Thread: dark side of global variables

Originally Posted by matsp

That does make the variable a non-global, as it's now a Thread-Local-Storage (TLS) variable - and I much prefer to not use compiler-specific extension to make TLS work - as most OS's and compilers support TLS, but no other compiler than MS supports this keyword (hence the __ to indicate that it's non-standard).

While TLS is not standard, it is certainly an option that defies the problems posed by multi-threading without simply hoping volatile will save the day. I never said I was pro-globals. I am simply offering solutions posed by the original poster's scenario.

Originally Posted by Matus

While TLS is not standard, it is certainly an option that defies the problems posed by multi-threading without simply hoping volatile will save the day. I never said I was pro-globals. I am simply offering solutions posed by the original poster's scenario.

Yes, but it's no longer a global variable - it's a variable accessed through a pointer (in some way - it may be an offset from a segment variable in x86, but that's for all intents and purposes a pointer anyways) - and you can always do that yourself with a little bit of extra pointer stuff.

And volatile has nothing at all to do with TLS - it removes the need to use volatile because the memory is no longer accessible from multiple threads, but it's otherwise unrelated to volatile. Volatile simply means "compiler, please be aware that the use of this variable may change even if you don't think it will from looking at the code", which essentially means "do not optimize away reads/writes of the variable", and it was originally introduced in C as a means to allow the compiler to optimize your normal variables, but tell the compiler that hardware registers that aren't memory needs to be treated differently. It may come in handy with threads too, but it's a side-effect.

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Mats

Originally Posted by matsp

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Mats

Kk Mats no problemo, good reading, good posts!

Just a point on "reading through the code": I did "dir /s *.h *.cpp" on ONE of the directories of the code I'm working on, that I have worked a bit with: 2.9MB. Another one, that I know very little of: 11.3MB.

Entire tree of our groups work: 35 MB (in about 3000 files).

That's an awful lot of code to "read through" - in fact, I couldn't do my job if I read through all the code before I worked on it - even in a small project with, say, a few hundred kilobytes of code, to understand the use of all variables in all places becomes an impossible task.

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Mats