I had to use SWIG to access a kernel module’s chardev interface through python and found SWIG examples are not enough, so adding my own.<\/p>\n
Lets take the following example header file. \/* test.h *\/<\/p>\n #ifndef _TEST_STRUCT_H \/* a simple structure – no problem with SWIG *\/ typedef struct tlv_base { typedef struct tlv_type1 { \/* relatively complex C structure. *\/ simple_struct_t embedded;<\/p>\n int pointer_array_count; tlv_base_t tlv; \/\/How do cast this to derived_struct_1 ? complex_struct_t * alloc_complex_struct(int array_count); #endif [c tab=’source’]<\/p>\n \/* test.c *\/<\/p>\n #include #include “test.h”<\/p>\n complex_struct_t * result = (complex_struct_t *)malloc(sizeof(complex_struct_t) + sizeof(tlv_type1_t)); result->tlv.type = 1; size = sizeof(simple_struct_t) * array_count; return result; void static inline void void printf(“String = %s\\n”, cs->string); [\/c]<\/p>\n [raw tab=’interface’] %include “test.h” [shell tab=’commands’] [raw tab=’makefile’] SWIG_SRCS = test_swig.i OBJS = $(patsubst %.c, $(BLD_DIRECTORY)\/%.o, $(C_SRCS)) \\ $(BLD_DIRECTORY)\/%.o: %.c %.h $(BLD_DIRECTORY)\/%.o: $(BLD_DIRECTORY)\/%.c $(BLD_DIRECTORY)\/%_wrap.c: %.i $(BLD_DIRECTORY): clean: all: $(BLD_DIRECTORY) $(OBJS) $(C_SRCS) $(SWIG_SRCS) .PHONY: all clean<\/p>\n .DEFAULT_GOAL := all With this simple interface file, SWIG would be able to create a _test_struct.so and test_struct.py which is perfect for most cases. %include “test.h” [python] This shows SWIG’s ability to convert C string to python string and vice versa. [raw] It also shows how to call function call into C library. If you have noticed this program looks more like a C program rather than a python program – mainly because it manages the memory allocation\/free. Python can informed that [c] alloc_complex_struct() [\/c] returns a new object and it is the responsibility of the caller to free it by using the SWIG typemap [python] newobject [\/python]. Now python garbage collector will free the object when there is no reference. But python does not know how to free the object(complex_struct_t) – this can be done by using [python]newfree[\/python] typemap.<\/p>\n By adding the following to the test_swig.i, we can avoid calling free_complex_struct() in python program. Lets modify the program a little bit and access the pointer_array elements.<\/p>\n [python] This will fail with the following error:
\nI will explain how to access all the members in complex_struct_t<\/em> from python.
\nAlso extend these structures so that python code would look little better.
\n[codegroup]
\n[c tab=”header”]<\/p>\n
\n#define _TEST_STRUCT_H<\/p>\n
\ntypedef struct simple_struct {
\n int int_var;
\n long long_var;
\n float float_var;
\n} simple_struct_t;<\/p>\n
\n int type;
\n int length;
\n unsigned char value[];
\n} tlv_base_t;<\/p>\n
\n tlv_base_t base;
\n int stat;
\n int info;
\n long something;
\n} tlv_type1_t;<\/p>\n
\ntypedef struct complex_struct {
\n char string[10]; \/\/SWIG considers this as null terminated string
\n unsigned char bytes[10]; \/\/SWIG wont considers this as string<\/p>\n
\n simple_struct_t *pointer_array; \/\/SWIG supports only accessing first element.<\/p>\n
\n} complex_struct_t;<\/p>\n
\nvoid free_complex_struct(complex_struct_t *cs);
