Python ctypes string to char An explicit cast (from char array to char pointer) is required; For the 1 st argument, I create singleton CAxClass object that is returned by every createObject call. It's up to you to manage the objects in the global keep-alive unsigned char array[channels*width*height]; in C. f_handle type is <ctypes. ; str For Unicode strings, Python's built-in str type provides a convenient way to work This attribute contains the actual value of the instance. I am creating a string buffer using ctypes library in python. some_func(param_to_some_func) fails with exception "unicode string or integer address expected instead of bytes instance" The documentation for ctypes and ctypes. dirname(os. As-is, some code somewhere needs to do a conversion of the Python strings to low-level C ones, and hold on to that memory for the duration of the call. myfunc(p2, 1, None, None); The first function argument has the type constchararrayiwant * which is char [33] * which is char**. What is needed is a pointer to a c_char array:. fileName]). The ctypes default conversions were removed, as was the function set_conversion_mode. c_int(len(shellcode)), Instead of using a Python string, we use a char pointer (the string equivalent) from ctypes. p = lib import ctypes gp = ctypes. Improve this Does anyone know a way to convert a string of hexadecimal characters into particular ctype variables? To elaborate, I need create a function which takes in a hexadecimal list of characters and a string representing the datatype and produces a value according to the datatype specified. windll. Your problem is that ctypes tries to do some magic for you with char arrays, auto-converting them into NUL-terminated strings. It corresponds to the char data type in C. You can get around this magic by using the ctypes. c_char * (len(bstr) + 1))(); buf. abspath('nodispersion. value b'T' The thing behind the scenes is a ctypes char array: (Pdb) Cname <ctypes. 2. fileopen(ctypes. ctypes has the c_char_p and c_wchar_p types which represent const pointers to zero terminated strings in C: const char * and const wchar_t *. A simple way to get around this is to use a void * and cast the result: You can also use a subclass of c_char_p. from_address(a) x. The eval() command does solve the example's question, but it does not answer above mentioned problem. For example, in >>> import ctypes >>> s = ctypes. c_char_p lib. The result of the multiplication is a new array type which can be used e. Follow C Method returning string to Python via ctypes. getText. I know that python strings are utf-8 by default. VCS_OpenDevice VCS_OpenDevice. printf(b'hello %s\n', ctypes. I more or less understand what is the cause of the problem (my C function returns a pointer, which my Python function can't handle as a useful data). value attribute is printed to show the C string representation of the bytes. First: ctypes. For a string declared as. This payload always has an 8 byte header that I would like to skip over. Improve this answer. If so, then . It works for me. b"Python string" is working with c_char_p and "Python wide string" is working with c_wchar_p. 6. c_char_p instead helps. def _createShader(self, strings, shadertype): # create the shader handle shader = gl. Only use buf = ctypes. c_char*len(buf))(*buf) and then cast it to a pointer using wchar_t **foo(const char *); /* The structure of the return value is a NULL terminated (wchar_t **), each of which is also NULL terminated (wchar_t *) strings */ Now I want to call the function through this API from python using It would be easier if you posted runnable code: getting a suitable volume name for this call is a bit of a pain. compare libc. Rather than using create_string_buffer(9) you'll need to use ctypes. c_char_Array_1024 object at 0xb6a1ad1c> Mark's answer is quite helpful in that it passes a character array to the C function, which is what the OP really wanted, but in case there's folks finding their way here who really want to pass a byte-array, an approach seems to be to build a ctypes. The _type_ code of c_char_p is 'z'. so') a = gp. if the data contains nulls), then string slicing works to read the data as a Python byte string. test_lib. h> #include <string> #include <iostream> using namespace std; extern "C" string return_string(char* name){ cout<<strlen(name)<<endl; cout<<name<<endl; string s = "hello "; s += name; return s; } @HelinWang Even in the general case, if C allocates it, C must free it. value) I create a new variable that no longer points to a. As was stated in the comments, a Python string is immutable and could be interned or used as a dict key. gp_library_version(0) x = ctypes. B. POINTER(c_char_p) is equivalent to char**. c : z_set, you see that for an str object it simply uses the underlying ob_sval I'm using ctypes in order send the request to C++ application. c_float) to use in the prototype readparameter. However, if I try the same in Python, only the first character in my string is printed. In the Python version, I want to be able the string 'apple' consists of the following chars: a p p l e the string 'pears' consists of the following chars: p e a r s the string 'cherry' consists of the following chars: c h e r r y the string 'pear' consists of the following chars: p e a r the string 'blueberry' consists of the following chars: b l u e b e r r y the string 'strawberry c_char_p takes bytes object so you have to convert your string to bytes first: ct. wchar_p object representing the wide character string. #include <Python. import ctypes import cdll buf_c = (ctypes. typedef struct { int msg_id; unsigned char name[20]; } c_struct; from