Index

1. Creating shared libraries under GNU/Linux

• focus on the major advantage
• interfacing C, C++, Modula-2 with scripting languages
  • Python, Perl, Ruby, TCL
  • further focus examples around Python
• Python's modules are either written in Python or are implemented as a shared library
  • or a combination of both
• we will briefly examine the following tools
  • gcc , libtool , swig , make and gm2

2. Simple pedagogical example

• let us create a module to sum two integers, we will use swig to call C functions from Python

• mymodule.i

  %module mymodule
  %{
  extern int sum (int a, int b);
  %}
  extern int sum (int a, int b);

• 
  mymodule.c

  int sum (int a, int b)
  {
    return a + b;
  }

• $ swig -python mymodule.i

• generates the following files:
  • mymodule_wrap.c and mymodule.py
    

3. Use gcc and libtool to compile and link the shared library

• $ libtool --tag=CC --mode=compile gcc -g -I/usr/include/python2.7 \
    -c mymodule_wrap.c -o mymodule_wrap.lo
  $ libtool --tag=CC --mode=compile gcc -g -I/usr/include/python2.7 \
    -c mymodule.c -o mymodule.lo
  $ libtool --tag=CC --mode=link gcc -g mymodule.lo mymodule_wrap.lo \
    -L$HOME/opt/lib64 -rpath ‘pwd‘ -lc -lm -o libmymodule.la
  $ cp .libs/libmymodule.so _mymodule.so
  $ python testsum.py
  3

4. libtool on GNU/Linux

• notice the file extensions .lo and .la
• libtool is told about the library dependents and where other shared libraries reside

5. Output from running previous libtool command

• libtool: compile:  gcc -g -I/usr/include/python2.7 -c mymodule_wrap.c  -fPIC -DPIC -o .libs/mymodule_wrap.o
  libtool: compile:  gcc -g -I/usr/include/python2.7 -c mymodule_wrap.c -o mymodule_wrap.o >/dev/null 2>&1
  libtool: compile:  gcc -g -I/usr/include/python2.7 -c mymodule.c  -fPIC -DPIC -o .libs/mymodule.o
  libtool: compile:  gcc -g -I/usr/include/python2.7 -c mymodule.c -o mymodule.o >/dev/null 2>&1
  libtool: link: rm -fr  .libs/libmymodule.a .libs/libmymodule.la .libs/libmymodule.lai \
    .libs/libmymodule.so .libs/libmymodule.so.0 .libs/libmymodule.so.0.0.0
  libtool: link: gcc -shared  -fPIC -DPIC  .libs/mymodule.o .libs/mymodule_wrap.o \
     -L‘pwd‘/lib64 -lc -lm    -Wl,-soname -Wl,libmymodule.so.0 -o .libs/libmymodule.so.0.0.0
  libtool: link: (cd ".libs" && rm -f "libmymodule.so.0" && ln -s "libmymodule.so.0.0.0" "libmymodule.so.0")
  libtool: link: (cd ".libs" && rm -f "libmymodule.so" && ln -s "libmymodule.so.0.0.0" "libmymodule.so")
  libtool: link: ar cru .libs/libmymodule.a  mymodule.o mymodule_wrap.o
  libtool: link: ranlib .libs/libmymodule.a
  libtool: link: ( cd ".libs" && rm -f "libmymodule.la" && ln -s "../libmymodule.la" "libmymodule.la" )

6. More complex example

• passing data from Python into C, C++, Modula-2 shared library
  • can pass int , float , double and enums easily enough
• strings are also reasonably well supported
  • binary strings of data can be passed
• build a sequence of bytes using the Python struct module
  • uses a printf formatting structure to pack and unpack binary data

7. Passing binary data from C, C++, Modula-2 into Python

• harder - but it can be done
  • the .i file needs extra information to say which functions give binary data and its length

• %cstring_output_allocate_size(char **start, int *used);
  
  
  %{
  extern "C" void get_ebuf (char **start, int *used);
  extern "C" void get_fbuf (char **start, int *used);

• swig has many mechanisms to allow binary strings of data to be retrieved
  • above is the safest - as it contains the length
    

8. Case study: pge

• #!/usr/bin/env python
  
  import pge
  
  print "starting exampleBoxes"
  pge.batch ()
  
  
  t = pge.rgb (1.0/2.0, 2.0/3.0, 3.0/4.0)
  wood_light = pge.rgb (166.0/256.0, 124.0/256.0, 54.0/256.0)
  wood_dark = pge.rgb (76.0/256.0, 47.0/256.0, 0.0)
  metal = pge.rgb (0.5, 0.5, 0.5)
  ball_size = 0.04
  boarder = 0.01

• def placeBoarders (thickness, color):
      print "placeBoarders"
      e1 = pge.box (0.0, 0.0, 1.0, thickness, color).fix ()
      e2 = pge.box (0.0, 0.0, thickness, 1.0, color).fix ()
      e3 = pge.box (1.0-thickness, 0.0, thickness, 1.0, color).fix ()
      e4 = pge.box (0.0, 1.0-thickness, 1.0, thickness, color).fix ()
      for e in [e1, e2, e3, e4]:
          e.on_collision (play_wood)
      return e1, e2, e3, e4

• def crate (x, y, w):
      c = pge.box (x, y, w, w, wood_dark).on_collision (crate_split).set_param (6)

• def crate_split (p):
      w = p.width () / 2
      wg = w - gap
      e = p.get_param ()
      if e != None:
          if e % 2 == 1:
              # subdivide into smaller crates, every odd bounce
              m = p[0].mass ()
              c = p[0].colour ()
              for v in [[0, 0], [0, w], [w, 0], [w, w]]:
                  b = pge.box (v[0], v[1], wg, wg, c).mass (m).on_collision (crate_split)
                  b.set_param (e-1)
              p.delete ()
          elif e == 0:
              # at the end of 6 collisions the crates disappear
              p.delete ()
          else:
              # allow collision (bounce) without splitting every even bounce
              p.set_param (e-1)

9. Overview of the Python interface to PGE

10. Conclusion

• work in progress
• having an easy mechanism to write to a (char *) memory buffer is very useful (Modula-2 MemStream)
• struct module in Python is also very useful
• wrapping low level shared library objects (represented by int s) is also good
  • can test parameter types at run time in Python before they are translated into the pge int objects

• swig -outdir . -o pgeif_wrap.cxx -c++ -python $(srcdir)/pgeif.i
  libtool --tag=CC --mode=compile g++ -g -c pgeif_wrap.cxx \
    -I/usr/include/python$(PYTHON_VERSION) -o pgeif_wrap.lo
  gm2 -c -g -I$(SRC_PATH_PIM) -fmakelist $(srcdir)/pgeif.mod
  gm2 -c -g -I$(SRC_PATH_PIM) -fmakeinit -fshared $(srcdir)/pgeif.mod
  libtool --tag=CC --mode=compile g++ -g -c _m2_pgeif.cpp -o _m2_pgeif.lo
  libtool --tag=CC --mode=link gcc -g _m2_pgeif.lo $(PGELIBSOURCES:%.mod=%.lo) \
     pgeif_wrap.lo buffers.lo \
    -L$(prefix)/lib64 \
    -rpath ‘pwd‘ -lgm2 -liso -lgcc -lstdc++ -lpth -lc -lm -o libpgeif.la \
  cp .libs/libpgeif.so _pgeif.so

Index

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