\nvoid print_complex_struct(complex_struct_t *cs);<\/p>\n
\n[\/c]<\/p>\n
\n#include
\nalloc_complex_struct(int array_count)
\n{
\n complex_struct_t *result;
\n size_t size;<\/p>\n
\n if (result == NULL){
\n return NULL;
\n }<\/p>\n
\n result->tlv.length = sizeof(tlv_type1_t);<\/p>\n
\n result->pointer_array = (simple_struct_t *)malloc(size);
\n if (result->pointer_array == NULL) {
\n free(result);
\n return NULL;
\n }
\n memset(result->pointer_array, 0, size);
\n result->pointer_array_count = array_count;<\/p>\n
\n}<\/p>\n
\nfree_complex_struct(complex_struct_t *cs)
\n{
\n free(cs->pointer_array);
\n free(cs);
\n}<\/p>\n
\nprint_simple_struct(simple_struct_t *ss)
\n{
\n printf(“int %d long %ld float %f\\n”, ss->int_var, ss->long_var, ss->float_var);
\n}<\/p>\n
\nprint_complex_struct(complex_struct_t *cs)
\n{
\n int i;<\/p>\n
\n printf(“Embedded : “);
\n print_simple_struct(&cs->embedded);
\n printf(“External : \\n”);
\n for (i=0; i
\n printf(“%d) “, i + 1);
\n print_simple_struct(&cs->pointer_array[i]);
\n }
\n}<\/p>\n
\n\/\/ test_swig.i
\n%module test_struct
\n%{
\n #include “..\/test.h”
\n%}<\/p>\n
\n[\/raw]<\/p>\n
\n# Commands to make the shared library
\n$ mkdir -p _build
\n$ gcc -I \/usr\/include\/python2.7\/ -fPIC -c -o _build\/test.o test.c
\n$ swig -python -outdir _build -o _build\/test_swig_wrap.c test_swig.i
\n$ gcc -I \/usr\/include\/python2.7\/ -fPIC -c -o _build\/test_swig_wrap.o _build\/test_swig_wrap.c
\n$ ld -shared _build\/test.o _build\/test_swig_wrap.o -o _build\/_test_struct.so
\n$ rm _build\/test_swig_wrap.c
\n[\/shell]<\/p>\n
\n#Makefile
\nBLD_DIRECTORY = _build<\/p>\n
\nC_SRCS = test.c
\nCFLAGS = -I \/usr\/include\/python2.7\/ -fPIC -c -Wall<\/p>\n
\n\t\t $(patsubst %.i, $(BLD_DIRECTORY)\/%_wrap.o, $(SWIG_SRCS))<\/p>\n
\n\tgcc $(CFLAGS) -o $@ $<<\/p>\n
\n\tgcc $(CFLAGS) -o $@ $<<\/p>\n
\n\tswig -python -outdir $(BLD_DIRECTORY) -o $@ $<
\n\tcp $@ $@.bak<\/p>\n
\n\tmkdir -p $(BLD_DIRECTORY)<\/p>\n
\n\trm -rf $(BLD_DIRECTORY)<\/p>\n
\n\tld -shared $(OBJS) -o $(BLD_DIRECTORY)\/_test_struct.so<\/p>\n
\n[\/raw]
\n[\/codegroup]<\/p>\n
\n[raw tab=’interface’]
\n%module test_struct
\n%{
\n #include “..\/test.h”
\n%}<\/p>\n
\n[\/raw]<\/p>\n
\nimport test_struct as ts
\ncs = ts.alloc_complex_struct(1)
\ncs.string = “Hello”
\ncs.embedded.int_var = 9
\ncs.embedded.long_var = 10
\ncs.embedded.float_var = 11.23
\nts.print_complex_struct(cs)
\nts.free_complex_struct(cs)
\n[\/python]<\/p>\n
\nSimilarly accessing primitive structure members is very easy.
\nHere is the output of the program when ran from _build directory. <\/p>\n
\nString = Hello
\nEmbedded : int 9 long 10 float 11.230000
\nPointer Array :
\n1) int 0 long 0 float 0.000000
\n[\/raw]<\/p>\n
\n[c]
\n%typemap(newobject) alloc_complex_struct;
\n%typemap(newfree) complex_struct_t * {
\n free_complex_struct($1);
\n}
\n[\/c]<\/p>\n
\nimport test_struct as ts
\ncs = ts.alloc_complex_struct(5)
\nprint ‘Pointer array count ‘, cs.pointer_array_count
\nprint cs.pointer_array[0]
\nts.free_complex_struct(cs)
\n[\/python]<\/p>\n
\n[raw]
\nPointer array count 5
\nTraceback (most recent call last):
\n File “.\/test.py”, line 4, in
\n print cs.pointer_array[0]
\nTypeError: ‘simple_struct_t’ object does not support indexing
\n[\/raw]<\/p>\n