ctypes import * names = c_char_p * 4 # A 3 times for loop will be written here. in structures or to create a new array in memory. bar = create_string_buffer(b"My string", 10) I have a ctypes structure (for example): from ctypes import * class Foo(Structure): _fields_ = [('f1',c_uint), ('f2',c_uint)] I would like to copy an instance of that structure in to a buffer that was created from create_string_buffer (that is larger in I need to pass two Python strings, representing an input file and an output file, to a C function named decrypt_file. printf(b'hello %sn', s) and libc. 8 bits). value # => 'foo' Converting String() to c_char array in Python. byref(s)). Call with, for example: setParameters(['abc','def','ghi']) I haven't looked at the source, but ctypes on Python 2. Sending a string from Python to C++ using Python's ctypes module requires you to parse it as a c_char_p (a char *). Ask Question Asked 14 years, 6 months ago. 5, I wish to implement the following C code in python: C code: #include <wtypes. So if I am using a String array, how can I change this code? I am using a vector since I do not pass the length of the vector to c++ function. I want to read the returned value of the function in a python script using ctypes:. cpp. abspath("nodispersion. That could segfault, so yes, that's an important distinction for a result type. create_string_buffer(bstr) when the function modifies the string. Stack Overflow for Teams Where developers & technologists share private knowledge with coworkers; Advertising & Talent Reach devs & technologists worldwide about your product, service or employer brand; OverflowAI GenAI features for Teams; OverflowAPI Train & fine-tune LLMs; Labs The future of collective knowledge sharing; About the company It's already been copied. Docs]: ctypes - A foreign function library for Python. Key points. I am trying to pass a Python string to a c function (DLL) using ctypes. Share. Along the lines of my original example, a solution would look like this: Previous variant doesn't behave well when there are NUL (0, \x00) chars in the string (text = b"abcd\x001234"), because ctypes. The C library expects function signature of void (*p_func)(char stat, char * buf, short buf_len) so I register an appropriate python function. So in your case, you'd want to do something like: z. c_wchar is a powerful tool for interacting with C-style wide character strings in Python, there are alternative approaches that might be suitable for certain I have a ctypes wrapper around an internal library. Windows example: DLL code (compiled on MSVC as cl /LD test. These descriptors also allow you to write a string to the buffer: >>> hid = (c_char * 24)() >>> hid. For this I created the same structure in Python and C header file like this. Most systems use little-endian byte order, so "test" is internally represented as b"t\x00e\x00s\x00t\x00". Python strings objects are immutable; you c_char_p is a wrapper over NUL terminated (8bit) strings (that work with strlen, strcpy, ) and has a nice Python interface, while POINTER(c_char) is a generic char pointer (that might contain 0s). That is, the function is interpreting the string 'Martin\0\0' as pointer to MyStruct. Here's an example: C++ I am exploring the use of ctypes in Python to create a char * array that will be passed to a library for storing strings. Notes:. That code works. 1) does not automatically convert str (now Unicode) to bytes. c_char_p (a. All the C function sees is a pointer to the first byte of the You can use malloc() but if the memory is allocated by your test() function, you will need to provide some way to free the memory to avoid memory leaks (by saving the pointer returned by sh_obj. c_char_p print testlib. join([chr(i) for i in f. Below is functioning DLL code to write to the types you want, but I am assuming char WString Access in Ctypes . I've been trying to return an array of strings from C++ to python a follows: // c++ code extern "C" char** queryTree(char* treename, float rad, int kn, char*name, char *hash){ //. dylib") s1 = create_string_buffer(b"abc") # create a null terminated string buffer s2 = create_string_buffer(b"bc") # same at above g = libc. dll")] public static extern Util_func(byte[] clearText, byte[] encodedText, ref int l);` My iunderstanding is that the firts part is the input, the second is a bufer and the third kind of pointer, the function receives a string and should return You should be careful, however, not to pass them to functions expecting pointers to mutable memory. encode('utf-8')) Another solution is using the c_wchar_p type which takes a string. POINTER(c_char) is for any buffer of char data. Essentially, the solution is to pass into the C function an extra char * buff buffer that has already been created using ctypes. Python ctypes in_dll string assignment. c_char(a),ctypes. You can use ctypes. Cast the pointer to (single) wchar to a pointer to an array of wchar. ctypes. Using buffer. CDLL('libgphoto2. It's also recommended to set . CDLL(LIB_PATH) lib. Don't declare pBuf as c_char_p. Basically the only progress I'm made is getting Python to crash. getText() print testlib. And wchar is utf-16. (index 0) A cast() should get a pointer to see the data's type,. Only use create_string_buffer(255) for example to make Python Ctypes - String to c_char_p to c_void_p and back. c_char I have a ctypes array that I defined like this: buff= (c_ubyte*buff_size)() After filling the buffer up with data, I need to have this data in bytes format. You can create C-style Learn how to use Python's ctypes library to pass a char * array to a C function and store the output values. cdll. Docs]: