[fpc-pascal] TFileStream.SetSize sometimes fails

Martin lazarus at mfriebe.de
Fri Aug 14 18:28:48 CEST 2009

Michael Van Canneyt wrote:
> On Thu, 13 Aug 2009, Mattias Gaertner wrote:
>> I want to replace the content of an existing file (keeping the type and
>> rights). I used:
>> fs := TFileStream.Create(Filename, fmOpenWrite or fmShareDenyNone);
>> fs.Size := 0;
>> fs.Write(.....);
>> fs.Free;
>> But it does not always work. The line fs.Size does not clear the file on
>> every file system. So the fs.Write only replaces the first part of the
>> file.
> If it depends on the filesystem, then you should complain with the linux
> kernel people ?
> Size=0 results in a ftruncate(Handle,0) call
> Part of a strace (ex20 from the baseunix unit docs, fptruncate call)
> open("test.dat", O_WRONLY|O_LARGEFILE)  = 3
> ftruncate(3, 60)                        = 0
> close(3)                                = 0
> So we can state that FPC's ftruncate does work, it passes the args to 
> the kernel.
> If the kernel does not actually truncate the file, there is little FPC 
> can do.

Well the only case were it fails (at least were we know that it is 
truncate that fails) is VBox shared folders. And indeed it seems they do 
have a bug.  http://www.virtualbox.org/ticket/3712
or the one I can reproduce http://www.virtualbox.org/ticket/4771

However in c I can specify
   open(fname, O_WRONLY | O_TRUNC );
which does work (bypasses the bug)

I haven't seen an equvivalent to O_TRUNC in fpc ?


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