When is it ok to use a global variable in C?

Variables should always have as small a scope as possible. The argument behind this is that every time you increase the scope, you have more code that potentially modifies the variable, thus more complexity is induced in the solution.

It is thus clear that avoiding using global variables is preferred if the design and implementation naturally allow that. Due to this, I prefer not to use global variables unless they are really needed.

I can not agree with the 'never' statement either. Like any other concept, global variables are something that should be used only when needed. I would rather use global variables than using some artificial constructs (like passing pointers around), which would only mask the real intent.

Some good examples where global variables are used are singleton pattern implementations or register access in embedded systems.

On how to actually detect excessive usages of global variables: inspection, inspection, inspection. Whenever I see a global variable I have to ask myself: Is that REALLY needed at a global scope?

Consider this koan: "if the scope is narrow enough, everything is global".

It is still very possible in this age to need to write a very quick utility program to do a one-time job.

In such cases, the energy required to create safe access to variables is greater than the energy saved by debugging problems in such a small utility.

This is the only case I can think of offhand where global variables are wise, and it is relatively rare. Useful, novel programs so small they can be held completely within the brain's short-term memory are increasingly infrequent, but they still exist.

In fact, I could boldly claim that if the program is not this small, then global variables should be illegal.

• If the variable will never change, then it is a constant, not a variable.
• If the variable requires universal access, then two subroutines should exist for getting and setting it, and they should be synchronized.
• If the program starts small, and might be larger later, then code as if the program is large today, and abolish global variables. Not all programs will grow! (Although of course, that assumes the programmer is willing to throw away code at times.)