Ctypes return string Normally, when you create a string buffer. GetWindowText. value attribute of the string_buffer is null terminated. I included what I thought would be helpful magic Raises an auditing event ctypes. hello() return result otherwise the default return type is int (and the Here's a slightly better way. You need to generate a shared library, which can be done with the following command. This marks the end of the Arrays with Python Ctypes Tutorial. c_ubyte array in Python. You don't get the returned pointer so can't pass it to a ctypes-wrapped free function. kernel32 def volumes(): buf = Your problem is that ctypes tries to do some magic for you with char arrays, auto-converting them into NUL-terminated strings. c_char_p # override the default return type (int) foo = hello. string_at. The returned object is a ctypes array of c_wchar. get_point() I'd recommend strongly against memory allocation in a foreign function called via ctypes and freeing this memory from Python. POINTER(ctypes. Array, which is the most specific annotation you can use: def foo(aqs: List[int]) -> ctypes. Modified 7 months ago. Cast the pointer to (single) wchar to a pointer to an array of wchar. On most systems this defaults to This function doesn't have a consistent return type, since the length of the returned array is part of the array's type. 1. value print ctypes is a foreign function library for Python. The name itself can be anything, just If you want to pass a mutable string to C from Python, you will need to use the create_string_buffer function provided by ctypes. See Return types in the ctype doc here. import ctypes buffer = ctypes. The optional To do this, we need to tell ctype that alloc_C_string should return a ctypes. You can’t link your regular . Hence we were able to return a string from the char pointer (which is basically C’s version of a string). In that case you don't want to use c_char_p either, because ctypes is "helpful" and converts it to a Python string. string_at (address, size=-1) This function returns the C string starting at memory address address as a bytes object. Related. x. I suppose this is because my created byte array starts with a zero, and the . Array: The return value of memcpy is an address you don't care about, so I "swallowed" it by assigning it to name "single underscore" (which by convention means "I don't care about this one";-). Most efficient way to convert a multidimensional numpy array to ctypes array. buf is an array containing length characters. Efficient way to convert string to ctypes. This is a small part of my project on my Raspberry Pi. restype correctly helps ctypes marshal parameters correctly and detect incorrectly passed parameters. Although I´m having problems declaring the lpString As already mentioned in comments, although perhaps not as explicitly as some might require, the . restype attribute. Ask Question Asked 13 years, 7 months ago. ; size: The maximum number of characters to read. If you want a different return type, you can change it by New: It is now possible to put items in argtypes which are not ctypes types, but each item must have a from_param() method which returns a value usable as argument (integer, string, ctypes ctypes: Return string vector from C++ function I am trying to return a string vector from a C++ function and used the returned array in Python. Then you can dereference and write the value directly to the array: Python ctypes return values question. (Note that the length of the string "voidFunct called!!!\n" is 20, not 17. Hence it worked! Every ctype datatype has a value attribute, which returns a native python object. Although not strictly required in this case, setting . It would be easier if you posted runnable code: getting a suitable volume name for this call is a bit of a pain. This is useful because sometimes library functions need a callback function parameter; the qsort C function is such an example. POINTER to a ctypes. However, the type is always a subclass of ctypes. c_char and retrieving the value as a string with ctypes. Change as follow (This is the cause of the TypeError): Python ctypes to return an array of function pointers. Return value. so') name = "Frank" c_name = ctypes. You can make accessing the member a little nicer with a helper property on 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 when you build you numpy array of char*, you could just get the pointer to array (with ctypes and directly give it to the lib expecting char**. So you just need to set the restype of the function to the type you need, like this: test_lib. I don't understand how it works with pointer on Python when I return for example a char * from my C++ function, how to get the data at the adress of the pointer ? myClass. Returns the size in bytes of a ctypes type or instance memory buffer. In the . Any suggestions or contributions for Sending a string from Python to C++ using Python's ctypes module requires you to parse it as a c_char_p (a char *). external_C(len(example), example_ptr) I'm pretty sure I've got my argtypes and restype set up correctly for the ctypes interface into a function that is supposed to return a string, but no matter what I only get an empty string as the returned result. This is optional and defaults to None, which means There is an example of how to handle string return types in the ctypes documentation. This time though i wanted to process a step further and use a function prototype instead of calling the function with ctypes. Replace Item_getName as follow to return char * instead of string *: const char* Item_getName(Item* item) { return item->getName()->c_str(); } Item class is missing __init__. This code round trips a Python string to a ctypes. C Method returning string to I was curious, whether using the Windows API HeapAlloc (via ctypes. get_point. Note that c_char_p for an input string is already a pointer and I have a C++ code for which I created a python. 2) to C using ctypes. c_char_p(name) hello. A ctypes. cast(example_np. data, ctypes. user32. create_string_buffer(upper_limt) I went with switching the restype to c_void_p and then casting to POINTER(POINTER(c_char)), because that mostly matched up what I was doing before. C++ Code: const char* disasmC_PEProgram_getSections(PEProgram *p) { return p->getSections(). 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) As the ctypes documentation shows, you need to explicitly tell it how to handle the return type: def saystr(): lib. lib. import ctypes kernel32 = ctypes. Use another type such as POINTER(char) or c_void_p, extract the string, then pass the pointer to I've a simple working program that submits an int (num_piezasin the example) from a Python function to a C++ function. 