[Vm-dev] [squeak-dev] opening a write-only file
Douglas McPherson
djm1329 at san.rr.com
Fri Jun 6 19:33:30 UTC 2014
Sounds good to me. But the comment should perhaps be "/* and if w+b fails, try ab to open a write-only file in append mode (not wb which opens a write-only file but overwrites its contents */ "
Thanks,
Doug
On Jun 6, 2014, at 11:25 , Eliot Miranda wrote:
>
>
>
> On Fri, Jun 6, 2014 at 9:33 AM, Douglas McPherson <djm1329 at san.rr.com> wrote:
>
>
> On Jun 5, 2014, at 21:03 , David T. Lewis wrote:
>
> > On Thu, Jun 05, 2014 at 06:50:31PM +0200, Bert Freudenberg wrote:
> >>
> >>
> >> On 04.06.2014, at 18:19, Douglas McPherson <djm1329 at san.rr.com> wrote:
> >>
> >>> Hi All,
> >>>
> >>> I've tested a fix for this on OS X and Linux. The fix simply adds an extra attempt to open the file after the "w+b" attempt using this code:
> >>>
> >>> if (getFile(f) == NULL) {
> >>> setFile(f, fopen(cFileName, "wb"));
> >>> }
> >>>
> >>> If the previous attempts to open the file failed (because the file exists with write-only permission, no read permission) then the above will actually open the file write-only. Adding the extra attempt preserves existing behaviour for the success paths, so the change should be transparent to existing code.
> >>>
> >>> BTW, AFAICT the same issue exists in Pharo.
> >>>
> >>> Thoughts?
> >>>
> >>> Doug
> >>
> >> Sounds reasonable to me.
> >>
> >> - Bert -
> >
> > I have not tested it yet, but I think that this would truncate an existing file,
> > so that if the write-only file exists and contains data, the data would be lost.
> >
> > Doug, could you check this with your patched VM?
> >
> > Thanks,
> > Dave
> >
>
> Hi Dave,
>
> Yes you are right, I didn't think of that. That is indeed what happens.
>
> We could use "ab" instead of "wb" to get append behaviour if the file already exists. I think "a" is still a write-only mode. This is probably safer.
>
> OK. Then I propose to commit the following:
>
> --- platforms/Cross/plugins/FilePlugin/sqFilePluginBasicPrims.c (revision 2888)
> +++ platforms/Cross/plugins/FilePlugin/sqFilePluginBasicPrims.c (working copy)
> @@ -245,6 +245,9 @@
> try opening it in write mode to create a new, empty file.
> */
> setFile(f, fopen(cFileName, "w+b"));
> + /* and if w+b fails, try ab (not wb which deletes read-only files) */
> + if (getFile(f) == NULL)
> + setFile(f, fopen(cFileName, "ab"));
> if (getFile(f) != NULL) {
> char type[4],creator[4];
> dir_GetMacFileTypeAndCreator(sqFileName, sqFileNameSize, type, creator);
>
> The larger issue of course is that FileStream and its subclasses have no protocol for the user to specifically request opening of a a write-only file, whether in append mode or overwrite mode. Currently all files that are opened must have at least read permission. Should we add something? If so, any suggestions? I'm ok with the simple tweak above rather than adding new protocol, though it is really a hack.
>
> IMO, some time we should replace the FilePlugin. The directory interface sucks. Buffering sucks. We should go the VW route of having proper buffered files above the raw system calls (ReadFile & WriteFile in win32/64, read & write in unix et al).
>
> Once we have ephemerons then closing buffered files through finalization is easier, cuz one can flush the buffer on the about-to-be-collcted file. That means one can have a registry of open files and easily flush them before quit (ephemerons are fancy Associations).
>
>
> Thanks,
> Doug
>
>
>
>
>
> >>
> >>
> >>>
> >>>
> >>> On Jun 1, 2014, at 19:44 , Douglas McPherson wrote:
> >>>
> >>>> It turns out #forceNewFileNamed: does not work either.
