Visual Studio C++ preprocessor-define fails with path starting on "u"

Question

That's right I want to supply a path as preprocessor define (properties->configuration->c/c++->preprocessor)

MY_PATH=c:\$(WindowsSdkDir)\um

But this hits me upon use with

E1696 cannot open source file "C:\asdf\u0000m\xyz.h"
E0992 command-line error: invalid macro definition: MY_PATH=c:\asdf\um

Because visual studio seemingly sees \u as a unicode escape. However, there is no way to escape the backslash, so now I cant specify any path that contains a directory starting on u. I also cant switch to / as a path separator because I pull in environment variables that use .

What to do?

I am on latest Windows 10 with latest SDK and Visual Studio 2019.

`MY_PATH=c:\$(WindowsSdkDir)\um` is a statement of what language? Doesn't look like C++. Is is a Makefile? Is it a powershell script? The preprocessor by itself is just fine with paths like this, literal or passed via a command line option '/Dvar=value' — n. m. could be an AI, Aug 18 '22 at 13:58
weird, preprocessor macro has nothing to do with source opening so `E1696 cannot open source file "C:\asdf\u0000m\xyz.h"` is probably unrelated to it — phuclv, Aug 18 '22 at 14:13
n.1, that would be what vs calls a "macro". It can be used in msvc preprocessor defines, at least if they are specified in the VS GUI / in the vcxproj file. phuclv is right, i then go on to use the define in an include statement. I get it both as intelisense and compile error. — bernd feinman, Aug 18 '22 at 14:22
Yes. Under . Can be edited via GUI right-click on projectfile properties -> configuration-> c/c++ -> preprocessor — bernd feinman, Aug 18 '22 at 17:48
I would try to replace either \ or `u` with their `%xx` equivalents as per [msbuild docs](https://learn.microsoft.com/en-us/visualstudio/msbuild/how-to-escape-special-characters-in-msbuild?view=vs-2022) — n. m. could be an AI, Aug 18 '22 at 17:59
Can you see the actual command line that runs the compiler? Should be in the build logs. — n. m. could be an AI, Aug 19 '22 at 18:14

score 2 · Answer 1 · answered Aug 18 '22 at 11:12

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You should use raw string literals for anything that requires escaping instead

char const * ddd = R"(C:\asdf\u0000m\xyz.h)";

No more escaping required and the result is much more readable. So in this case on the command line you'll use

 -DMY_PATH=R\"\(C:\\asdf\\u0000m\\xyz.h\)\"

because you only need to escape for the shell and not the C++ source code

Demo on Godbolt

answered Aug 18 '22 at 11:12

phuclv

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Thanks! But it seems this isnt supported when I need to add the ppdefine to my .vcxproj or use the built-in property editor in VS... Also I need to resolve environment variables in the path... – bernd feinman Aug 18 '22 at 11:24
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@berndfeinman Entering `MY_PATH=R"(c:\$(WindowsSdkDir)\um)"` in the properties in Visual Studio works for me. Printing `std::string s = MY_PATH;` results in `c:\C:\Program Files (x86)\Windows Kits\10\\um`, as expected. (Naturally, prepending "c:\" makes no real sense, but that is what you wrote in the original post.) In the vcprojx file, it looks the same: `MY_PATH=R"(c:\$(WindowsSdkDir)\um)";%(PreprocessorDefinitions)`. If it doesn't work for you, you need to post more information (e.g. the relevant part of the vcprojx and the source). – Sedenion Aug 18 '22 at 19:41
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Thanks! I am getting the same issue though - but! meanwhile I have found out the bug is related to msvc two phase lookup. So sometimes you can get lucky and it will magically work with "rebuild". Even better turning off /Zc:twoPhase- ... I will report more tomorrow. Meanwhile, can you try to hit rebuild a couple of times or build new with minor modification on your end and see if it works always? – bernd feinman Aug 18 '22 at 19:58
It's a header name. Header names are *not* string literals though they look pretty similar. – n. m. could be an AI Aug 19 '22 at 20:31

score 1 · Answer 2 · answered Aug 18 '22 at 10:50

1

Use four backslashes: -DMY_PATH=\"C:\\\\asdf\\\\u0000m\\\\xyz.h\"

https://godbolt.org/z/9Yn4csjv1

answered Aug 18 '22 at 10:50

Osyotr

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Thanks, but this still doesnt work. Perhaps unclear - the 0000 are inserted by VS mistakenly, the actual directory name is "um". If i rename it "vm" it works fine. With four slashes, the 0000 are still inserted... I need to put the ppdefine inside my .vcxproj. – bernd feinman Aug 18 '22 at 11:26
Hmm that sounds like file encoding problem. Make sure your vcxproj is saved in utf-8 – Osyotr Aug 18 '22 at 12:17
It is. It even happens when I input the path with the VS GUI. Seems like a VS bug? – bernd feinman Aug 18 '22 at 12:22
Yeah, I bet it is – Osyotr Aug 18 '22 at 12:48

Visual Studio C++ preprocessor-define fails with path starting on "u"

2 Answers2