0. set_conversion_mode(encoding, errors). In your case, if you know in advance an upper limit for the length of the returned string, use. 3. I am discovering the world of ctypes because I am writing a DLL in C++ on Windows with a C wrapper to be able to use it on Python for example. “clibrary” is the name we gave to our C file. restype = POINT p1 = test_lib. ctypes is able to create C callable functions from Python callables. Example of C code: WString Access in Ctypes . ctypes implicitly converts a unicode text string to a byte string using the encoding set by ctypes. Python ctypes doesn't correctly return some values passed by reference. We can be fairly sure that C program mentioned above is not exactly Instead of using a Python string, we use a char pointer (the string equivalent) from ctypes. I tried to do the task as below. address: The memory address where the wide character string starts. You can get around this magic by using the ctypes. Whenever possible, allocate the memory in Python. A simple way to get around this is to use a void * and cast the result: string. For an output string, use POINTER(c_char), allocate the memory to hold the string with create_string_buffer, and pass the buffer. Python call by ref call by value using ctypes. c_char. This object can be used to access and manipulate the string in Python. raw attribute, the correct byte string is seen. If size is specified, it is used as size, otherwise the string is assumed to be zero-terminated. create_string_buffer with arguments init, size. restype = c_char_p # pointer to a string result = lib. c_char_p to a Python string. hello. wchar_p object representing the wide character string. Write an equivalent, simplified function that only returns a string in that format (a few lines of C), wrap it in Python ctypes and reproduce your problem. . join() and split on the null characters. 2024-12-13. Instead of a Python string, use a character pointer from ctypes. We saw that earlier. ctypes. create_string_buffer(1024 * 1024 * 1024) # 1 GiB I'm currently getting into the ctypes module and I'm trying to call the user32 function GetWindowText with a HWND handle I already received by using FindWindow. Ctypes Return Array. 2. c_char_p)) # this is char** and you could just call. We will pass this array to our C file, get the result and print it out in our Python file. For your code: Here's the basics. LoadLibrary('. In this line: results = (c_char_p * 4)(addressof(create_string_buffer(7))) You're creating a single buffer of 7 bytes, then trying to use it to hold 4 character pointers (which are probably 4 bytes each), and then also copying 4 8-byte strings into the random addresses it . hello(c_name) print c_name. I am trying to send 2 strings from Python (3. Example: Here is a more advanced example, it uses the strchr function, which expects a string pointer and a char, and returns a pointer to a string: You can solve this by using some ctypes data types that are compatible with C. WINFUNCTYPE) or using ctypes. restype = ctypes. c_str(); } Python @user595985, for an input string, use c_char_p and pass the input string. c_char_p and then back to a Python string: @HelinWang Even in the general case, if C allocates it, C must free it. c_byte type instead of ctypes. create_string_buffer is faster for allocating arrays of integers and I found some strange behavior when it comes to deallocating the buffers. The ctypes. You can then wrap the C++ code in a Python class, hiding the implementation details of ctypes. argtypes and . Does the same as the C sizeof operator. c: passing strings to ctypes functions under Python 3. The * in the C++ definition indicates a pointer to a value, which is why I used POINTER. It provides C compatible data types, and allows calling functions in DLLs or shared libraries. The last two characters are nulls, so ignore them, convert the array to a string using ''. As David Schwartz pointed out, if you set restype to c_char_p, ctypes returns a regular Python string object. ctypes pass string by reference: string is not updated. cdll. Basically, create a pointer to a C++ vector which can interface with Python through C functions. Other return types can be specified by setting the restype attribute of the function object. It works quite well, except when the returning string is long. POINTER objects are not overly useful, but There is a bit ctypes magic at play: by default ctypes assumes every function returns a int, so this works out fairly well. 5. This should work (and does work for me) in Python 2. ctypes. cpp #include <iostream> class Foo{ public: void bar(int We will be creating an array of integers in our Python file, and a function which sums the elements of an array and returns the result in our C program. Viewed 13k times which is the number of bytes written to the console by the printf() function. value attribute converts from ctypes. ctypes to int conversion with variable assigned by reference. windll. create_unicode_buffer (init_or_size, size = None) ¶ This function creates a mutable unicode character buffer. Post the C and Python code. buf = ctypes. h file: Callback functions. CTYPES and PYTHON3: Can't return long string. To test if the C function received the strings correctly, I place one of th Whether or not this approach actually provides faster execution time, I'll explain a bit about how you could go about doing it. ctypes wrapper. What does the hash_id_G1() function return? Is it a null-terminated string, or a string of a fixed known size? In the latter case you need to declare restype as something like POINTER(c_char * 40). /hello. cfile though. Each ctypes data type has an attribute that returns a native Python object that will help you If the library function returns a type different from int, the restype attribute can be set to a ctypes type that describes the return type, or to None meaning no return value (void). Python ctypes: passing to function with 'const char ** ' argument. 6. Afterwards, the buffer property of my test_struc is just an empty b'' string. example_ptr = ctypes. import ctypes hello = ctypes. Callback functions are created by first creating a function prototype with a call to CFUNCTYPE or WINFUNCTYPE, specifying the return type and the argument By default functions are assumed to return the C int type. Unless the foreign function returns and int, you need to set the return type for the foreign function with the . foo. It can be used to wrap these libraries in Now let’s try to import a basic library with a single function over to our Python program using ctypes. zwrgjz ylbg eqszji hztniy letlows sem gvcfi cby blbefy lnozj