I'm using C (not C++) and I'm unsure how to avoid using global variables.
I have a pretty decent grasp on C, its syntax, and how to write a basic application, but I'm not sure of the proper way to structure the program.
How do really big applications avoid the use of global variables? I'm pretty sure there will always need to be at least some, but for big games and other applications written in C, what is the best way to do it?
Is there any good, open-source software written strictly in C that I could look at? I can't think of any off the top of my head, most of them seem to be in C++.
Thanks.
Edit
Here's an example of where I would use a global variable in a simple API hooking application, which is just a DLL inside another process.
This application, specifically, hooks API functions used in another application. It does this by using WriteProcessMemory to overwrite the call to the original, and make it a call to my DLL instead.
However, when unhooking the API function, I have to write back the original memory/machine code.
So, I need to maintain a simple byte array for that machine code, one for each API function that is hooked, and there are a lot.
// Global variable to store original assembly code (6 bytes)
BYTE g_MessageBoxA[6];
// Hook the API function
HookAPIFunction ( "user32.dll", "MessageBoxA", MyNewFunction, g_MessageBoxA );
// Later on, unhook the function
UnHookAPIFunction ( "user32.dll", "MessageBoxA", g_MessageBoxA );
Sorry if that's confusing.
Global variables can be altered by any part of the code, making it difficult to remember or reason about every possible use. A global variable can have no access control. It can not be limited to some parts of the program. Using global variables causes very tight coupling of code.
The simplest way to avoid globals all together is to simply pass your variables using function arguments. As you can see, the $productData array from the controller (via HTTP request) goes through different layer: The controller receives the HTTP request. The parameters are passed to the model.
Use Local Variables Instead If your global variable is only ever accessed from one script, make it a "script local" instead.
Parameter passing - allows the values of local variables within the main program to be passed to sub-programs without the need to use global variables. The value of these variables (or a copy of the value of these variables) is passed as a parameter to and from sub-programs as necessary.
"How do really big applications avoid the use of global variables?"
Use static variables. If a function needs to remember something between calls, use this versus global variables. Example:
int running_total (int num) {
static int sum = 0;
sum += num;
return sum;
}
Pass data via parameters, so that the value is defined one place, maybe main()
and passed to where it is needed.
If all else fails, go ahead and use a global but try and mitigate potential problems.
Gbl_MyApp_DeveloperName
. So all global variables would start with the Gbl_MyApp_
part -- where "MyApp" was something descriptive of your app.extern
keyword).There are some valid uses for global variables. The schools started teaching that they were evil to keep programmers from being lazy and over using them. If you're sure that the data is really globally needed then use them. Given that you are concerned about doing a good job I don't think you'll go too far wrong using your own judgment.
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