Re: Defining a section that isn't file-backed (with an additional wrinkle)

[Ian Lance Taylor <ian at wasabisystems dot com>]

Christopher Faylor <cgf@redhat.com> writes:

> I'd like to have a similar type of layout but have the memory reserved
> but not allocated, i.e., I'd like the OS to set aside the memory addresses
> automatically but not associate any physical memory with the space until
> the equivalent of an 'sbrk' is called.
> 
> The distinction between reserved and allocated space is probably not a
> UNIX-like concept although it is easy to do in Windows.  I can't find
> any way in mmap or sbrk that would allow this, so I am probably out of
> luck.

It's true that Unix doesn't really distinguish between reserved and
allocated space.  When you allocate empty space in the program
segments, Unix doesn't associate any physical memory with that address
space.  It just marks up the page table entries.  The space is only
allocated when the program references the page.  You can do a bit of
tweaking here on some systems with stuff like mlock, the mmap
MAP_NORESERVE flag, and (on Linux) /proc/sys/vm/overcommit_memory.

> My reason for wanting this is to reserve a guaranteed section of memory
> in every cygwin process but not necessarily use all of the section --
> like for a heap.
> 
> So, is it possible in, ld or gas, to set aside a region of memory as
> reserved but not allocated?

I guess my question in return is: how do you want to express this?
The assembler and linker are only able to express information which
can be put into the object file.  Does the PE format have a way to
express this information?

What you are asking for can more or less be described in a linker
script using a MEMORY region.  But right now that information won't be
put into the object file, because Unix object files don't have any
place for the information to go.

Ian