Can gcc output C code after preprocessing?

Question

I'm using an open source library which seems to have lots of preprocessing directives to support many languages other than C. So that I can study what the library is doing I'd like to see the C code that I'm compiling after preprocessing, more like what I'd write.

Can gcc (or any other tool commonly available in Linux) read this library but output C code that has the preprocessing converted to whatever and is also readable by a human?

Solution 1

Yes. Pass gcc the -E option. This will output preprocessed source code.

Solution 2

cpp is the preprocessor.

Run cpp filename.c to output the preprocessed code, or better, redirect it to a file with cpp filename.c > filename.preprocessed.

Solution 3

-save-temps

This is another good option to have in mind:

gcc -save-temps -c -o main.o main.c

main.c

#define INC 1

int myfunc(int i) {
    return i + INC;
}

and now, besides the normal output main.o, the current working directory also contains the following files:

• main.i is the desired prepossessed file containing:

  # 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"
  
  
  int myfunc(int i) {
      return i + 1;
  }
  

• main.s is a bonus :-) and contains the generated assembly:

      .file   "main.c"
      .text
      .globl  myfunc
      .type   myfunc, @function
  myfunc:
  .LFB0:
      .cfi_startproc
      pushq   %rbp
      .cfi_def_cfa_offset 16
      .cfi_offset 6, -16
      movq    %rsp, %rbp
      .cfi_def_cfa_register 6
      movl    %edi, -4(%rbp)
      movl    -4(%rbp), %eax
      addl    $1, %eax
      popq    %rbp
      .cfi_def_cfa 7, 8
      ret
      .cfi_endproc
  .LFE0:
      .size   myfunc, .-myfunc
      .ident  "GCC: (Ubuntu 8.3.0-6ubuntu1) 8.3.0"
      .section    .note.GNU-stack,"",@progbits
  

If you want to do it for a large number of files, consider using instead:

 -save-temps=obj

which saves the intermediate files to the same directory as the -o object output instead of the current working directory, thus avoiding potential basename conflicts.

The advantage of this option over -E is that it is easy to add it to any build script, without interfering much in the build itself.

Another cool thing about this option is if you add -v:

gcc -save-temps -c -o main.o -v main.c

it actually shows the explicit files being used instead of ugly temporaries under /tmp, so it is easy to know exactly what is going on, which includes the preprocessing / compilation / assembly steps:

/usr/lib/gcc/x86_64-linux-gnu/8/cc1 -E -quiet -v -imultiarch x86_64-linux-gnu main.c -mtune=generic -march=x86-64 -fpch-preprocess -fstack-protector-strong -Wformat -Wformat-security -o main.i
/usr/lib/gcc/x86_64-linux-gnu/8/cc1 -fpreprocessed main.i -quiet -dumpbase main.c -mtune=generic -march=x86-64 -auxbase-strip main.o -version -fstack-protector-strong -Wformat -Wformat-security -o main.s
as -v --64 -o main.o main.s

Tested in Ubuntu 19.04 amd64, GCC 8.3.0.

CMake predefined targets

CMake automatically provides a targets for the preprocessed file:

make help

shows us that we can do:

make main.i

and that target runs:

Preprocessing C source to CMakeFiles/main.dir/main.c.i
/usr/bin/cc    -E /home/ciro/bak/hello/main.c > CMakeFiles/main.dir/main.c.i

so the file can be seen at CMakeFiles/main.dir/main.c.i

Tested on cmake 3.16.1.

Solution 4

I'm using gcc as a preprocessor (for html files.) It does just what you want. It expands "#--" directives, then outputs a readable file. (NONE of the other C/HTML preprocessors I've tried do this- they concatenate lines, choke on special characters, etc.) Asuming you have gcc installed, the command line is:

gcc -E -x c -P -C -traditional-cpp code_before.cpp > code_after.cpp

(Doesn't have to be 'cpp'.) There's an excellent description of this usage at http://www.cs.tut.fi/~jkorpela/html/cpre.html.

The "-traditional-cpp" preserves whitespace & tabs.

Solution 5

Run:

gcc -E <file>.c

or

g++ -E <file>.cpp

View more solutions

Comments

[Tor Klingberg]

If your compiler commands already has a parameter like -o something.o you may also want to change it to -o something.i. Otherwise the preprocessed output will be in the .o file.

[user2808264]

@TorKlingberg Can I do this for multiple files at a time?

[amirouche]

Many thanks, this is very helpful to generate python cffi cdef!

[lee8oi]

I think this is the best answer because it demonstrates cpp directly. Linux systems (at least Manjaro) seem to have -E by default too. I get the same results from this command either way. diff turns up no difference in the files. This is also looks like a useful way to preprocess the code looking for errors in your macros. Great question and a great answer (IALCTHW).

[Matthieu]

@user2808264 gcc -E file1.c file2.c ...

[EvertW]

Much more elegant than -E because I can just add -save-temps to CFLAGS without changing the overall behaviour of the build script. Thank you!

[C--]

This is indeed very useful, and -E is very convenient for single files.

[mercury0114]

Is there an option for the preprocessor to expand only macroses like #define SIZE 1000 or #ifdef Something #endif but not #include <other_file.h> I want to see a preprocessed file but without external functions imported into a single file.

[user64742]

@lee8oi I was curious what IALCTHW meant but my attempt to web search for it only resulted in this page as a search result. What does that acronym mean? I am quite curious.

[Alex Measday]

A little late! You can skip steps 2 and 3 -- g++/gcc recognize .i files as preprocessed source code. First, #define the PQR macro in the source file. Then: Step 1: g++ -E Message.cpp > Message.i Step 2: g++ Message.i > MessageApp

[MrMesees]

cpre article link jkorpela.fi/html/cpre.html I am guessing this is what is filling the suggested edit queue...

[Pedro Perez]

Incredibly useful! thanks!