When would one use Un-named shared memory?

Question

When would you choose to use un-named shared memory in windows?

it seems to me that message passing between threads is not very useful. One can instead pass a pointer to a struct/variable to the worker threads, and use that as shared memory instead, rather than calling the CreateFileMapping system call.

Answer 1

One reason to use unnamed shared memory is to restrict access to the file mapping to only those processes who are given a handle to it by the creating process. This avoid two problems:

• any process that knows the name and has sufficient access to create a mapped file can squat on your named object, preventing or interfering with its legitimate use - this allows a denial of service attack.
• accidental rather than malicious name clashes.

When you don't use a name, you can be sure that only processes that you want to have access, get it. From the MSDN docs for CreateFileMapping:

A single file mapping object can be shared by multiple processes through inheriting the handle at process creation, duplicating the handle, or opening the file mapping object by name.

Answer 2

Section objects (aka "file mapping objects") are not just used to share memory between processes. The most obvious use of section objects is to map in a file to do I/O, and giving the objects names wouldn't be very useful in most cases. For unnamed pagefile-backed sections ("shared memory") you can still make child processes inherit the handle so they can use the sections.