope variable of the same name.
shared = 1000;
}
static void ParamShadow(int shared) {
// This will affect only the parameter and will have no effect on the file-
// scope variable of the same name.
shared = -shared;
}
int main() {
// Reference to the file-scope variable.
printf("%d\n", shared);
ChangeShared();
printf("%d\n", shared);
LocalShadow();
printf("%d\n", shared);
ParamShadow(1);
printf("%d\n", shared);
return 0;
}
As the variable is an external one, there is no need to pass it as a parameter to use it in a function besides main. It belongs to every function in the module.
The output will be:
3 5 5 5
Java
Some languages, like Java, don't have global variables. In Java, all variables that are not local variables are fields of a class. Hence all variables are in the scope of either a class or a method. In Java, static fields (also known as class variables) exist independently of any instances of the class and one copy is shared among all instances; hence public static fields are used for many of the same purposes as global variables in other languages because of their similar "sharing" behavior:
public class Global {
public static int a;
}
PHP
PHP has a global
keyword and a number of unusual ways of using global variables.
Variables declared outside functions have file scope (which is for most purposes the widest scope). However, they are not accessible inside functions unless imported with the global
keyword (i.e., the keyword accesses global variables, it does not declare them).
However, some predefined variables, known as superglobals are always accessible.
They are all arrays. A general purpose one is the $GLOBALS
superglobal, which contains all the variables
defined out of function scope. Changes to its elements change the original variables, and additions create new variables.
The superglobals $_POST
and $_GET
are widely used in web programming.
Other languages
- In Python and MATLAB a global variable can be declared anywhere with the
global
keyword.[4][5] - Ruby's global variables are distinguished by a '
$
' sigil. A number of predefined globals exist, for instance$$
is the current process ID.
See also
References
- ^ "Fortran 77 Tutorial".
- ^ "First Steps: Stack & Heap Objects".
- ^ C in a Nutshell, P.Prinz & T Crawford, 2006, O'Reilly, Ch 11
- ^ "What are the rules for local and global variables in Python?". docs.python.org. Python Software Foundation. Retrieved 4 June 2020.
- ^ "Declare variables as global". in.mathworks.com. The MathWorks, Inc. Retrieved 7 February 2015.
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