Quoting Command-line Arguments

Raymond Chen recently blogged about the way CommandLineToArgvW treats quotes and backslashes. Parsing the command-line into argv[] is something I have had to fight with as well, so besides pointing to Raymond’s excellent post, I wanted to add a few comments of my own here.

We are examining how command-line arguments with spaces and quotes are handled. Part of the problem comes from the fact that DOS/Windows uses backslash as separator in paths. On systems like Unix, where forward slash is used instead, using backslash to escape special characters is less of a problem. But if you ever put a Windows path in a C string literal, you may have run into LTS — the situation where a string becomes unreadable due to escape characters.

Microsoft fixed this in C# with verbatim string literals. C# also implements a simpler method of escaping a quote inside a quoted string — doubling it — which is used in languages like Pascal and BASIC, and is what Raymond’s second hypothetical set of rules suggest.

The compromise we get for parsing command-line arguments in the C runtime library (and CommandLineToArgvW) is documented on MSDN. What the MSDN documentation does not tell you is that there is a second mechanism for inserting a literal quote in a quoted string — or at least there might be, depending on which version of the C runtime library.

In Visual C++ 2.0 from 1994, the command-line parser handles two quotes inside a quoted string as an escape. What it does is that it copies the second quote and switches to unquoted mode. Thus you can insert a literal quote inside a quoted string by putting three quotes (two to insert the literal quote, and one more to put you back into quoted mode).

This code is present until Visual C++ 8.0 from 2005, where it is changed so the two quote escape sequence no longer switches mode. That means that to insert a literal quote inside a quoted string, you now only need two quotes.

This change is probably what is causing some confusion in the comments on Raymond’s post — the twelve literal quotes in foo""""""""""""bar will result in five quotes with the current parser (opening quote, five pairs, closing quote), but only four with the older method (opening quote, three triples, and a double quote).

The implementation of CommandLineToArgvW in current versions of shell32.dll use the older method. The same goes for the command-line parser used in GCC on Windows (I checked 4.5.1).

Looking at the two code excerpts, one could wonder if the old handling was in fact just a bug introduced by the else part not being in braces, and left in for over ten years. Or maybe it was intentional, and the change in Visual C++ 8.0 was to support the way two quotes are used as escape in other languages.

Either way, I find it interesting that this functionality was added (it wasn’t supported in Visual C++ 1.52), and then changed in the C runtime library but not in the API function, and not mentioned in the documentation of either.

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