How do you pass a bytes value from Python (like data loaded from a file with open('my file.dat', 'rb').read()) to a C/C++ function using swig?
When I try using char * or uint8_t * and then a size parameter it gives me an error like this:
TypeError: in method 'processData', argument 3 of type 'char *'
I've tried using %pybuffer_mutable_binary and %pybuffer_binary but they don't seem to change the definition of the wrapper and I still get the same error.
Without code can't diagnose what is wrong, but likely you didn't declare %pybuffer lines before the function definitions. If declared after the generated wrappers won't use them when processing the functions, which would explain "they don't seem to change the definition of the wrapper".
Here's a working example. Note that passing an immutable item to a function that modifies the string will crash Python. It would be nice if the commands from pybuffer.i type-checked the Python object for mutability. If you want that don't use pybuffer.i.
test.i
%module test
%{
#include <stdlib.h>
#include <string.h>
%}
%include <pybuffer.i>
%pybuffer_mutable_string(char* str1)
%pybuffer_string(const char* str2)
%pybuffer_mutable_binary(char* str3, size_t size)
%pybuffer_binary(const char* str4, size_t size)
%inline %{
void funcms(char *str1) {
strupr(str1);
}
size_t funcs(const char *str2) {
return strlen(str2);
}
void funcmb(char* str3, size_t size) {
memset(str3,'A',size);
}
size_t funcb(const char* str4, size_t size) {
size_t tmp = 0;
for(size_t i = 0; i < size; ++i)
tmp += str4[i];
return tmp % 256;
}
%}
Demo:
>>> import test
>>> b=bytearray(b'abc') # mutable string (nul-terminated)
>>> test.funcms(b)
>>> b
bytearray(b'ABC')
>>> test.funcs(b'abc') # immutable string (nul-terminated)
3
>>> b=bytearray(b'ab\0cd\0ef') # mutable data (includes nulls)
>>> test.funcmb(b)
>>> b
bytearray(b'AAAAAAAA')
>>> test.funcb(b'ab\0cd\0ef') # immutable data (compute byte checksum)
85
>>> sum(b'ab\0cd\0ef')%256 # verify result
85
I think the best way to do this is a type map using the Python buffer interface. This passes a pointer to your data to the C/C++ function without any copying of data. For example:
%typemap(in, numinputs=1) (const char *data, unsigned long int size) {
Py_buffer view;
if (PyObject_CheckBuffer($input) != 1) {
PyErr_SetString(
PyExc_TypeError,
"in method '$symname', argument $argnum does not support the buffer interface");
SWIG_fail;
}
if (PyObject_GetBuffer($input, &view, PyBUF_SIMPLE) != 0) {
PyErr_SetString(
PyExc_TypeError,
"in method '$symname', argument $argnum does not export a simple buffer");
SWIG_fail;
}
$1 = view.buf;
$2 = view.len;
PyBuffer_Release(&view);
}
%typemap(doc) const char *data, unsigned long int size "$1_name: readable buffer (e.g. bytes)"
Related
I have two functions in my C library:
void passBytes(char *data, int size);
void returnBytes(char **data, int *size); //dynamically allocates
I successfully implement the wrapper for the first function using this post:
%typemap(in) (char *data, int size) {
Py_ssize_t len;
PyBytes_AsStringAndSize($input, &$1, &len);
$2 = (int)len;
}
But can't figure out how to implement the second wrapper with typemaps. Can't use SWIG_PYTHON_STRICT_BYTE_CHAR because I need mix of str and bytes.
You are almost there
%typemap(in, numinputs=0) (char **data) (char *temp) {
$1 = &temp;
}
%typemap(argout) (char **data) {
if (*$1) {
$result = PyUnicode_FromString(*$1);
free(*$1);
}
}
Note that the two typemaps combined do the job. The length is not needed if the character array is null terminated. The parameter temp is created and the reference to this is passed to your C function allocating the array.
Let's say we have a following struct in C:
typedef struct buffer
{
uint8_t* const data;
const size_t maxSize;
size_t size;
} buffer_t;
How can I make a SWIG wrapper for this so that when buffer_t is created in Python it allocates given number of bytes to data pointer and sets maxSize accordingly?
Details
So basically the problem here is related to constant struct members. By default SWIG initializes structs with default values. This leads into problems:
StructWithConsts_t struct;
struct.constant = 5; // Error. This should've been set when creating the struct.
The accepted answer with constructors and destructors provide solution for this problem.
You can treat C structs as C++ classes in SWIG and extend them with creators and destroyers methods.
