Is there a way for CMake to utilize dependencies generated by `swig -MM`?

Question

SWIG generates wrapper code from your C/C++ in a desired target language (Python, Java, C#, etc) using an interface (.i) file that specifies the input code to be wrapped as described in the SWIG tutorial . CMake can be used to call swig in order to generate target code from the .i interface, as described in the SWIG documentation .

However, using this method CMake only generates a dependency for the interface file itself, but not for the source files it includes. One can manually add dependencies , but SWIG can generate dependencies automatically with the -MM option, and I would like for these to be utilized by CMake.

There was a commit to CMake that utilized dependencies generated by swig -MM but it was later reverted due to a problem with generated sources that didn't exist at the time of the call to swig. At this time the problem seems to remain unsolved.

So I put the issue to the brilliant StackOverflow community: Is there a way with the current CMake to utilize dependencies generated by swig -MM when the interface file (a) does not include generated code (eg config.h), and (b) includes generated code?

Here is a small example that can be used for experimentation ( download it here ).

// swig_example.h
int foo(int n);
//*** comment this declaration after compiling once to witness dependency failure ***/
int another_function();

// swig_example.cpp
#include "swig_example.h"
int another_function() {return -1;}
int foo(int n) 
{
    if (n <= 1) return 1;
    else return another_function();
}

// swig_example: example.i
%module example
%{
#include "swig_example.h"
%}
%include "swig_example.h"

# swig_example: CMakeLists.txt
FIND_PACKAGE(SWIG REQUIRED)
INCLUDE(${SWIG_USE_FILE})
FIND_PACKAGE(PythonLibs)
INCLUDE_DIRECTORIES(${PYTHON_INCLUDE_PATH})
INCLUDE_DIRECTORIES(${CMAKE_CURRENT_SOURCE_DIR})
SET(CMAKE_SWIG_FLAGS "")

SET_SOURCE_FILES_PROPERTIES(example.i PROPERTIES CPLUSPLUS ON)

#add manual dependencies (must be called before SWIG_ADD_MODULE)
#SET(SWIG_MODULE_example_EXTRA_DEPS ${CMAKE_SOURCE_DIR}/swig_example.h)

SWIG_ADD_MODULE(example python example.i swig_example.cpp)
SWIG_LINK_LIBRARIES(example ${PYTHON_LIBRARIES})

Compile it once, then comment the declaration of another_function and try to compile again. Because the swig interface is not regenerated an error occurs trying to compile examplePYTHON_wrap.cxx.

examplePYTHON_wrap.cxx:3220:17: error: use of undeclared identifier 'another_function'
  result = (int)another_function();

Uncomment the add manual dependencies line of the CMakeLists.txt and the interface will be properly regenerated. However, I want this to work using dependencies generated from swig -MM instead of needing to manually specify dependencies.

$ swig -python -MM -c++ ../example.i
../example_wrap.cxx: \
  ../example.i \
  ../swig_example.h \

Answer 1

Turning my comments into an answer

I don't think - if you want to do this automatically and eg want to utilize swig -MM - that this can be done without changing the UseSWIG.cmake code.

When I look at why the previous attempt you have linked was reverted - namely the discussion of on "0012307: regression in 2.8.5 rc2: UseSWIG.cmake broken" - I don't believe that SWIG_GET_WRAPPER_DEPENDENCIES() was actually broken it just introduced a new restriction: "this simply require all headers for swig module to be present" before calling SWIG_ADD_MODULE() .

So I suggested to add the -ignoremissing SWIG option, but this would need further testing.

---

Update (April 2017)

With CMake version 3.8.0 there came a fix "Automatically scan dependencies of SWIG files for Makefile generators" that works for makefile generators .

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Reference

The general discussion of how to fix this (including my suggestion) is discussed at "Issue #4147: [MODULES][UseSWIG] Use swig to compute dependencies" . The ticket is still open (was reopened), so please feel free to add your support, suggestions or test results there.