Claus Brod: Don't quote me on this (18 Mar 2006)

Don't quote me on this (18 Mar 2006)

Let us assume that I'm a little backward and have a peculiar fondness for the DOS command shell. Let us further assume that I also like blank characters in pathnames. Let us conclude that therefore I'm hosed.

But maybe others out there are hosed, too. Blank characters in pathnames are not exactly my exclusive fetish; others have joined in as well (C:\Program Files, C:\Documents and Settings). And when using software, you might be running cmd.exe without even knowing it. Many applications can run external helper programs upon user request, be it through the UI or through the application's macro language.

The test environment is a directory c:\temp\foo bar which contains write.exe (copied from the Windows system directory) and two text files, one of them with a blank in its filename.

Now we open a DOS shell:

C:\>dir c:\temp\foo bar
 Volume in drive C is IBM_PRELOAD
 Volume Serial Number is C081-0CE2

 Directory of c:\temp

File Not Found

 Directory of C:\

File Not Found

C:\>dir "c:\temp\foo bar"
 Volume in drive C is IBM_PRELOAD
 Volume Serial Number is C081-0CE2

 Directory of c:\temp\foo bar

03/18/2006  03:08 PM    <DIR>          .
03/18/2006  03:08 PM    <DIR>          ..
01/24/2006  11:19 PM             1,516 foo bar.txt
01/24/2006  11:19 PM             1,516 foo.txt
03/17/2006  09:44 AM             5,632 write.exe
               3 File(s)          8,664 bytes
               2 Dir(s)  17,448,394,752 bytes free

Note that we had to quote the pathname to make the DIR command work. Nothing unusual here; quoting is a fact of life for anyone out there who ever used a DOS or UNIX shell.

Trying to start write.exe by entering c:\temp\foo bar\write.exe in the DOS shell fails; again, we need to quote:

C:\>"c:\temp\foo bar\write.exe"

And if we want to load foo bar.txt into the editor, we need to quote the filename as well:

C:\>"c:\temp\foo bar\write.exe" "c:\temp\foo bar\foo bar.txt"

Still no surprises here.

But let's suppose we want to run an arbitrary command from our application rather than from the command prompt. The C runtime library provides the system() function for this purpose. It is well-known that under the hood system actually runs cmd.exe to do its job.

#include <stdio.h>
#include <process.h>

int main(void)
{
  char *exe = "c:\\temp\\foo bar\\write.exe";
  char *path = "c:\\temp\\foo bar\\foo bar.txt";

  char cmdbuf[1024];
  _snprintf(cmdbuf, sizeof(cmdbuf), "\"%s\" \"%s\"", exe, path);

  int ret = system(cmdbuf);
  printf("system(\"%s\") returns %d\n", cmdbuf, ret);
  return 0;
}

When running this code, it reports that system() returned 0, and write.exe never starts, even though we quoted both the name of the executable and the text file name.

What's going on here? system() internally runs cmd.exe like this:

  cmd.exe /c "c:\temp\foo bar\write.exe" "c:\temp\foo bar\foo bar.txt"

Try entering the above in the command prompt: No editor to be seen anywhere! So when we run cmd.exe programmatically, apparently it parses its input differently than when we use it in an interactive fashion.

I remember this problem drove me the up the freakin' wall when I first encountered it roughly two years ago. With a lot of experimentation, I found the right magic incantation:

  _snprintf(cmdbuf, sizeof(cmdbuf), "\"\"%s\" \"%s\"\"", exe, path);
  // originally: _snprintf(cmdbuf, sizeof(cmdbuf), "\"%s\" \"%s\"", exe, path);

Note that I quoted the whole command string another time! Now the executable actually starts. Let's verify this in the command prompt window: Yes, something like cmd.exe /c ""c:\temp\foo bar\write.exe" "c:\temp\foo bar\foo bar.txt"" does what we want.

I was reminded of this weird behavior when John Scheffel, long-time user of our flagship product OneSpace Designer Modeling and maintainer of the international CoCreate user forum, reported funny quoting problems when trying to run executables from our app's built-in Lisp interpreter. John also found the solution and documented it in a Lisp version.

Our Lisp implementation provides a function called sd-sys-exec, and you need to invoke it thusly:

(setf exe "c:/temp/foo bar/write.exe")
(setf path "c:/temp/foo bar/foo bar.txt")
(oli:sd-sys-exec (format nil "\"\"~A\" \"~A\"\"" exe path))

Kudos to John for figuring out the Lisp solution. Let's try to decipher all those quotes and backslashes in the format statement.

Originally, I modified his solution slightly by using ~S instead of ~A in the format call and thereby saving one level of explicit quoting in the code:

  (format nil "\"~S ~S\"" exe path))

This is much easier on the eyes, yet I overlooked that the ~S format specifier not only produces enclosing quotes, but also escapes any backslash characters in the argument that it processes. So if path contains a backslash (not quite unlikely on a Windows machine), the backslash will be doubled. This works surprisingly well for some time, until you hit a UNC path which already starts with two backslashes. As an example, \\backslash\lashes\back turns into \\\\backslash\\lashes\\back, which no DOS shell will be able to grok anymore.

John spotted this issue as well. Maybe he should be writing these blog entries, don't you think? smile

From those Lisp subtleties back to the original problem: I never quite understood why the extra level of quoting is necessary for cmd.exe, but apparently, others have been in the same mess before. For example, check out this XEmacs code to see how complex correct quoting can be. See also an online version of the help pages for CMD.EXE for more information on the involved quoting heuristics applied by the shell.

PS: A very similar situation occurs in OneSpace Designer Drafting as well (which is our 2D CAD application). To start an executable write.exe in a directory c:\temp\foo bar and have it open the text file c:\temp\foo bar\foo bar.txt, you'll need macro code like this:

LET Cmd '"C:\temp\foo bar\write.exe"'
LET File '"C:\temp\foo bar\foo bar.txt"'
LET Fullcmd (Cmd + " " + File)
LET Fullcmd ('"' + Fullcmd + '"')  { This is the important line }
RUN Fullcmd

Same procedure as above: If both the executable's path and the path of the data file contain blank characters, the whole command string which is passed down to cmd.exe needs to be enclosed in an additional pair of quotes...

Revision: r1.7 - 05 Jun 2011 - 15:10 - ClausBrod

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