In your case your .i should have this
typedef struct
{
uint8_t* const data;
const size_t maxSize;
size_t size;
} buffer_t;
%extend buffer_t {
buffer_t(size_t size)
{
buffer_t* pBuffer = malloc(sizeof *pBuffer);
pBuffer->data = malloc(size * sizeof (*pBuffer->data));
pBuffer->maxSize = size;
return pBuffer;
}
void ~buffer_t()
{
buffer_t *pBuffer = ($self);
free(pBuffer->data);
free(pBuffer);
return;
}
};
In python it will be used like this
from yourmodule import buffer_t
buffer = buffer_t(10)
The garbage collector will take care of freeing the memory.
There are other ways of doing this but you don't have a reproducing example to test it out.
I found one solution, but it is rather hackish due to pointer typecasts. I'm not convinced that this is the proper way to do this, but it works:
In the .i file I created a wrapper using %inline:
%inline %{
buffer_t* new_buffer(size_t size)
{
buffer_t* pBuffer = (buffer_t*)malloc(sizeof(buffer_t));
*(uint8_t**)&pBuffer->data = (uint8_t*)malloc(size * sizeof(uint8_t));
*(size_t*)&pBuffer->maxSize = size;
return (buffer_t*)pBuffer;
}
void free_buffer(buffer_t* pBuffer)
{
free(pBuffer->data);
free(pBuffer);
return;
}
%}
I'm using Swig to generate a python wrapper for a DLL file. What I do is:
Generate the wrapper and loader file using swig -c++ -python
myfile.i
Create a new DLL file, include myfile_wrapper.cxx and
compile to _myfile.pyd.
Load the module myfile.py created by Swig in Idle and try to use it.
The interface file looks like:
%module myfile
/* Make a test with cpointer - needed?? */
%include cpointer.i
%pointer_functions(MyHandle, my_handle_p);
%pointer_class(int, intp);
%{
#define SWIG_FILE_WITH_INIT
#include "MyFile.h"
}%
%include "MyFile.h"
The function looks like
typedef struct tagMyHandle
{
void* reserved;
} *MyHandle;
int OpenFile(const char *szPath, MyHandle* pFile); // pFile is an out parameter
int GetNumberOfItems(MyHandle hFile, int *pnNrOfItems); // pnNrOfItems is an out parameter
If I try to use this from Python I have to do like this:
import myfile
handle_p = myfile.new_my_handle_p()
myfile.OpenFile("Path", handle_p)
handle = myfile.my_file_p_value(handle_p)
num_items_p = myfile.new_intp()
myfile.GetNumberOfItems(handle, num_items_p)
num_items = num_items_p.value()
Am I using Swig incorrectly? It feels that it's a very cumbersome way to call the functions that are supposed to be wrapped for Python.
I would like to do something like:
result, handle = OpenFile("path")
result, items = GetNumberIfItems(handle)
I can't change the source code for myfile.h.
I'm looked at input/output parameters, but do I have to define them for each output type? MyFile.h have hundreds of functions with different output types. And it only supports primitive data types, but most types in MyFile.h are not primitive types, but like the struct MyHandle.
I have looked at SWIG function with pointer struct and http://www.swig.org/Doc3.0/Python.html#Python_nn18 as well, but without any good solution.
Update 1
After a lot of help, I've solved most problems, but I still have a few left that I don't understand.
Problem 1:
// For the out parameter, shouldn't be needed?
%typemap(in,numinputs=0) MyHandle* pOutParam (MyHandle h) %{
$1 = &h;
%}
// For all others
%typemap(in,numinputs=0) MyHandle* (MyHandle h) %{
$1 = &h;
%}
// For the return type
%typemap(argout) MyHandle* pOutParam (PyObject* o) %{
o = PyLong_FromVoidPtr(*$1);
$result = SWIG_Python_AppendOutput($result,o);
%}
%typemap(in) MyHandle %{
$1 = reinterpret_cast<MyHandle>(PyLong_AsVoidPtr($input));
%}
and the code
int OpenFile(const char *szPath, MyHandle* pOutParam);
int DoSomething(MyHandle* pInParam);
OpenFile works like charm, but DoSomething still tries to return MyHandle instead of taking it as an in parameter, and I don't understand why. %typemap(argout) MyHandle* is only defined for pOutParam.
Problem 2:
I don't understand how to make the type map for something like
int GetFileName(char *szPathBuffer, int iLength);
how to create a char buffer and send that in, like I C:
char szBuffer[MAX_PATH]; GetFileName(szBuffer, MAX_PATH);
Maybe something together with cstring_bounded_output or should I do something like
%typemap(in) (char*, int) {
$2 = PyString_Size($input);
$1 = (char*) malloc($2 * sizeof(char*));
}
but where is it deallocated?