ctypes - A foreign function library for Python states, you must not use c_char_p with binary data. Assuming your function is in test. so. Working with ctypes. cCreateObject. If it's not const, use ctypes. main(ctypes. c_char_p is used for NUL terminated strings. As a parameter type it does not matter one bit. restype correctly helps ctypes marshal parameters correctly and detect incorrectly passed parameters. I tried the solution here: Passing string to Fortran DLL using ctypes and Python. value = bstr. I'm trying to patch together a motor control system using python and ctypes and one of the things I need to do is take an text input and convert it to an 8-bit signed integer. dat") as the following: import ctypes as ctypes dll_name = "IO/pyaio. names[0] = c_char_p('hello') With regard to Python 2. So in argtypes, POINTER(c_char) works the same as c_char_p-- except for argtypes of a callback (i. In Python 3, the z_set function (3. The * in the C++ definition indicates a pointer to a value, which is why I used POINTER. Hot Network Questions 2012 vs 2022 Chevrolet Vehicle and Coolant Consumption How to construct A HashMap in an inductive type? How to make machine always turn on after a power outage Countable translations of a positive measure set cover an interval? This is where conversion to c_void_p is done. string_at. import ctypes as ct dll = ct. We can use create_string_buffer to create a mutable character buffer which is a ctypes array of c_char. address: The memory address where the wide character string starts. Then array is a pointer to N*types unsigned char pointing to first byte of the array. Using ctypes. value 'This is a test. # The last array will assign to a null pointer. c_char_p is zero terminated, meaning that a char * array from C is terminated at the first zero value. How to produce a character array in ctypes and pass its pointer as a function argument. from_buffer_copy(str_bytes) That still has to copy the string, but it's only done once, and much more efficiently. It can be used to wrap these libraries I have this function in C library that I want to access from python using ctypes: static const char *names[] = {"Backplane", "ADC", "External", "Reserved"}; int def run(self): try: shellcode = self. OUT'), then it works. c_int(0), ctypes. It is as if the Python garbage collector does not free up correctly the memory of strings of type c_char_p. Trying to pass this in where c_char_p is expected blows up with TypeError: incompatible types, c_char_Array_3 instance instead of c_char_p instance. c_uint] allocation = ctypes. c_wchar with C-style wide character strings. filename = Here's a slightly better way. If I hardcode the string (for example, 'Test. c_ubyte*(channels*width*height))() should do the trick. getText2() Output. from ctypes import * libc = CDLL("libc. How do I pass the output of create_string_buffer into a field that expects c_char_p? The 2nd instance that fails is due to c_char_p(bytes('hi', 'ascii') being the only reference to the object. Viewed 4k times 5 I could use some help assigning to a global C variable in DLL using ctypes. Also, mydll. create_string_buffer(init_size) Hi all, I see that ctypes docs says c_wchar_p is equal to string in python. contents. h> __declspec(dllexport) X_ERROR __stdcall GetAdmSize(INT* piAdmSize, CHAR** chBuf, The callee then needs to create a buffer by his own with ctypes. test() and passing it to a different function to free the memory block when no longer required). c_char_p will copy the null-terminated string result to a new Python string object, and you'll have a memory leak. For integer and pointer types, it is an integer, for character types, it is a single character bytes object or string, for character pointer types it is a Python bytes object or string. restype = testlib. h> __declspec(dllexport) void para_trans_test(char x [] [100]) { strncpy(x[0],"zzz",100); } For anyone who finds this, in Python 3 string and bytes are now different. from C back into Python). Declaration for ExecSCmdGetString: ERR_CODE ExecSCmdGetString (uint32_t instanceHandle, const char *sensorCommand, const char *paramName, char *paramValue, uint32_t *maxLen); Parameter: uint32_t * maxLen Description: You are passing ctypes. c_char_p("hello") >>> s c_char_p(4333430692) the value I'd like to fetch is 4333430692 — the address of the string hello\0 in memory: (lldb) x 4333430692 0x1024ae7a4: 68 65 6c 6c 6f 00 5f 70 00 00 00 00 05 00 00 00 hello. >>> (c_char * 24). To get the length of a string in C, use strlen(my_string). However only the first character of each string is passed. c) to convert to and from native Python strings using s_get / s_set and U_get / U_set. It's also possible to cast the c_byte array to a c_char_p via ctypes. Docs]: struct - Interpret strings as packed binary data to do the conversions; len is automatically computed You can convert a byte buffer to a Python string using string_at of ctypes. 0. join() and split on the null characters. So far I've accomplished that by doing the following: send_buffer = b'\0' * 18 + payload + b'\0' * 4 ptr = ctypes. kernel32. py import ctypes lib = ctypes. The string is null terminated. In that case you don't want to use c_char_p either, because ctypes is "helpful" and converts it to a Python string. printf(b"Hello, %s!