SWIG for HDF5

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SWIG for HDF5

• Choonghwan Lee
• The HDF Group
• August 6, 2008

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Motivation

• Python users want to write an application using
  HDF5 library

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SWIG(Simplified Wrapper and Interface Generator)
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What is SWIG?

• Takes C declarations and turns them into the glue
  needed to access them from higher-level
  languages.
• Lower-level libraries
• C, C, Objective-C
• Higher-level languages
• Python, Perl, Tcl, C, Java,
• We can manually do this, but it is too tedious
• As lex and yacc make our lives easier

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SWIG is not

• Library application users dont need SWIG
• Interpreter add-on
• Generated wrapper may be still low-level
• PyHDF

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What does SWIG rely on?

• Higher-level languages can call C function from
  shared object and dynamically linked library
• Python extension module
• C - P/Invoke
• Java JNI
• Reading and writing variables can be wrapped by
  getter and setter functions

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How to use SWIG
SWIG Interface
SWIG
C Wrapper
Python Wrapper
swig python hello.i
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How to use SWIG
gcc shared \ hello_wrap.c \ impl.c \ -o
_hello.so
SWIG Interface
SWIG
C Wrapper
Python Wrapper
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How to use SWIG
SWIG Interface
SWIG
python gtgtgt import hello gtgtgt print
hello.literal 1 gtgtgt print hello.constvar 1
C Wrapper
Python Wrapper
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HDF5 Wrapper
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Goal

• Does not introduce new names
• readonly variables or constants

hdf5.H5Dcreate(group, name,
hdf5.H5T_NATIVE_INT, dataspace,
hdf5.H5P_DEFAULT)
hdf5.H5Fopen(fname,
hdf5.H5F_ACC_RDWR, hdf5.H5P_DEFAULT)
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Problem

• Some macros are too complicated for SWIG

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What can SWIG recognize?

• Global variables
• Preprocessor macros

/put in cvar/ extern int globalvar1_g const
int constvar 3
/ignored/ define funccall_globalvar \
(init(), globalvar1_g) /ignored/
define funccall_literal \
(check(), 2) define literal 4 /ignored/
define globalvar globalvar1_g
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Augment SWIG Interface file
module hdf5 ... ... init H5open()
H5check() undef H5T_NATIVE_INT constant
hid_t H5T_NATIVE_INT H5T_NATIVE_INT_g undef
H5F_ACC_RDWR define H5F_ACC_RDWR 0x0001u
/ HDF5 headers / extern hid_t
H5T_NATIVE_INT_g define H5T_NATIVE_INT \
(H5OPEN H5T_NATIVE_INT_g) define H5F_ACC_RDWR
\ (H5CHECK 0x0001u)
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Evaluation

• Dont need to modify HDF5
• Need to distribute SWIG Interface file
• Hard-coded macros are not manageable
• define H5F_ACC_RDWR 0x0001u

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Discussion

• Can we distribute SWIG Interface?
• Can we modify HDF5 library?
• Backward compatible?