> >>>>
> >>>> I believe the immediate cause of the problem is in the function you pointed me at: sqFileOpen() in platforms/Cross/plugins/FilePlugin/sqFilePluginBasicPrims.c:
> >>>>
> >>>> if (writeFlag) {
> >>>> /* First try to open an existing file read/write: */
> >>>> setFile(f, fopen(cFileName, "r+b"));
> >>>> if (getFile(f) == NULL) {
> >>>> /* Previous call fails if file does not exist. In that case,
> >>>> try opening it in write mode to create a new, empty file.
> >>>> */
> >>>> setFile(f, fopen(cFileName, "w+b"));
> >>>> if (getFile(f) != NULL) {
> >>>> char type[4],creator[4];
> >>>> dir_GetMacFileTypeAndCreator(sqFileName, sqFileNameSize, type, creator);
> >>>> if (strncmp(type,"BINA",4) == 0 || strncmp(type,"????",4) == 0 || *(int *)type == 0 )
> >>>> dir_SetMacFileTypeAndCreator(sqFileName, sqFileNameSize,"TEXT","R*ch");
> >>>> }
> >>>> }
> >>>>
> >>>> The first call to fopen() indeed fails if the file does not exist. But it also fails if the file exists but does not have both read and write "r+b" permission. So in that case the intent, as you mentioned, is to try to open in write-only but "w+b" is not the write-only mode, it is also a read and write mode, see [1]. So this fopen() attempt also fails if the file has write-only permission. A fix may be to use mode "wb" instead of "w+b", but I'm not sure if there are unintended consequences to doing this.
> >>>>
> >>>> [1] http://en.wikibooks.org/wiki/C_Programming/C_Reference/stdio.h/fopen
> >>>>
> >>>> On May 31, 2014, at 14:45 , Douglas McPherson wrote:
> >>>>
> >>>>> Ahh, thanks. I'll try it out when back in front of my machine.
> >>>>>
> >>>>> Thanks,
> >>>>> Doug
> >>>>>
> >>>>> Sent from my iPhone
> >>>>>
> >>>>> On May 31, 2014, at 14:23, Eliot Miranda <eliot.miranda at gmail.com> wrote:
> >>>>>
> >>>>>> Hi Doug,
> >>>>>>
> >>>>>>
> >>>>>> On Sat, May 31, 2014 at 10:00 AM, Douglas McPherson <djm1329 at san.rr.com> wrote:
> >>>>>> Is there a way to open a write-only file in Squeak? On both Mac and Linux attempting to open such a file fails.
> >>>>>>
> >>>>>> Squeak has forceNewFileNamed: & forceNewFileNamed:do:. If you look at sqFileOpen in platforms/Cross/plugins/FilePlugin/sqFilePluginBasicPrims.c you'll see it first attempts to open read-write and if that fails, opens write-only. So it should work.
> >>>>>>
> >>>>>> HTH
> >>>>>>
> >>>>>> For example, if you create a write-only file, i.e. in a shell, after cd to default directory
> >>>>>>
> >>>>>> echo nada > foo
> >>>>>> chmod 200 foo
> >>>>>>
> >>>>>> Then from Squeak
> >>>>>>
> >>>>>> StandardFileStream fileNamed: 'foo'
> >>>>>>
> >>>>>> returns nil.
> >>>>>>
> >>>>>> If the file is given read permission (i.e. chmod 600 foo) then Squeak can open it.
> >>>>>>
> >>>>>> Anyone know a workaround?
> >>>>>>
> >>>>>> Thanks,
> >>>>>> Doug
> >>>>>>
> >>>>>>
> >>>>>> --
> >>>>>> best,
> >>>>>> Eliot
> >>>>>>
> >>>>>
> >>>>
> >>>>
> >>>
> >>>
> >>
> >>
> >>
> >
> >
> >
> >
>
>
>
>
> --
> best,
> Eliot
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