Does not it affect the efficiency to declare a function variable as local instead of global?

2
votes

I've actually seen some results by testing it, but I want to know which way is better and why.

Question #1: Do local variables get declared every time when I call that function again and again? I know that it is better to declare variables in the narrowest scope possible. But I can not stop myself thinking about declaring it as a global variable and make it get declared only once, not in every function call. Or, does it get declared again in every function call? I know that the scope of a local variable is only that function. So when it leaves that function, it must forget that variable as it is going out of its scope right?

Question #2: When I have some function variables which need to store its previous content(e.g. timer counter variables), which way is better: to declare them as a global variable or to declare them as a static local variable? I don't need them to get their initial values whenever I call that function, I am already setting them to zero or etc whenever I need.

3
cvariable-declaration
Question1: what is the problem if space for variable are stack allocated each time you call the function? Question2:If those variables are used in function scope only, I'd declare them static inside the function scope. – LPs
Show code and test results for it, and explain how you obtained them. Otherwise it's shooting in the dark. – n. 1.8e9-where's-my-share m.
I meant I have seen that both ways are working well, declaring as global or static local. But I wanted to know which one is better to go. If you meant a test result in terms of speed etc, I don't actually know how or where to do that. – abdullah cinar
Efficiency analysis may be difficult. Creating and deleting is not an efficiency issue. Local variables may appear as less efficient because they are addressed relatively to a register value (SP or BP). They require a numeric addition to get their address (contrary to globals and static locals). However, local variables are usually better register optimized and cause less cache misses. The resulting efficiency will depend on particular case, but I'd bet in favor of local variables. Not speaking about program clarity, non-pollution of global name space, etc. – Marian

3 Answers

5
votes

Question #1: Do local variables get declared every time when I call that function again and again?

A1: Yes, but it's not an issue really.

Declaring a local variable means that space is made for that variable on the stack, within the stack frame of that function. Declaring a variable global means that space is made for that variable in the data section of the executable (if the variable is initialized), or the BSS section (if not).

Allocating on the stack comes at zero cost. At function entry, the stack frame is sized to make room for all local variables of the function. One more or less does not matter. Statically allocating (for a global variable) is a tad quicker, but you only get that one variable. This can become a huge issue at some later point, e.g. if you want to make your program multithreaded, your function re-entrant, or your algorithm recursive. It can also become a major hassle during debugging, wasting hours of unproductive time while you are hunting down that bug.

(This is the main point of it all: The performance difference is really negligible. The time you can waste on a suboptimal design riddled with globals, on the other hand, can be quite significant.)

Question #2: [...] which way is better: to declare them as a global variable or to declare them as a static local variable?

A2: From an architectural standpoint, avoid globals wherever possible. There are a few specific cases where they make sense, but you know them when you see them. If you can make it work without globals, avoid them. (The same is true, actually, for static locals. They are better than globals as they are limited in scope, and there are cases where they make sense, but local variables should really be the "default" in your mind.)

1
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Global variable - declared at the start of the program, their global scope means they can be used in any procedure or subroutine in the program

It is seldom advisable to use Global variables as they are liable to cause bugs, waste memory and can be hard to follow when tracing code. If you declare a global variable it will continue to use memory whilst a program is running even if you no longer need/use it.

Local variable - declared within subroutines or programming blocks, their local scope means they can only be used within the subroutine or program block they were declared in

Local variables are initiated within a limited scope, this means they are declared when a function or subroutine is called, and once the function ends, the memory taken up by the variable is released. This contrasts with global variables which do not release memory.

Question #1: YES. Local variables get declared every time when you call that function again and again. After it leaves the function it forgots the variables that you declared in that scope. You must also remember that when some variable faced, the program will start to search for it. So when it is closer like declaring in the same scope, it will find faster and be able to continue. Also this will be more efficent while you are coding and will cause less bugs and mistakes etc.

Question #2: If you use the same variable with different functions, I strongly suggest you to declare them as global or define, this will lead the program to carry your "counter" with it. So it can be fastly use it when you need between the scopes you travel. But after these conditions I must strongly suggest you to:

avoid globals wherever possible (as @DevSolar said)

1
votes

Q2:

It is usually more preferable to use static variables in your function. The main reason is that since all functions can access global variables, it is very hard to keep track of and debug your program.

Q1:

Yes, local variables are created every time the function it belongs to is run, and deleted when the ends.

Suppose your program has 5 functions (that are rarely used), and each function uses 6 local variables. If you change them all to global variables, you will have all 30 variables taking up space for the entire duration of your program, instead of only have 5 variables occasionally being created and destroyed. Moreover, allocation does not really take much time.