C++ Preprocessor doesn't process define as expected

Question

I'm having problems with cpp Preprocessor. I have this Input.h file like this:

#ifndef PLATFORM_MOBILE1111
    #define MyTest WEB111
#endif 

int MyTest;

I process it with this command (on OSX):

cpp -E -P Source/Input.h Generated/Output.h

I get this:

    #define MyTest WEB111


int MyTest;

i.e. macro MyTest is not getting applied. Why?

After a bunch of experimentation, I found that if I insert an empty line, variable definition, a comment or any other line after #ifndef line - then it works fine.

#ifndef PLATFORM_MOBILE1111

    #define MyTest WEB111
#endif 

int MyTest;

So the input above gets processed correctly into:

int WEB111;

Can someone explain to me why that is happening? and how to solve that? Is there an option that I can pass?

Edit: I also found that ## (concatenation operator) doesn't work too!

Symbols beginning with an underscore and followed by an uppercase letter are reserved for the implementation, so you should rename your macro. See: https://stackoverflow.com/questions/228783/what-are-the-rules-about-using-an-underscore-in-a-c-identifier — Cody Gray - on strike, Sep 01 '17 at 13:34
@Bathsheba I thought your comment was a clever pun on undefined behavior. Perhaps a bit too obtuse if you don't already know why, though. — Cody Gray - on strike, Sep 01 '17 at 13:36
It's essentially unanswerable until `_Test` gets renamed to something else, and the problem shown to still persist. — Bathsheba, Sep 01 '17 at 13:41
As others have said, change the name of that macro. But that business with the behavior changing when you add a blank line is **definitely wierd**, and almost certainly **not** the result of using a reserved name. — Pete Becker, Sep 01 '17 at 13:43
I changed the name and I can confirm that the problem persists. I updated my sample code too. — Paulius Liekis, Sep 01 '17 at 21:58
Perhaps your file has one or more characters that is undisplayed (or undisplayable) by your text editor? — , Sep 01 '17 at 22:01
@Bathsheba The standard doesn't define *any* behaviour for a standalone preprocessor. — n. m. could be an AI, Sep 01 '17 at 22:17
You aren't passing `-dD` or similar, so you shouldn't be getting any `#` lines in the output at all. Make sure that's really a `#`. — o11c, Sep 01 '17 at 22:39

Johannes Overmann · Answer 1 · 2017-09-22T23:23:53.543

Strange! You clearly found a bug in the clang preprocessor!

I can reproduce this as well (Apple LLVM version 8.1.0 (clang-802.0.42)) on OSX.

This also starts working as expected when you remove the spaces before #define (indentation), but leading spaces should not matter at all, and a lot of editors indent #defines inside #ifdefs.

One hint on what is going on is that in the bogus versions the #define is still present in the processed source code, while in the correct version it is not. So apparently the preprocessor did not recognize the #define as such at all.

Even this fails (note the leading space):

 #define MyTest WEB111
int MyTest;

While this works as expected:

#define MyTest WEB111
int MyTest;

And this results in WEB222:

 #define MyTest WEB111
 #define MyTest WEB222
int MyTest;

While this results in WEB111:

#define MyTest WEB111
 #define MyTest WEB222
int MyTest;

Crazy!1!! :-)

It seems that always the first indented #define gets ignored.

Now the 'solution': Use:

gcc -E Source/Input.cpp -P -o Generated/Output.cpp

This works as expected, although it is using the same LLVM.

So I assume the gcc executable initializes the preprocessor differently than the cpp. This is still a bug in cpp.

score 1 · Accepted Answer · answered Jan 06 '18 at 19:30

I found out that running preprocessor through c++ rather than cpp solves both of my problems:

c++ -E -P Source/Input.h Generated/Output.h

BTW.: you need to add -x c++ flag if you file is not using regular c/c++ extention for c++ command. cpp works fine without that.

C++ Preprocessor doesn't process define as expected

2 Answers2