Sharing extern Variables among Load Modules

Global extern Variables

If the rent or rentext compiler option is used, the compiler generates an object code data type called a pseudoregister for each extern variable. If the norent compiler option is used, the compiler generates a pseudoregister only for extern variables declared as __rent or whose names begin with an underscore (such as _options ). When the program is linked, the linker assigns an offset to each pseudoregister and accumulates the total length. At execution time, storage is dynamically allocated for all the pseudoregisters in the load module. This storage is known as the pseudoregister vector.

In a program consisting of several load modules, each load module has its own pseudoregister vector that is independent of the pseudoregister vector of any other load module. This means that, in general, extern variables cannot be shared between load modules.

However, certain extern variables need to be available in all the load modules in a multi-load-module program. An extern variable of this sort is known as a global extern . During dynamic loading, the loadm function copies the values of the global extern from the pseudoregister vector of the main load module to the psuedoregister vector of the newly loaded module. Of course, loadm must be able to determine the length and offset of each global extern both in the calling and called load modules. This copy takes place only when the load module is loaded. Changes in the value of the global extern after that point are not reflected back to any other load module.

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