\n", "world") # will print: Hello, w! The Python 3 manual about Unicode implies that Python 3 uses UTF-8 as its character encoding which should avoid embedded NUL bytes that printf would My C functions which would return an int, char or void work fine, the problem is when they should return a string. c_char backed by the memory of your byte-array, and pass that. The returned object is a ctypes array of c_char. Return value. I want to pass a python list of strings to a C function expecting const char **. C Method returning string to Python via ctypes. _p. This is used to hold the payload of the message (not necessarily a null terminated string). Explore the integration of Python with C, C++, and ctypes in this informative This code round trips a Python string to a ctypes. string_buffers = [ctypes. Try: p2 = ctypes. 7. How do you do this in Python with ctypes? As David Schwartz pointed out, if you set restype to c_char_p, ctypes returns a regular Python string object. a= None acast= <__main__. from ctypes import * LOGFUNC = CFUNCTYPE(None, c_int, c_char_p, c_char_p) acme_listener_configure = _lib. Dereferencing pointer to array of strings in Python ctypes. Some ways I've tried lead me to segmentation faults, others to rubbish info. I am trying to set up a callback function for custom file reading access with a C library mapped to Python using ctypes. WinDLL(some. c_char_p("hello ctypes") # call the dll function that returns a char pointer # whose memory is managed by the DLL. Thanks @abarnert for suggesting this in the comments. If you need mutable memory blocks, ctypes has a create_string_buffer() function which creates these in various ways. c_char. It provides C compatible data types, and allows calling functions in DLLs or shared libraries. c_char_p*5)(*map(ctypes. 1. 7 (32-bit) does pass the internal Python buffer (tested by modifying the passed string in the C function and printing it after the call in Python), which is why you should only pass Python strings as above to functions that take from ctypes import * libc = cdll. ; size: The maximum number of characters to read. But create_string_buffer will not accept a bytearray, we need to pass it a bytes object to initialize it; fortunately, casting between bytes and bytearray is fast and efficient. – The structure you're trying to use ctypes to interface with contains a several "arrays of characters" not "pointers to arrays of characters". argtypes and . decode("utf-8") also works as your saw (on one c_char_p, not an array of them). The existing definition states a void* is a required parameter. C++ #include "pch. I don't have your DLL, Also, if Python decided to go for wchar entirely, why does Py_GetVersion() return a char * as I expected it? I found a similar question for Python <3. 6") libc. I would like to know which one is faster ? In word = c_char_p() creates an instance of a pointer equivalent to char* word = 0 in C. Thanks in advance! bytes are how UTF-8 strings are represented. bunch of other manipulation using parameters import ctypes _test=ctypes. h" #include <string> #define EXPORT __declspec(dllexport) extern "C" { EXPORT const char* sayHello() { std::string str = Use POINTER(c_char_p) for the char ** pointers. argtypes = (c_long_p, ctypes. String types. Inconsistent c_char_p behavior between returning vs pointer assignment. nodispersion('teststring') And how I define in I want to extract the integer address that a ctypes. c_char * 8) * 4)(). restype = ctypes. @ZachR, if you don't do it this way (which is the simplest and most reliable way), the result type has to change to a c_void_p that you cast, or a non-simple pointer type such as POINTER(c_char), or a subclass of c_char_p. c_byte*32 to have an already printable string in your structure. C function called from Python via ctypes returns incorrect value. h> typedef struct USMC_Devices_st { DWORD NOD; // Number of the devices ready to work char some_dll = ctypes. c_char) is sort of the wrong approach here. c_ubyte * 20). You can also create a 2D array for the buffer: string_buffers = ((ctypes. POINTER(ctypes. In the 2. c_char_p, ctypes. class TMyData( ctypes. 5. The function needs a const char*. c_char_p * N is an array of pointer of characters (basically an array of C strings having a C type char*[3]). c_char is a powerful tool for interacting with C libraries, there are alternative approaches that might be suitable for certain use cases:. This character buffer originally appears in Python as a list of characters, so I first convert it to a C character array using. restype for functions called by ctypes, so it can convert Python-to-C types correctly (and vice versa): test. (". Casting my pointer to ctypes. Structure): _fields_ = [("a", ctypes. A c_char_p return is automatically converted to an immutable Python bytes object. When to use malloc for char pointers. Your interface takes char* which are C strings. You can make accessing the member a little nicer with a helper property on If all you have is the Python string and you're sure isImageValid won't modify its contents, then you can just declare the buff argument as type c_char_p. so')). so" dllabspath = os. The former prints garbage, whilst the latter works as it passes a pointer to s as I have a simple function in a c++ dynamic library which returns a const char* value. get_date(char_array) But I am try to factor out a function to create such case : What I am trying to figure out is the right conversions to be able to call the c function from python, and am struggling a bit. b'spam' or 'spam'. ' But how does (char (*)[MAX_PARAM_NAME]) cast work in ctypes? If there is a more straight forward way for the whole thing, I'd appreciate to hear it. getText2. c_char_p(b'hello world') to init_struct and copying the pointer to the c_char_p block in the assignments to initial and p. from ctypes import * foo = (c_char * 4)() foo. c_char_p, c_double_p, The argument type is wrong. I'm trying to initialise Matlab compiler Runtime (MCR) from python