Why are global variables bad, in a single threaded, non-os, embedded application

Question

Most of the objections I see to using global variables make sense since they refer to issues of multiple threads, thread safety, etc.

But in a small, single threaded, non-OS, case, what objections do you have? In my case, I'm writing my embedded system in "C", if it matters. I'm also the only developer on the product.

Why would eliminating global variables make my code better?

(After reading several responses, I realize I also should have pointed out that this system has no dynamic memory allocation (e.g. malloc). All the memory is statically allocated at compile time.)

Answer 1

It wouldn't.

The two fundamental issues with global variables is simply cluttering the namespace, and the fact that "no one" has "control" over them (thus the potential collisions and conflict with multiple threads).

The "globals are bad", like pretty much every other computer programming idiom is a guideline, not a hard and fast rule. When these kinds of "rules" are made, its best rather than simply adopting the rule by rote to understand the circumstances and motivations behind the creation of the rule. Don't just take them blindly.

In your case, you seem to understand the nature of your system and the arguments around the rule and decided that it doesn't apply in this case. You're right, it doesn't.

So, don't worry about it.

Answer 2

Global variables are not necessarily bad, just as macros are not necessarily bad, and enriched uranium is not necessarily bad. As long as you make a conscious decision about the pros and cons of a given design choice, you should be all right.

Some of the arguments against global variables are:

1. They violate good object-oriented design
2. They make your code difficult to unit-test, since you can not test individual blocks of code without setting up all of the global variables that the code expects to see
3. They increase coupling in your code: actions in one block of code may affect things in another block of code in unpredictable ways via a shared global variable

Some of the arguents for global variables:

1. They make it easy to share a single resource between many functions
2. They can make code easier to read

If, in your design, global variables make sense, and can be used to make your code simpler to read or easier to maintain, without setting yourself up for random errors and testing headaches, then by all means use them.

I write a lot of C code for embedded microcontrollers, and I use global variables all the time. In such a rigid system, global variables make sense. I'm aware of the potential drawbacks, but I have analyzed the pros & cons and I write my code so as to guard against the major pitfalls.

Bottom line: There is no hard and fast rule. Just make the best decision for your particular project or platform, based on the best information that you have.