[Vm-dev] Why StandardFileStream fileID is 16 bytes in Linux while 20 bytes in OSX?

[David T. Lewis]

On Sat, Jan 09, 2016 at 07:46:28PM -0300, Mariano Martinez Peck wrote:
>  
> On Sat, Jan 9, 2016 at 12:45 PM, Eliot Miranda <eliot.miranda at gmail.com>
> wrote:
> 
> >
> > My guess is that the VM is being built, wrongly, without 64-bit file
> > sizes.  In the Squeak VM this is definitely not 16 bytes.
> >
> >
> Hi Eliot,
> 
> Thank you very much!! You nailed it :)  It is indeed that the fileSize in
> Linux VM is 4 bytes and not 8 as for OSX.

I don't think so. Details below.

> 
> You said that in the SqueakVM it is 64 bits. By SqueakVM you mean your Cog
> (and not our Pharo builds) or you mean the old interpreter VM?

There are two other factors at work here. In earlier days, sizof(SQFile)
on a 32-bit host system was 16. At some point, the isStdioStream member got
added to the data structure (<OT>horrible, what were we thinking?</OT>) and
the size became 20, which is what you will now see with any recent VM on a
32-bit host.

If you are using a VM for which the size is 16, this suggests that it
is a VM that was compiled from a code base that does not include the
isStdioStream member in the data structure.

The other factor is pointer size on the host platform. If you compile a
VM in 64-bit mode, then sizeof(SQFile) is 32.

On a non-Spur 64-bit image on 64-bit host, the size is also 32. So you
should expect this to be the case for 64-bit Spur (because the VM for
64-bit Spur will also be compiled in 64-bit mode).

The size of 32 is what I see on my Linux system, and it is probably what
you will see on most other systems. But there is no guarantee, the compiler
is free to organize data structures as it sees fit, and it is certainly
possible that factors such as word alignment optimizations might lead to
different results on different platforms.

> 
> Should I open an issue somewhere so that we can make it 64 bits file size?
> 

Probably not. This looks like normal expected variation that is (and should
be) under the control of the host platform and compiler. If the VM that you
are using is compiled from outdated platforms source code, that might be
a concern, but even so there should be no expectation that sizeof(SQFile) is
the same on all platforms. That definitely is not the case.

Compiling for 64-bit file size does not have any impact on sizeof(SQFile).
If I compile a Linux VM with or without that option, the size of SQFile
is the same in either case.

Dave

> Thanks advance,
> 
> 
> _,,,^..^,,,_ (phone)
> >
> > On Jan 9, 2016, at 7:06 AM, Mariano Martinez Peck <marianopeck at gmail.com>
> > wrote:
> >
> > Hi guys,
> >
> > In OSX, StandardFileStream byteID size is 20, while in Linux, it is 16. I
> > see this definition in  ./Cross/plugins/FilePlugin/FilePlugin.h
> >
> >
> > /* squeak file record; see sqFilePrims.c for details */
> > typedef struct {
> >   int                    sessionID;     /* ikp: must be first */
> >   void                  *file;
> >   squeakFileOffsetType   fileSize;      /* 64-bits we hope. */
> > #if defined(ACORN)
> > // ACORN has to have 'lastOp' as at least a 32 bit field in order to work
> >   int lastOp; // actually used to save file position
> >   char writable;
> >   char lastChar;
> >   char isStdioStream;
> > #else
> >   char                   writable;
> >   char                   lastOp; /* 0 = uncommitted, 1 = read, 2 = write */
> >   char                   lastChar;
> >   char                   isStdioStream;
> > #endif
> > } SQFile;
> >
> > And I see no override from UNIX platform code. So...how can that be 16
> > bytes in Linux?
> >
> > Thanks in advance,
> >
> > --
> > Mariano
> > http://marianopeck.wordpress.com