Compiling previously preprocessed file changes out

问题:

I have a source file which I preprocess using the options -E and -P (using GCC 4.1.2 for a vxWorks-based embedded platform). All other options are the same as when I compile the file. These options are:

-Wall  
-march=pentium 
-nostdinc 
-O0 
-fno-builtin 
-fno-defer-pop
-g  
-c
-o

as well as all include-paths. Now when I compile this preprocessed file, the resulting object-file is much smaller (about 30%) than when I compile the original directly. And when I then link the program, the linker complains about missing symbols (all in user-code), which again does not happen when using the original source-file. Why is there a difference? Is there any way to make this work?

回答1:

You're sure you're not missing any -D defines from your command line? Your result would be consistent with parts not being compiled due to conditionals.

Another possibility (since you don't name the compiler specifically) is that you're using a generic gcc -E rather than the arch-specific cross compiler for your vxWorks environment. The cross-gcc will predefine some variables that you'll need for gcc -E.

回答2:

When compiling the preprocessed output, try passing the -fpreprocessed option to tell GCC not to preprocess again.

The only difference I can think of is macros that result in expanding to an identifier that's a macro name that has already been expanded - the preprocessor stops expansion at that point, but if you ran the preprocessor again, the identifier would be expanded again. I would have expected any instances of this to probably cause a compiler error, but who knows?