Scenario: Global variables in DLL which is used by Multi-threaded Application

Question

Few months back, I had come across this interesting scenario asked by a guy (on orkut). Though, I've come up with a "non-portable" solution to this problem (have tested it with small code), but still would like to know what you guys have to say and suggest.

Suppose, I created a DLL, exporting some functionalities, written in C++, for single threaded client. This DLL declares lots of global variables, some maybe const variables (read-only) and others are modifiable.

Anyway, later things changed and now I want the same DLL to work with multi-threaded application (without modifying the DLL); that means, several threads access the functions and global variables from the DLL, and modify them.. and so on. All these may cause global variables to hold inconsistent values.

So the question is,

Can we do something in the client code to prevent multi-access of the DLL, and at the same time, ensuring that each thread runs in it's own context (meaning, when it gets access to the DLL, the DLL's global values are same as it was before)?

Answer 1

Sure, you can always create a wrapper-layer handling multi-threading specific tasks such as locking. You could even do so in a second DLL that links with the original one, and then have the final project link with that new DLL.

Be aware that no matter how you implement it, this won't be an easy task. You have to know exactly which thread is able to modify which value at what time, who is able to read what and when etc. unless you want to run into problems like deadlocks or race conditions.

If you're solution allows it, it's often best to assign a single thread to modify any data, and have all others just read and never write, as concurrent reading access is always easier to implement than concurrent writing access (Boost provides all basic functionality to do so, for example shared_mutex).

Answer 2

Can we do something in the client code to prevent multi-access of the DLL, and at the same time, ensuring that each thread runs in it's own context (meaning, when it gets access to the DLL, the DLL's global values are same as it was before)?

This is the hard part. I think the only way top do this would be to create a wrapper around teh existing DLL. When it is called, it would restore the state (global variables) for the current thread, and save them when the call to the DLL returns. You would need to know all of the state variables in the DLL, and be able to read/write them.

If performance is not an issue, a single lock for the entire DLL would suffice, and be the easiest to implement correctly. That would ensure that only one thread was accessing (reading or writing) the DLL at one time.