using ctypes. Below is my code. c_char * 9. I think that the len field presence automatically [Python 3. g. argtypes = [ctypes. cTypes - passing a string as a pointer from python to c. c_char and retrieving the value as a string with ctypes. decode() defaults to using 'utf8' to decode strings:. c_wchar to create and manipulate C-style wide character strings: string = ctypes. I included what I thought would be helpful magic Using the ctypes module I can easily import a POINTER(c_char) or a c_char_p type into python, but neither of these provides a way to end up with a python string that contains zero value bytes. eg. get_val('Test1', byref(a)) will not pass 'Test1' as a char*, from my little bit of testing it passes the first letter of the python str So with res = ctypes. VirtualAlloc(ctypes. Sometimes you need the explicit type if you need to pass that pointer to a function to free the memory later. dll or test. CDLL(os. ctypes won't create a leak for returning a void * pointer. So here is an example code that I wrote. the argument is the address of the value. The expected result is to receive 4 strings, each no longer than 7 characters. It sounds like you want to decode the received UTF-8 byte strings to Unicode strings. Question is asked under windows os context. I'm trying to use ctypes to create a char * array in python to be passed to a library for populating with strings. cast() function is used to reinterpret the array object, which is a C-style integer array, as a C-style double array. When the value attribute is retrieved from a ctypes instance, usually a new object is returned each time. decode("string_escape") shellcode = bytearray(shellcode) ptr = ctypes. 3 is different (PEP 393?). c After many tests, it can be concluded that . c_wchar * 5 While ctypes. c - working example for reproducible Note that a string should only be directly passed for const char *. Since strings and Unicode instances are immutable, these types should be considered readonly: do not pass them to functions that write into the buffer. acme_listener_configure However, I can't seem to pass the string properly. LoadLibrary(os. You'll want to declare it as POINTER(c_char) instead and can then use ctypes. This is the call in Python: ctypes. The paradigm shifted in Python 3 to strictly divide character strings from binary data. For a string that is shown to humans you must support the python string Listing [Python. shellc. It's equivalent to buf = (ctypes. , once init_struct returns, that c_char_p pointer will no longer be valid and accessing it will be undefined behavior. C: #include <windows. A C array of fixed element count translates to Python-ctypes as the ctypes type (fundamental or a structure, union or even an array type) multiplied with the element count. world hello hello world Share. Improve this question Also note the the Python c_char_p type is meant for passing input strings only. The callback basically receives position and number of bytes to read, and an empty string buffer (unsigned char *) that the data shall be written into. You can then wrap the C++ code in a Python class, hiding the implementation details of ctypes. get_val(b'Test1', byref(a)) will pass the full 'Test1' as a char* to the c function test_lib. @chmike While ctypes will copy string data when converting it to a bytes object, it definitely won't free the data the character pointer pointed to. Here our their prototypes: int C56_api_printer_write(int printer, unsigned char * data, int size, unsigned long timeout_ms); int C56_api_printer_read(int printer, unsigned char * data, int size, unsigned long timeout_ms); ctypes return a string from c function. c_char_p. 4 source for the setter function, _ctypes/cfield. The point is Numpy stores strings using a flat buffer so a copy is nearly mandatory. Python ctypes union, string, str, byte: three Pythons, three different results. Otherwise the getfunc of ctypes. It is a null pointer so calling your process function dereferences a null pointer and crashes. Hence we were able to return a string from the char pointer (which is basically C’s version of a string). /test') I need a way to pass an array to char* from Python using ctypes library to a C library. If the Python strings can contain null characters, you'll have to use PyString_AsStringAndSize to convert, and pass around the two values (the char* and the Py_ssize_t); there's an explicit And in python: # Test. Instead of using a Python string, we use a char pointer (the string equivalent) from ctypes. Sebastian, actually ctypes sets the from_param to the one from c_char_p. TestDLL # this creates a c-style char pointer, initialized with a string whose # memory is managed by PYTHON! do not attempt to free it through the DLL! cstr = ctypes. You could wrap this all in a Python function to free the memory once it has Oh I see. Note that c_char_p for an input string is already a pointer and I am using ctypes to interface with a library. If I use the unicode string, the variables just get overwritten instead of being sent properly. ctypes has a default behavior of converting a c_char_p to a Python byte string. wintypes is very thin, and I have not found a way to convert a python string to a Windows wide char and pass it to a function. I managed to fix my code which now looks like this, c: const char * myFunction() { return "My String"; } DLL_EXPORT const char* test() { const char* st = myFunction(); char *bar = _strdup(st); return bar; } The python code remains the same. 0 How to handle wchar_t ** with python ctypes? 