Use Shared Objects on Linux

By Sachin O. Agrawal
2005-05-27


Make shared memory work for you, not against you

In terms of time and space, shared memory is probably the most efficient inter-process communication channel provided by all modern operating systems. Shared memory is simultaneously mapped to the address space of more than one process: a process simply attaches to the shared memory and starts communicating with other processes by using it as it would use ordinary memory.

However, in the object-oriented programming world, processes prefer to share objects rather than raw information. With objects, there is no need to serialize, transport, and de-serialize the information contained within the object. Shared objects also reside in shared memory, and although such objects "belong" to the process that created them, all processes on the system can access them. Hence, all of the information within a shared object should be strictly process-neutral.

This is a direct contradiction to the C++ object model currently adopted by all popular compilers: C++ objects invariably contain pointers to various Vee-Tables and sub-objects that are process-specific. For such objects to be sharable, you need to make sure that the target of these pointers resides at the same address in all the processes.

With the help of a small sample program, this article illustrates cases where the C++ model succeeds, where it fails to work with the shared memory model, and where possible workarounds exist. The discussion and the sample program are limited to non-static data members and virtual functions. Other situations are not as relevant to the C++ object model as these are: static and non-static non-virtual member functions do not have any issues in shared environment. Per-process static members do not reside in shared memory (and thus have no issues), while shared static members have issues similar to those discussed here.

Tutorial Pages:
» Make shared memory work for you, not against you
» Environmental assumptions
» Sample program
» Making shared memory work
» Resources


First published by IBM DeveloperWorks

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