How to only process the macros in a C/C++ project without compiling it?

Question

I'm trying to review a C/C++ project which is heavily dealing with macros and function-like macros. What I would like to do is to replace the define and function-like macros with their replacement.

For example I have this file:

#include <iostream>

#define SUM(a,b,c,d) a+b+c+d
using namespace std;

int main(){

    cout << SUM(1,2,3,4) << endl;

}

And I want to reach to this file:

#include <iostream>

using namespace std;

int main(){

    cout << 1+2+3+4 << endl;

}

Please note that I'm not looking to replace the #include lines.

EDIT:

gcc -E expands the #define macros but it will also expand the #include macros as well. I DO NOT want the #include to be expanded.

@FiddlingBits It won't directly help this case because it will also expand #include <iostream>. — MikeCAT
– MikeCAT, Commented May 4, 2020 at 13:32
@MikeCAT you could first remove all lines with an include, then "compile" it — RoQuOTriX
– RoQuOTriX, Commented May 4, 2020 at 13:33
The duplicates proposed so far are not correct, because this question asks to expand the macros defined with #define but not to process #include statements. I do not think GCC has switches for that. One might do it by creating a copy of the source file with #include lines altered to suppress them, then using GCC, then restoring the #include lines. — Eric Postpischil
– Eric Postpischil, Commented May 4, 2020 at 13:48
untested -- I think I used this before -- gcc -no-stdinc -E ... or something similar — pmg
– pmg, Commented May 5, 2020 at 9:05

pmg · Accepted Answer · 2020-05-05 09:57:23Z

4

I've used gcc -nostdinc -E ... (or something similar) before.

Can't test this at the moment; don't really remember exactly why I used it either

answered May 5, 2020 at 9:57

pmg

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3CEZVQ · Accepted Answer · 2020-05-05 08:04:51Z

3

Use gcc -E to preprocess your source file. Then find the first line of the original source file after all #include and remove all lines before that first line. Then restore #include from the original source file.

Preprocessor marks original source lines for debugger like below so you can easily find the proper original first line after #include.

# 1 "main.c"
# 1 "<built-in>"
# 1 "<command-line>"
# 31 "<command-line>"
# 1 "/usr/include/stdc-predef.h" 1 3 4
# 32 "<command-line>" 2
# 1 "main.c"

What do the numbers mean in the preprocessed .i files when compiling C with gcc?

edited May 5, 2020 at 8:04

answered May 5, 2020 at 7:57

3CEZVQ

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Comments

Pat Pac · Accepted Answer · 2020-05-04 13:36:32Z

1

GCC has the -E flag which makes it output preprocessed source code.

answered May 4, 2020 at 13:36

Pat Pac

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2 Comments

Eric Postpischil Over a year ago

This is incorrect because gcc -E does the full preprocessing, including #define and #include directives, but the question asks not to process #include directives.

Pat Pac Over a year ago

I agree. I've misunderstood. The -E flag might still be useful though if You don't have to go through tones of includes appended in the output. I don't believe there's any ready solution like that for what OP needs.

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