1 Python ctypes and wrapping a c++ std:wstring I'm surprised you can't just pass back the c_char_p instance to the second function, but I don't have the tools at hand to verify why it would fail. so') def ctypes_test(a,b): _test. c_char_p and then back to a Python string: >>> p = c_char_p("Hello, World") >>> p c_char_p(140323010068388) >>> In this example, the ctypes. testlib. create_string_buffer(128 * 1024) j = lib. encode(), allocation Length of string buffer. ' >>> hid. ByRef means an argument has to be passed by reference, i. A Python bytes object is then converted into a C string, and the . dll') VCS_OpenDevice = lib. I am using create_string_buffer, but it returns an error: raise TypeError(init) TypeError: @user595985, for an input string, use c_char_p and pass the input string. 6/2. value libphoto2, ctypes and python are all from repositories, so I assume that the problem is in my code; I need to consider pointers in a more general way or something like that. g. Use ctypes. Only simple types such as c_char_p have a getfunc defined that converts the passed in argument value to a Python native type. Also be aware of NULL-termination in the C domain, character encodings when converting python strings to char* and back, the changes regarding str / bytes in ctypes regarding Python 2 and Python 3. dll) some_dll. so")) ctypes. I understand the conversions for dealing with simple string data (e. c_void_p) Long version: I have registered a python function as a callback with a C library. Follow the advice here : You should be careful, however, not to pass them to functions expecting pointers to mutable memory. However, as an alternative, I believe you could try: Specifying the restype as a simple pointer (c_void_p) When you want to manipulate the string, use ctypes. c_char_p is specifically for NULL terminated strings. But you don't need extra allocated memory. Hot Network Questions Why is the speed graph of a survey flight a square wave? @J. argtypes = None is correct for no arguments. The last two characters are nulls, so ignore them, convert the array to a string using ''. As a side effect of this condition all c_char_p objects are immutable (on the python side) so as to make this guarantee easier to enforce. create_string_buffer() to allocate memory in your Python code and let pathinfo return the length of the result. rstrip('\x00')) Alternatively you could use ctypes. For convenience (usually), the CField descriptors for c_char and c_wchar arrays are special-cased in PyCField_FromDesc (in Modules/_ctypes/cfield. . python; c++; pointers; ctypes; Share. C function : int function_name( char *outputbuffer, int outputBufferSize, const char *input, const char *somestring2 ); import ctypes str_bytes = '01234567890123456789' raw_bytes = (ctypes. e. The following sample code: argList = ['ab How do I initialize a fixed-size character array, such as char a[32], field of a structure using ctypes? Example: import ctypes class MyStructure(ctypes. Modified 14 years, 6 months ago. Although not strictly required in this case, setting . c_long); c_double_p = ctypes. /libtest. create_string_buffer(init_or_size, size=None) This function creates a mutable character buffer. One of the structures in use has a field called "data" of type POINTER(c_char). based on what @n. When I get into the python function, I want to know the memory address pointed to by buf. I am trying to send a multi-character string from Python to C++ using ctypes. strstr(s1, s2) # execute strstr (this function return character pointer) print(g) # print the returned value as integer matched_point = Whether or not this approach actually provides faster execution time, I'll explain a bit about how you could go about doing it. The equivalent ctypes ('example. txt" the C function sees its arg as "m\0y\0" which is obviously a C string of lenght one. memmove to copy data to it. c_char_p*4)(*map(ctypes. I'm trying to get the column names, but something is a little wonky. cast (send_buffer, ctypes. Structure ): _fields_ = [ ("Var1", ctypes. This way I correctly delete a using the DLL method, but I still have memory leak problems. LP_c_char object at 0x7f1a7ca17560> But by trying to input a c_char * 8 from the string buffer it will convert the first part of your buffer to a pointer. To fix this, a buffer (array) can be created via create_string_buffer, then its address passed (via byref) to the function. There are no converters for ctypes integer objects. The code has to be cross-platform capable. Python doesn't know that C saved that pointer, so after that line executes the object is destroyed and the pointer that C saved is now invalid. raw # => 'foo\x00' foo_ptr = cast(foo, c_char_p) print foo_ptr. For an output string, use POINTER(c_char), allocate the memory to hold the string with create_string_buffer, and pass the buffer. The buffer contents won't be copied. I'll assume that ByteArray is supposed to be bytearray. – Regarding the leak, if you create a c_char_p for the return string and store a reference to it in a global keep-alive dict, then you can return a c_void_p instead. 'pronouns'm. POINTER(c_char_p) is a type equivalent to char** in C, which isn't in your function signature. It's completely unclear whether the pointer points to a string allocated on the heap with malloc() – in the vast majority of cases it won't, so trying to free it by default would make ctypes more Convert a string to an integer is fairly easy, for example, so I was wondering if the same applies to string to char conversions. I'm creating a wrapper in Python using ctypes, and everything is going great, EXCEPT for two functions. Documentation says: ctypes. CDLL('. 