Global or not Global

**c_geek** · 12-19-2007

Hi Guys
This is my first post on this forum. I am trying to get save some information to be used later on. I have two threads (program/processes) running at the same time. I need to get a variable A value to save in variable B which i can use in my second thread later on. for example

In Thread A

B = A;

In Thread B

something = B;

Both thread A and B are independent to each other. My problem is how to share this information between these two threads. I have tried to use global variable but it didn't work. I defined variable B as global in one of the threads and then redefine the same variable with "extern" keyword in one of the header files which is common to both threads. With some testing i could see the value assigned to B in thread A(by printf) but then in thread B when i do a printf on B variable it is printed as 0. Can u guys help me out in this. I appreciate your time and consideration in this regard.

**iMalc** · 12-19-2007

It sounds to me like your trying to share information between processes. You seem to be under the impression that this is how multi-threading is done.
What are you really doing? How are you creating these "threads"? What operating system? Can you post some code?

**c_geek** · 12-19-2007

Originally Posted by iMalc

It sounds to me like your trying to share information between processes. You seem to be under the impression that this is how multi-threading is done.
What are you really doing? How are you creating these "threads"? What operating system? Can you post some code?

Hi iMalc
All i am trying to do is to save variable 'A' value in thread 'A' in Variable 'B' so that i can use this value later on anywhere i want to use. As long as the code is concerned, i am just trying to test this in already written code (which is quite big) and i am not having success in it. I am running this multithreading code in XP.

**matsp** · 12-19-2007

Are there DLL's involved in this [and by that I don't mean "system" DLLs such as the MSVC runtime DLL's, but DLL's produced by the source from your project] - if so, you can't trivially share global variables between a DLL and the main executable. This may be the source of your problems, but without more details, it's hard to say.

There is no reason why global variables can't be used between threads as such - although you may need to make sure that the compiler doesn't optimize the access to your variables into registers - so a volatile keyword could be necessary too.

--
Mats

**c_geek** · 12-19-2007

Originally Posted by matsp

Are there DLL's involved in this [and by that I don't mean "system" DLLs such as the MSVC runtime DLL's, but DLL's produced by the source from your project] - if so, you can't trivially share global variables between a DLL and the main executable. This may be the source of your problems, but without more details, it's hard to say.

There is no reason why global variables can't be used between threads as such - although you may need to make sure that the compiler doesn't optimize the access to your variables into registers - so a volatile keyword could be necessary too.

--
Mats

Yes there are the DLL involved in the whole source code. So again in this situation how can i accomplish what i am trying to do.
Thanks

**Elysia** · 12-19-2007

Perhaps you should post some code showing the problem?
It's difficult to understand the problem. Also, it's also proffered to not to use global variables, but passing a pointer to your thread instead. You may have to use locking to avoid problems too, though.
But again, show a little snippet of what you're doing and maybe we can analyze it as see what needs to be done.

**matsp** · 12-19-2007

Originally Posted by c_geek

Yes there are the DLL involved in the whole source code. So again in this situation how can i accomplish what i am trying to do.
Thanks

The easiest way to share something between two units like that (an .EXE and a .DLL) is to have a function call that you perform to hand over the data from one side to the other.

Another option is to use the parameter(s) of the thread creation to pass a data structure which has amongst it's content the address of some data to be shared.

Or you can set up various named entities, such as named semaphores, named pipes or named shared memory, that each thread will "open" or "create" as necessary.

Which is the best choice for your depends a lot on what the ACTUAL problem you are trying to solve is.

--
Mats

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