Re: streams and file locks, ancient OS history, ARM is sort of channeling

On Mon, 1 Jul 2024 02:10:43 -0000 (UTC)
John Levine <johnl@taugh.com> wrote:

According to MitchAlsup1 <mitchalsup@aol.com>:

In fact, you can ftruncate arbitrarily large at the start -
backing sectors for the mapped pages will only be allocated when
a page is referenced for the first time. 

>
Can you ftruncate( 0x7FFFFFFFFFFFFFFFF ); ?? ½ of the address
space 

>
Every filesystem has a maximum file size. Exceed that and you'll
get an EFBIG error. 

>
Then it is not capable of ftruncate arbitrarily large in a hardware's
view of arbitrarily large which tends to be 1-bit smaller than the
largest container. 

 
The goal here is to map a file into the address space. What's your use
case for mapping a file-like thing that is bigger than any real file
and can't be written out to the disk?
 

Consider that configurations in which maximal file size is bigger than
half of address space are quite common. And not just on 32-bit HW.

Of course, what is considered "half of address space" is not a very
simple question by itself. Take x86-64. Is its address space 2**64 or
2*48? Take aarch64. Is its address space 2**64 or 2**49? Or, may be,
2**56?