Problem 3:
What is the correct mapping for enum values. If I have
typedef enum tagMyEnum {
MyTrue = 1,
MyFalse = 0 } MyEnum;
and the function
int IsCorrect(MyEnum* pOutValue);
#Mark Tolonen:
Thanks for all help! I really appreciate it! I've learned so much new things about Swig!
Here's an example with a interface similar to what you want to illustrate using typemaps to redefine an interface:
myfile.h
typedef struct tagMyHandle
{
void* reserved;
} *MyHandle;
int OpenFile(const char *szPath, MyHandle* pFile);
int GetNumberOfItems(MyHandle hFile, int *pnNrOfItems);
// Added this to free the allocated handle.
void CloseFile(MyHandle hFile);
myfile.cpp
A hack implementation of the header...
#include "myfile.h"
int OpenFile(const char *szPath, MyHandle* pFile)
{
*pFile = new tagMyHandle;
(*pFile)->reserved = new int(7);
return 1;
}
int GetNumberOfItems(MyHandle hFile, int *pnNrOfItems)
{
*pnNrOfItems = *reinterpret_cast<int*>(hFile->reserved) + 5;
return 1;
}
// mirrors OpenFile to free the allocated handle.
void CloseFile(MyHandle hFile)
{
delete reinterpret_cast<int*>(hFile->reserved);
delete hFile;
}
myfile.i
%module myfile
%{
#include "MyFile.h"
%}
// An input typemap for the an output parameter, called before the C++ function is called.
// It suppresses requiring the parameter from Python, and uses a temporary
// variable to hold the output value.
%typemap(in,numinputs=0) MyHandle* (MyHandle h) %{
$1 = &h;
%}
// An output argument typemap, called after the C++ function is called.
// It retrieves the output value and converts it to a Python int,
// then appends it to the existing return value. Python will get a tuple of
// (return_value,handle).
%typemap(argout) MyHandle* (PyObject* o) %{
o = PyLong_FromVoidPtr(*$1);
$result = SWIG_Python_AppendOutput($result,o);
%}
// An input typemap that converts a Python int to a MyHandle*.
%typemap(in) MyHandle %{
$1 = reinterpret_cast<MyHandle>(PyLong_AsVoidPtr($input));
%}
// This applies a pre-defined int* output typemap to all int* parameters.
%apply int *OUTPUT {int *};
%include "MyFile.h"
Output
>>> import myfile
>>> s,h = myfile.OpenFile('path')
>>> s,h
(1, 7706832)
>>> s,v = myfile.GetNumberOfItems(h)
>>> s,v
(1, 12)
>>> myfile.CloseFile(h)
I have a function that returns a char pointer called loop_p and I call it many times on my main_thread like this to pass it to the py_embed thread:
HANDLE handle;
SENDTOPY *cmd=new SENDTOPY();
char* msg=loop_p(ac);
char *argv[4]={"PythonPlugIn2","bridge","test_callsign",msg};
cmd->argc=4;
for(int i = 0; i < NUM_ARGUMENTS; i++ )
{
cmd->argv[i] = argv[i];
}
handle=(HANDLE) _beginthread(py_embed,0,(void*)cmd);}
where SENDTOPY is a struct:
typedef struct{
int argc;
char *argv[4];
}SENDTOPY;
The message it sent to python like this and python receives it well:
SENDTOPY *arg=(SENDTOPY*)data;
pArgs2=Py_BuildValue("(s)",arg->argv[4]);
pValue2 = PyObject_CallObject(pFunc, pArgs2);
In order to avoid having memory allocation problems i modified the loop_p function to a function that returns a std::string. I then call that string in the main_threadwith some modifications:
...
std::string msg_python=loop_p(ac);
const char * msg2=msg_python.data();
char *argv[3]={"PythonPlugIn2","bridge","test_callsign"};
cmd->argc=3;
cmd->msg=msg2;
for(...
...
and i modify the struct SENDTOPYto this:
typedef struct{
int argc;
char *argv[3];
const char* msg;
}SENDTOPY;
I print it to a textfile in the main_thread and the message before and after the modifications are equal. But in the py_embedthread the const char is no longer what is was, is just a bunch of gibberish. What am I doing wrong?
Thank you in advance.
Edit:
loop_p code
std::string CNewDisplay::loop_p(int ac){
std::string res("Number of Aircrafts\nHour of simulation\n\n");
for (...
....
//Route
textfile<<fp.GetRoute()<<endl;
std::string route=fp.GetRoute();
std::replace(route.begin(),route.end(),' ',',');
res+=route;
res.append(",\n");
res.append("\n\n");
};
return res;
}
It appears to me that you are storing a pointer to the internal guts of a temporary string object created on the stack. If you make string static, then the string's guts will remain valid throughout program execution, and you can safely store pointer to string guts:
static std::string msg_python; // survives beyond local scope
msg_python=loop_p(ac); // set string to loop_p return value
const char *msg2=msg_python.c_str(); // get ptr each time since it could change
Also, ensure that you use .c_str() to get your c-style char string pointer so that you are assured the string is null-terminated. Using .data() does not guarantee null termination.
