Performance: should I use a global variable in a function which gets called often?

First off, let me get of my chest the fact that I'm a greenhorn trying to do things the right way which means I get into a contradiction about what is the right way every now and then.

I am modifying a driver for a peripheral which contains a function - lets call it

Send()

. In the function I have a timestamp variable so the function loops for a specified amount of time.

So, should I declare the variable global (that way it is always in memory and no time is lost for declaring it each time the function runs) or do I leave the variable local to the function context (and avoid a bad design pattern with global variables)?

Please bear in mind that the function can be called multiple times per milisecond.

TRKemp

Answer Source

Speed of execution shouldn't be significantly different for a local vs. a global variable. The only real difference is where the variable lives. Local variables are allocated on the stack, global variables are in a different memory segment. It is true that local variables are allocated every time you enter a routine, but allocating memory is a single instruction to move the stack pointer.

There are much more important considerations when deciding if a variable should be global or local.

Performance: should I use a global variable in a function which gets called often?

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