opening a write-only file

On Sat, May 31, 2014 at 10:00:06AM -0700, Douglas McPherson 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.

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?

Sure, you can use FileStream readOnlyFileNamed: 'foo'.

Dave

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 <[hidden email]> wrote:

Hi Doug,

On Sat, May 31, 2014 at 10:00 AM, Douglas McPherson <[hidden email]> 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

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 <[hidden email]> wrote:

Hi Doug,

On Sat, May 31, 2014 at 10:00 AM, Douglas McPherson <[hidden email]> 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

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

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 <[hidden email]> wrote:

Hi Doug,

On Sat, May 31, 2014 at 10:00 AM, Douglas McPherson <[hidden email]> 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