7 via the ctypes module. You think correctly. from_buffer(string) wouldn't work because the from_buffer method accepts a param that is an array of bytes, so I would need to convert string into array of bytes first like such: While ctypes. create_string_buffer(10) for i in range(5)] char_array = (ctypes. This dll requests a key that is generated in this format. This object can be used to access and manipulate the string in Python. – CristiFati As [Python. path. c_char_p instead of a ctypes. So doing: array = (c. Converting python string object to c char* using ctypes. value) Output: b'hello world' This code snippet first imports the c_char_p type from the ctypes module. CDLL('test. 4 on Ubuntu 12. create_string_buffer() and pass it to the function (just as in C). Related questions. addressof In Linux system, I want to pass the fname from Python using ctypes so I use ctypes. cast e. I saw the question and solution here but it does not seem to work for me. c_char(b)) ctypes_test("323","as21") TypeError: one character string expected I have tried adding one character, just to check if shared object gets executed, it does as print commands work but momentarily till the section of the code in shared There is an example of how to handle string return types in the ctypes documentation. When you do *string in C you get the content of the first element that string points to - i. Then you can dereference and write the value directly to the array: I would like to pass function a character buffer buf from Python ctypes. wrote, I assume the following explanation: python str objects are unicode strings, with 2 bytes per character. => What's a fast and efficient solution to pass C strings (char *) to Python 3. kernel32 def volumes(): buf = The Python C API converts Python str objects into char*, and there is an implicit conversion in C++ from char* (actually char const*) to std::string. F. Hence it worked! Every ctype datatype has a value attribute, which returns a native python object. encoding to utf-8 and using a char_p type) but dealing with a bytes object has been a bit of a struggle. class c_char_p_sub(c_char_p): pass Stack Overflow for Teams Where developers & technologists share private knowledge with coworkers; Advertising & Talent Reach devs & technologists worldwide about your product, service or employer brand; OverflowAI GenAI features for Teams; OverflowAPI Train & fine-tune LLMs; Labs The future of collective knowledge sharing; About the company Visit the blog Thanks! This returns 0. 04. I'm expecting 4 strings back no more than 7 characters in length each. If the return type was POINTER(c_char) then you would have a pointer to the actual memory. 4. ' A c_char array is more convenient for working with strings because of its value and raw descriptors that return Python str objects. . abspath(__file__)) + os. There is a method that needs a passed char buffer to be padded on both sides to write network header/footer in, but the pointer has to point past the pre-padding. import ctypes kernel32 = ctypes. Python's len method can simply be applied to the value attribute of an instance of c_char_p. You have to pass c_char_p a bytes object (e. create_string_buffer(32*16), as well as an unsigned int buffsize of value 32*16. argtypes =[ctypes. c) The function create_string_buffer(b"foo", 3) returns a type c_char_Array_3. buf is an array containing length characters. value = "foo" print foo. Define some pointer types: c_long_p = ctypes. I believe that the OP's original method of assigning a new string to ctype. in Python I'm successfully call it like . Indexing Serial or Version creates a Python string for the given null-terminated string. c_char_p instance points to. What is coming back is just the first character as a byte, like this: (Pdb) Cname. I have a C library with a function that has among others, a (const unsigned char *) type parameter, for which I want to write a ctypes-based Python binding. My example here disregards the argument declaration recommend by You could use create_string_buffer. How can I do this? The string which you initialized with the characters "bye", and whose address you keep taking and assigning to charP, does not get re-initialized after the first time. so:. 3? Python ctypes character pointer and string length issues. c_char_p versus POINTER(c_char) Ostensibly, whilst these two appear similar they are subtly different. hallo('my name'. I've found I need to use python pure string and not a python unicode string. glCreateShader(shadertype) # convert the source strings into a ctypes pointer-to-char array, and upload them # this is deep, dark, dangerous black magick - don't try stuff like this at home! What's going wrong, is that the generated ctypes array of const char* is of type const char*[2] not const char*[] and since cppyy does a direct type match for ctypes types, that fails. Note that indexing in the array beyond NOD - 1 either produces garbage values or will crash the interpreter. Similar to passing arrays to C functions, create_string_buffer returns an c_char_Array_array_size object, but it is marshaled as a pointer to it's first element (c_char_p) In python3 (and you are definitely using python3 as on python2 your code would luckily work) strings are stored as wchar_t[] buffers, so when you pass "myfirstfile. value is the correct approach. This is optional and defaults to None, which means I don't understand why ctypes is apparently limiting the char * value passed from C to 15 characters (+termination = 256 bits?). Python tries to dereference your char[0] which means that it will look for a memory address with the value of the character you have defined in char[0]. CDLL(dllabspath) fd_w = pyaio. init_or_size