CODEWITHSUNDEEP
c-preprocessorpreprocessor-directive
owagh
owagh

Reputation: 3528

C/C++ file before preprocessing

Is it possible to look at a c/c++ file before preprocessing? Or rather after just a half-hearted pre-processing? Basically there is a 

#define <commonly_used_word> 0

in a third party library header and I want to figure out where it is. So basically, I just want the compiler to include all the headers but not the the preprocessor as such.

Upvotes: 1

Views: 301

Answers (4)

AnT stands with Russia
AnT stands with Russia

Reputation: 320481

Your original source file is file before preprocessing.

It sounds like you want your #include directives processed, yet you want to keep macros non-substituted. Both actions are carried out by the preprocessor.

In general case it is impossible, since in C and C++ it is legal to use macros as include file names, as in

#define INCLUDE_FILE "stdio.h"
#include INCLUDE_FILE

Achieving what you want would require a preprocessor specifically designed to satisfy your request. I, for one, don't know of any such preprocessor implementation.

If you want to find where a specific macro is defined, you might try the following trick: define your own macro with the same name before including any headers, and start compilation. The compiler (the preprocessor) should complain about macro redefinition when it encounters the library definition of the same macro and point out its location to you.

Upvotes: 5

Nicola Musatti
Nicola Musatti

Reputation: 18218

If you know which header files contain the definition you're looking for, e.g by using find and grep as suggested, you may be able to pinpoint which one is affecting the current source file by getting gcc to print the header inclusion tree. As described in gcc's documentation, you can achieve this by using the -H option, possibly combined with -MG to eliminate normal processing.

Upvotes: 1

Michał G&#243;rny
Michał G&#243;rny

Reputation: 19243

You can tell cpp to generate the list of included files using -M option:

$ cpp -M a.c
a.o: a.c /usr/include/stdio.h /usr/include/features.h \
 /usr/include/sys/cdefs.h /usr/include/bits/wordsize.h \
 /usr/include/gnu/stubs.h /usr/include/gnu/stubs-64.h \
 /usr/lib/gcc/x86_64-pc-linux-gnu/4.7.1/include/stddef.h \
 /usr/include/bits/types.h /usr/include/bits/typesizes.h \
 /usr/include/libio.h /usr/include/_G_config.h /usr/include/wchar.h \
 /usr/lib/gcc/x86_64-pc-linux-gnu/4.7.1/include/stdarg.h \
 /usr/include/bits/stdio_lim.h /usr/include/bits/sys_errlist.h

It gives you a Makefile rule format but you could ignore that and use with any other command.

For example, you can grep for the symbol (here I'm ignoring stderr because of \ and a.o: not being a real file names -- laziness):

$ grep '#\s*define\s*BUFSIZ' $(cpp -M a.c) 2>/dev/null
/usr/include/stdio.h:# define BUFSIZ _IO_BUFSIZ

You can also use a program like ctags to find the symbol for you:

$ ctags $(cpp -M a.c)
...
$ grep BUFSIZ tags 
BUFSIZ  /usr/include/stdio.h    128;"   d

Upvotes: 1

vines
vines

Reputation: 5225

There are GCC-specific -M and -MM options:

To list absolute paths of include files, use -M

Instead of outputting the result of preprocessing, output a rule suitable for make describing the dependencies of the main source file. The preprocessor outputs one make rule containing the object file name for that source file, a colon, and the names of all the included files, including those coming from -include or -imacros command line options. gcc -M test.c

If you dont want the system includes like #include <stdio.h>, then use -MM Like -M but do not mention header files that are found in system header directories, nor header files that are included, directly or indirectly, from such a header. gcc -MM test.c

That could significantly narrow down the search area.

http://www.math-linux.com/spip.php?article263

Upvotes: 2

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