Zhu Yong

$ echo "Be Persistent, Be Patient" >> ~/.life

Feb 20, 2014 - 6 minute read - Comments - vim clang

Hack YouCompleteMe And libclang For Lamely Structured Project

On 13 Feb 2014 I asked a question on StackOverflow on how to configure / modify vim plugin YouCompleteMe to fit special project structure. I guess my project structure is really lame, after two days waiting, some comments were given, but my problem not realy addressed. So I think I have to DO IT MYSELF. Then I read the source code for YouComeplteMe as well as libclang, because I only need the code completion function, lucky the related code are not much. Then I done experiments with different approaches, and today, finally, I had my hacking works. Then I think I should record this nice experience, that’s the purpose of this blog post.

My Question On StackOverflow

My C project has following structures. This is the structure for a large project, I can’t change the structure. I want to use YouCompleteMe for semantic code completion for this project.

main/                     // folder for C file to be compiled
  |- module1.c            // module main C file.
  |- module2.c
  |- .....
  |- mod1_func1.c         // function file to be included in main module C file.
  |- mod1_func2.c
  |- mod2_func3.c
  |- mod2_func1.c
  |- mod2_func2.c
  |- mod2_func3.c

Content for moduleX.c, this will include all related header files and module related C files.

#include "header1.h"
#include "header2.h"
#include "modX_func1.c"
#include "modX_func2.c"

Content for modX_funcX.c has one or few function definitions. Doesn’t have header included

// no header file included here
int modX_funcX(void) {.....}

Because there is not related header included, clang must parse moduleX.c in order to do code completion on modX_funcX.c, I have tried clang code completion from command line. Command below works

clang -cc1 -x c -fsyntax-only -code-completion-at mod1_func1.c:4:11 module1.c 

So my question: how to configure YouCompleteMe to do code completion when I edit the modX_funcX.c file?

I guess modification to YouCompleteMe source might required to do this job. My current idea is to add a file mapping database with format:


So before send the code completion request, get path_of_file_for_clang_to_parse from database based on current buffer name, pass this file name to libclang.

Is my idea workable? If Yes, where is exactly place to add this file mapping function?

My Analysis

To make job easy, I make a simple project structure to simulate my rare project structure.

$ cd ~/dev/c/clang
$ ls
a.c  a.h  b.c  b.h  c.c  c.h  main.c

content for a.h

typedef struct A {
    struct A * next;
    unsigned int data;

content for a.c

int test(void)
    A *list;

content for main.c

#include "a.h"
#include "b.h"
#include "c.h"

#include "a.c"
#include "b.c"
#include "c.c"

My ultimate goal is to let YouCompleteMe to do code completion when I type -> at line 4 of file a.c. To test with clang from command line

$ clang -cc1 -x c -fsyntax-only -code-completion-at a.c:4:11 main.c
COMPLETION: data : [#unsigned int#]data
COMPLETION: next : [#struct A *#]next

Analyze clang Command Line

So command line code completion works for this simulation structure. To understand why clang CLI works while YouCompleteMe doesn’t, I need to find the difference at the point of code completetion for CLI and libclang. So I need a debug version of clang. Below are the steps to download and build debug version of clang version 3.3.

$ mkdir -p /data/sourcecode
$ svn co http://llvm.org/svn/llvm-project/llvm/tags/RELEASE_33/final llvm3.3
$ cd llvm3.3/tools
$ svn co http://llvm.org/svn/llvm-project/cfe/tags/RELEASE_33/final clang
$ cd ../projects
$ svn co http://llvm.org/svn/llvm-project/compiler-rt/tags/RELEASE_33/final compiler-rt
$ cd /data/build
$ mkdir llvm3.3-linux
$ cd llvm3.3-linux
$ /data/sourcecode/llvm3.3/configure --prefix=/data/software/C-C++/llvm3.3-hack/ CC=/usr/bin/gcc CXX=/usr/bin/g++
$ make -j4
$ make install

Next step is to load debug version of clang with the working arguments. I want to break at function ASTFrontendAction::ExecuteAction() to check some variables.

