I'm trying to get a very basic Python-to-C interface working with SWIG where I can pass a pointer to a structure to a C function and the members get populated, and I can then access the members in python.
In the below example, everything works except when I try to print the structure members:
print swig_test.diags_mem0_get(var)
Results in:
$ ./runpython.py
Traceback (most recent call last):
File "./runpython.py", line 11, in <module>
print swig_test.diags_mem0_get(var)
AttributeError: 'module' object has no attribute 'diags_mem0_get'
Whereas this:
print var.mem0
Results in:
$ ./runpython.py
<Swig Object of type 'uint16_t *' at 0x7f8261e15b40>swig/python detected a memory leak of type 'uint16_t *', no destructor found.
I am following the SWIG 3.0 Documentation, specifically section "5.5 Structures and unions" here: http://swig.org/Doc3.0/SWIGDocumentation.html#SWIG_nn31
What am I doing wrong?
I have distilled the example down to bare bones:
swig_test.h
typedef struct diags_t {
uint16_t mem0;
uint16_t mem1;
} diags;
diags *malloc_diags(void);
void free_diags(diags *pdiag);
int get_diags(diags *pdiags);
swig_test.c
#include <stdlib.h> // malloc()
#include <stdint.h> // uint16_t
#include "swig_test.h"
int main(int argc, char **argv) {
return 0;
}
int get_diags(diags *pdiags) {
pdiags->mem0 = 0xdead;
pdiags->mem1 = 0xbeef;
return 0;
}
diags *malloc_diags(void) {
diags *dptr = malloc(sizeof(diags));
return dptr;
}
void free_diags(diags *pdiag) {
if (pdiag != NULL)
free(pdiag);
}
swig_test.i
%module swig_test
%{
#include "swig_test.h"
%}
%include "swig_test.h"
Makefile
CXX = gcc
INCLUDES = -I./
COMPFLAGS = -c -Wall -fPIC
PYINC = /usr/include/python2.7
SWIG = /usr/bin/swig
all: swig_test _swig_test.so
swig_test: swig_test.o
$(CXX) -Wall $^ -o $@
swig_test.o: swig_test.c
$(CXX) $(COMPFLAGS) $(INCLUDES) $^
_swig_test.so: swig_test_wrap.o swig_test.o
$(CXX) -shared $^ -L$(PYLIB) -lpython2.7 -o $@
swig_test_wrap.o: swig_test_wrap.c
$(CXX) $(COMPFLAGS) $(INCLUDES) -I$(PYINC) $^
swig_test_wrap.c: swig_test.i
$(SWIG) -python $(INCLUDES) $^
And finally the simple python example:
runpython.py
#!/usr/bin/python2
import swig_test
var = swig_test.malloc_diags()
if var == 'NULL':
print "Error, no memory left"
else:
ret = swig_test.get_diags(var)
if ret == 0:
print swig_test.diags_mem0_get(var)
print var.mem0
swig_test.free_diags(var)
The functionality you're looking for comes in typemaps. The documentation freely admits that "At first glance, this code will look a little confusing." Here's what worked for me.
In essence, a typemap is a few lines of code that SWIG swaps in when it needs to convert between Python and C. They're separately defined for Python to C ( %typemap(in)
) and C to Python ( %typemap(out)
). SWIG's documentation also defines a few magic variables:
$input
refers to an input object that needs to be converted to C/C++.
$result
refers to an object that is going to be returned by a wrapper function.
$1
refers to a C/C++ variable that has the same type as specified in the typemap declaration (an int in this example).
For unsigned integer support, you just need in and out maps for uint8_t
, uint16_t
, and uint32_t
The lines below provide that functionality. They can go into SWIG's .i file, or the main header (with an ifdef SWIG
guard around them).
/* uintXX_t mapping: Python -> C */
%typemap(in) uint8_t {
$1 = (uint8_t) PyInt_AsLong($input);
}
%typemap(in) uint16_t {
$1 = (uint16_t) PyInt_AsLong($input);
}
%typemap(in) uint32_t {
$1 = (uint32_t) PyInt_AsLong($input);
}
/* uintXX_t mapping: C -> Python */
%typemap(out) uint8_t {
$result = PyInt_FromLong((long) $1);
}
%typemap(out) uint16_t {
$result = PyInt_FromLong((long) $1);
}
%typemap(out) uint32_t {
$result = PyInt_FromLong((long) $1);
}
Resources I found useful:
One "fix" is to change the uint16_t
members to int
types. Then this syntax works:
print var.mem0
Clearly SWIG has some problems with non-integer types.
I'd love it if somebody proposed an alternative solution that lets me keep my uint16_t
types.
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