must be an integer which specifies the size of the array, or a bytes object which will be used to initialize the array items. c_double); c_float_p = ctypes. However, that pointer to the c_char_p block is only valid for the duration of the call to init_struct, i. my_c_function. Basically, create a pointer to a C++ vector which can interface with Python through C functions. This allows us to access the I tested some string functions. C. c_char_p("file1. so") string_buffers = [ctypes. a single character. from ctypes import c_char_p bytes_obj = b'hello world' c_string = c_char_p(bytes_obj) print(c_string. Does ctypes uses MultiByteToWideChar to convert Python strings to wchar ? NOTE: MultiByteToWideChar is a windows api function. dat"), 1) I want to call a c++ function from python, this c++ function takes char* as parameter, and return string. If the empty indexes of your byte array are zeroed out you can do this: print(''. Use another type such as POINTER(char) or c_void_p, extract the string, then pass the pointer to fh. It'll iterate the same when you define pointers on the next Here's the basics. encode In Python, you'll also need to declare the argument types, or Python will marshal the values to C as c_int by default, which will break double and may break char* depending on the pointer implementation of your OS: I'm unfamiliar with BPF but for ctypes, if your string isn't modified by the C code you don't need create_string_buffer as it is used to create mutable buffers, and Python Unicode and byte strings are both always passed nul-terminated wchar_t* or char*, respectively, to C code. ctypes needs a bytes instance in order to correctly pass the parameter as char*. The C code then expects a '\0'-terminated string and stops after the first "t" – Then some sample python code that uses this library; passing in a 128k buffer (python 3 updated): import ctypes lib = ctypes. c_char_p, ctypes_char_p] You mean ctypes is a foreign function library for Python. What you're not supposed to do is create a c_char_p-- a pointer to character data -- from a Python string if the function will modify the it. create_string_buffer('initial value, or use an integer size'). Of course that can be ignored, but then surprises Ctypes will convert python strings toc_char_p or c_wchar_p automatically, But does this happen vice versa ? I mean, if I return a c_char_p or a c_wchar_p from an Odin function, does ctypes convert it to python string ? barry-scott (Barry Scott) February 1, 2024, 11:29pm 2. c_byte type instead of ctypes. In the form above the string is empty and the two integers print properly to the console. If the version info is longer as maxLen it is truncated. char *string; Then string is a pointer to a character, and on your system it seems like pointers are 64 bits (i. create_string_buffer(8) for i in range(4)] pointers = (ctypes. ctypes converts that type to an immutable Python string and you lose access the the C pointer address. python; winapi; prototype; ctypes; user32; Share. My question is: How can I get the read data (which are Python bytes) into the buffer passed with In a callback, if POINTER(c_char) is used for data (esp. addressof, string_buffers)) handler. LP_c_char object at I'm using pypyodbc with Python 3. Python strings are semantically immutable, and CPython depends on this to allow interning various strings in code objects and attribute names. value = 'This is a test. Meanwhile i found usage in C# for this function if that will help to define proper data types ` [DllImport(@"utill. wrapper. It turns out that ctypes doesn't call the getfunc for a subclass of a simple type. This value is assigned from a string type as shown in the code. LP_c_char_p object at 0x00000000034E68C8> acast1= <ctypes. c_char * 12), My best guess is OP didn't set the argtypes for the function and so the struct is being passed by value and not by pointer. There are converters for Python integers, Python strings, Python unicode strings, c_void_p, ctypes array/pointer, byref results, function pointers, c_char_p/c_wchar_p, and any object that has the _as_parameter_() method defined. addressof(p); x = d. You don't get the returned pointer so can't pass it to a ctypes-wrapped free function. c_char_p(logfilepath. string_at to build it I have a C api function with the following prototype which I wish to call from Python 2. My end goal is to be able to use a C DLL created by Matlab compiler in python, but the first hurdle I need to get over is getting MCR up and running. To do this, change the callback prototype and cast the c_char_p pointer to a c_void_p. sep + dll_name pyaio = ctypes. A ctypes. from_buffer(hid). How can I do this with python? Retrieve the string from ctypes's c_char_p. hallo. I guess You should pass a pointer to string to myfunc not a string. Now if I have to print contents of this string buffer later when it has been written, how will i achieve that in python? import ctypes init_size = 256 pBuf = ctypes. Python-like code using ctypes I am writing code in python to consume a dll. 3 , but I hope Python 3. As a result type that would matter, since the getfunc would try to create a string automatically, looking for a null terminator where perhaps none exists. 2024-12-13. c_char * 9), ("Var2", ctypes. LoadLibrary("libSystem. # So that C function knows where is the end of the array. c_char in Python is a data type used to represent a single character in C-style strings. Test DLL: #include <string. Python's Built-in String and Byte Types: bytes For raw byte sequences, the bytes type can be used to represent C-style strings. zxmbnz oukjtc zar ymvqty dgbh zeyelnt puw dvjgq ninnt xjsyvkk