How Do Global and Static Variables Behave in Dynamically Linked Libraries?

The Impact of Global and Static Variables in Dynamically Linked Libraries

In the context of dynamic linking, the behavior of global and static variables within shared libraries can be confusing.

Load-Time Dynamic Linking (Module A)

With load-time dynamic linking, the DLL containing the global and static variables will be loaded into the application's address space. The operating system allocates memory for these variables within the DLL's own section.

Run-Time Dynamic Linking (Module B)

For run-time dynamic linking, the DLL is loaded as needed. The global and static variables within the DLL will be accessible to the application through the functions and symbols exported by the DLL.

Multiple Instances in Different Processes

If two modules in an application use the same shared libraries (A and B), each process will have its own instance of the global and static variables defined within A and B. This is due to the fact that process address spaces are separate.

Access to Application's Globals

DLLs A and B will not have direct access to the application's global variables. This is because the ODR (One Definition Rule) prevents multiple definitions of the same global variable. Additionally, different processes have separate address spaces, eliminating any possibility of shared memory.

Key Differences between Windows and Unix-Like Systems

Windows:

• Extern global variables are not exported by default.
• Explicit export directives are required to expose global variables.

Unix-Like:

• Extern global variables are exported by default.
• Load-time linking shares global variables across modules.

Conclusion

Understanding the behavior of global and static variables in dynamically linked libraries is crucial for effective module management. Properly considering the ODR and address space separation ensures stable and predictable code execution in different linking scenarios.

The above is the detailed content of How Do Global and Static Variables Behave in Dynamically Linked Libraries?. For more information, please follow other related articles on the PHP Chinese website!