You can use MAP_FIXED to get a range on a specific address. The address you specify that way does get rounded down to a multiple of the page size, but other than that it’s used as is.

A zero value for address has a special meaning, so if you absolutely want to map at address 0 you need to ask for address 1 rounded down. Though some kernels won’t permit that in the default configuration.

You can ask for more memory ahead of time. I am pretty sure the kernel only allocates the physical memory on the first write. However things get a little tricky with respect to over-commitment and such. As I understand it, the kernel will refuse the allocation if there is no way it could ever give you all of that memory. But I think the default is that when there is any doubt the kernel will let the allocation go through and kill the process later if it doesn’t have memory after all. (I am not saying that’s a good default.)

There is also the possibility of allocating a memory range with no permissions and then use mprotect to make parts of the range read and writable later. In that case it would make sense to me if the kernel only updates the count of committed memory once you make it writable, I don’t know if that’s actually what happens, but it should be easy to test.

I have used the approach of using mmap to allocate a range with no read or write permissions and then make a small range in the middle read-write with mprotect. My reason for using it has been to have guard pages around certain buffers as a security measure. It provides an extra layer of protection against buffer overflow vulnerabilities.

For some advanced use cases it can make sense to map a range with no privileges and later change the protection of the range from within a SIGSEGV handler when that address is accessed.