$ gdb --args /data/software/C-C++/llvm3.3-hack/bin/clang -cc1 -x c -fsyntax-only -code-completion-at a.c:4:11 main.c

(gdb) break z:\sourcecode\llvm3.3\tools\clang\lib\frontend\frontendaction.cpp:457

Breakpoint 1 at 0xd775d3: file /data/sourcecode/llvm3.3/tools/clang/lib/Frontend/FrontendAction.cpp, line 457.

(gdb) run

Starting program: /data/software/C-C++/llvm3.3-hack/bin/clang -cc1 -x c -fsyntax-only -code-completion-at a.c:4:11 main.c
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib/x86_64-linux-gnu/libthread_db.so.1".
Breakpoint 1, clang::ASTFrontendAction::ExecuteAction (this=0x513a5d0) at /data/sourcecode/llvm3.3/tools/clang/lib/Frontend/FrontendAction.cpp:457
457       if (hasCodeCompletionSupport() &&

(gdb) print CI.getFrontendOpts().Inputs[0]

$3 = (clang::FrontendInputFile &) @0x513b600: {File = {static npos = <optimized out>, _M_dataplus = {<std::allocator<char>> = {<__gnu_cxx::new_allocator<char>> = {<No data fields>}, <No data fields>}, _M_p = 0x513b5e8 "main.c"}},
Buffer = 0x0, Kind = clang::IK_C, IsSystem = false}

(gdb) print CI.getFrontendOpts().CodeCompletionAt

$4 = {FileName = {static npos = <optimized out>, _M_dataplus = {<std::allocator<char>> = {<__gnu_cxx::new_allocator<char>> = {<No data fields>}, <No data fields>}, _M_p = 0x513b568 "a.c"}}, Line = 4, Column = 11}

From gdb output, I know a.c is stored in CI.getFrontendOpts().CodeCompletionAt while main.c is stored in CI.getFrontendOpts().Inputs[0]

Analyze YouCompleteMe with libclang

Entry point for code completion from libclang is clang_codeCompleteAt and eventually it call to ASTUnit::CodeComplete(). And at end of this function, it also call ASTFrontendAction::ExecuteAction() same as clang from command line. It also has input file defined by Clang->getFrontendOpts().Inputs[0]. Up to this point, my thought become clear, my target is how to make Clang->getFrontendOpts().Inputs[0] set to main.c with all the passed in parameters.

My Hacking

The First Fail Hack

My first attemp is rather simple, as I stated in my StackOverflow question. When call clang_codeCompleteAt(), pass complete_filename in form of path_of_file_to_complete:path_of_file_for_clang_to_parse. This attempt failed because it won’t even run to the point of ASTUnit::CodeComplete(). Along the calling path, there are other function check and parse the input file, my hacking filename format is an invalid filename for those functions.

The Second Fail Hack

I understand that Clang->getFrontendOpts() is parsed from the passed flags, so my second attempt of hacking is to pass main.c together with flags. So I hardcode .ycm_extra_conf.py to append absolute path of main.c in flags, but Clang->getFrontendOpts().Inputs[0] was not set to main.c when I check at ASTUnit::CodeComplete(). So this attempt failed.

The Third Successful But Dirty Hack

After failed previous two attempts, I only have the option to do dirty hacking by change libclang API and related functions, as well as related parts in YouComplteMe. The idea was to pass in additional input_filename together with complete_filename. And at ASTUnit::CodeComplete(), if this input_filename is valid, use it to replace Clang->getFrontendOpts().Inputs[0]. This attempt works!!!


To solve my problem, I was forced to read source code of YouCompleteMe and clang. The hacking process takes time because of code reading, compilation and debugging, and it’s disappointed to find out my attempt didn’t work as expected. But I really enjoy the moment when the code worked after some days’ struggling. This whole hacking process is just another proof that open source is really good, you can do any hacking to fit your own need.

Update 2015-02-01

I have tried another hacking, a better hacking, click here for more info.