Regarding File mapping

Question

hello all
i want to know what actually creation of a file mapping object does.
does it loads the copy of specified bytes of file into physical memory and returns
the handle to it. so is that the handle is only there in the virtual address space of
the process that created it or whole of the file is loaded into the virtual address space
of that process.So if mapped file is not loaded in the virtual address space of calling process
that means it requires a contiguous memory space in physical memory.
this imposes a constraint to get that much contiguous memory.
so am i write in my concepts or misconcepts.
where can i get the whole information about varoius memory mamagement related sys calls
with description about what each call does right frm basic.
hope u will clearing my misconcepts.
thaking u in advance
mudit

Answer 1

A file mapping is an OS stuff that allows you use a file as a memory buffer, so you can read/write bytes from the file using a pointer. It is somewhat faster and easier to use than usual files, and they can alse be used as memory sharing. There is no native support for file mappings in C++ so this is quite OS based.
Every OS implements is different from each other so better ask for your specific OS.
In the case of Solaris[tm] file mapping, please read the man page on the mmap() system call for more detail.
Its a way to misuse the OS algorithms for swapping and memory managment for your convinience.
The OS (usually) will read in only those parts of the file you actually use. So if you only need small parts of a big file, this is way faster than reading it into memory first.
Secondly, since only the used parts are read, the file can be way bigger than the actual memory you have. The OS can also remove parts from memory again if you run short. It's kind of having your own private swapfile.
You can share the mapped file between threads and thus simulate shared memory with it. In fact (depending on the OS) you can map /dev/zero or /dev/null just for that purpose. The OS then shares the memory between the threads but forgets it on exit.