On Fri, 14 Aug 2009, Martin wrote:
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 ?
Sure there is:
uses baseunix;
Var
H : integer;
begin
h:=fpOpen(fname,O_WrOnly or o_trunk)
end.
But that is not relevant:
a) FPC itself should not work around kernel/vbox bugs.
b) It would not help in Mattias' case anyway...
Michael.
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