I'm new to swig and I have the following function which i cant fix:
int get_list(IN const char * string, OUT struct entry ** results);
where struct entry is defined:
struct flux_entry
{
char * addr_str;
char cc[2];
};
the entry struct is properly converted to a python class.
I googled but couldn't find any explanation i could use.
I want to make it return a tuple of: (original get_list int return value, python list of entry python objects, based on the results buffer), but don't know how to convert the C entry to a python object in the argout code snippet.
I've managed to get thus far:
%typemap(argout) struct entry **
{
PyObject *o = PyList_New(0);
int i;
for(i=0; $1[i] ; i++)
{
PyList_Append(o, SWIG_HOW_TO_CONVERT_TO_PYOBJECT($1[i]));
}
$result = o;
}
what should i replace SWIG_HOW_TO_CONVERT_TO_PYOBJECT with?
passed results is supposed to be a pointer to a (struct entry *) type, set to NULL before calling get_list and should be set to an allocated array of struct entry * pointers. maybe a small wrapper function could make that easier?
the struct entry array is allocated within the C function using malloc, after calculating (inside get_list) how many elements are needed, and ends with a NULL pointer to indicate the end of the array.
i'd also like to make sure it's freed somewhere :)
thanks!
This should at least give you a starting point that works. I still wasn't sure how the data was returned, since to return an array of pointers so that the final one was NULL I'd think you'd need a struct entry ***, so I just set addr_str = NULL on the last one as a sentinel, and just put some dummy data partially based on the input string into the fields. Modify as needed to suit your needs:
%module example
// Insert the structure definition and function to wrap into the wrapper code.
%{
struct entry {
char* addr_str;
char cc[2];
};
int get_list(const char* string, struct entry** results)
{
*results = malloc(3 * sizeof(struct entry));
(*results)[0].addr_str = malloc(10);
strcpy((*results)[0].addr_str,"hello");
(*results)[0].cc[0] = string[0];
(*results)[0].cc[1] = string[1];
(*results)[1].addr_str = malloc(10);
strcpy((*results)[1].addr_str,"there");
(*results)[1].cc[0] = string[2];
(*results)[1].cc[1] = string[3];
(*results)[2].addr_str = NULL;
return 0;
}
%}
#include <typemaps.i>
// Define the structure for SWIG
struct entry {
char* addr_str;
char cc[2];
};
// Define a set of typemaps to be used for an output parameter.
// This typemap suppresses requiring the parameter as an input.
// A temp variable is created and passed instead.
%typemap(in,numinputs=0) struct entry **OUTPUT (struct entry* temp) %{
$1 = &temp;
%}
// Build a list of tuples containing the two entries from the struct.
// Append the new Python list object to the existing "int" result.
%typemap(argout) struct entry **OUTPUT {
int i = 0;
PyObject* out = PyList_New(0);
while((*$1)[i].addr_str != NULL)
{
//PyObject* t = PyTuple_New(2);
//PyTuple_SET_ITEM(t,0,PyBytes_FromString((*$1)[i].addr_str));
//PyTuple_SET_ITEM(t,1,PyBytes_FromStringAndSize((*$1)[i].cc,2));
//PyList_Append(out,t);
//Py_DECREF(t);
PyObject* s = SWIG_NewPointerObj(*$1+i,$descriptor(struct entry*),0);
PyList_Append(out,s);
Py_DECREF(s);
++i;
}
$result = SWIG_AppendOutput($result,out);
}
// Since a Python object was created and the data copied for each entry struct,
// free the memory returned in the structure.
//%typemap(freearg) struct entry **OUTPUT {
// int i=0;
// while((*$1)[i].addr_str != NULL) {
// free((*$1)[i].addr_str);
// ++i;
// }
// free(*$1);
//}
// Apply the OUTPUT typemap set to the "results" parameter.
%apply struct entry **OUTPUT {struct entry** results};
// Finally, define the function for SWIG
int get_list(const char* string, struct entry** results);
Demo (Python 3.3):
>>> import example
>>> example.get_list('abcd')
[0, [(b'hello', b'ab'), (b'there', b'cd')]]
Hope that helps.
Edit:
I commented out the tuple creation and just save the entry* proxy instead. This doesn't leak Python objects, but the memory malloced for use by an entry* is not freed. I'm not sure where to put that, although I'm experimenting with %extend.