Assume we have a legacy C file with some functions to solve linear equations and several corresponding global variables.
lineq.c:
/* macro definitions */
...
/* global vars */
...
/* some functions that make use of above variables */
void solveLinEq(...);
Now we want to use this legacy library in a modern multi-threaded C++ application. Therefore, we want to write a kind of wrapper class LinEqSolver that provides an OO interface for solving linear equations and internally calls the functions of our legacy C library.
However, it should be possible that there are multiple instances of LinEqSolver used by different threads. This requires that each instance/each thread has its own copies of the global variables in lineq.c. How can this be achieved, if we don't want to modify the lineq.c?
A possible solution I can imagine is to copy the global variables and the functions from the C file into the class LinEqSolver, making them data and function members. Then, each instance of LinEqSolver would operate on its private copy of the former globale variables. However, this copy-paste programming style is rather bad, especially when there is an update to lineq.c and we need to copy-paste the changes into our code again.
What other possibilities do I have where the actual code to solve linear equations stays in lineq.c and is just called from LinEqSolver?
You can use thread_local keyword from C++11. This will work well if your function always initializes all needed static variables in the beginning.
If you have more complex picture - more work will be needed. For example:
int myVar1, myVar2;
void InitStaticVars()
{
....
}
void solveLinEq(...);
If you will simply add thread_local specifier to the variables above and call initialization function at the beginning of your program, it will initialize these variables only for the calling thread. On all other threads their initial values will be zeroes.