Ctypes return string. raw >>> x[0] = 255 >>> b = x.

Ctypes return string I have full control of both environme Listing [Python. Example of C code: In this case, the most recent use of the ax/eax register was the (unused) return value from the printf() statement - which is the number of bytes written to the console by the printf() function. GoString. In the . the argument is the address of the value. so" dllabspath = os. hello() return result otherwise the default return type is int (and the returned pointer is converted to an integer address). cdll. CDLL(dllabspath) fd_w = pyaio. If you don't want the behavior, use either: ctypes. c_char) to keep the pointer. Python call by ref call by value using ctypes. create_string_buffer(1) >>> a = x. c_char)) class PCHAR(ctypes. Use ctypes. join() and split on the null characters. The snprintf function can format it into a string for printing or conversion One of the functions returns a dynamically sized character array, and I would like to be able to read the contents of this array in Python, but also property handle freeing the memory of the array when I am done with it. Array, which is the most specific annotation you can use: def foo(aqs: List[int]) -> ctypes. The name itself can be anything, just C strings are not compatible with Python strings, so the output remains the same. In CTypes, this is achieved by specifying:. Everything worked fine, until I tried it with the threading module. argtypes=[ctypes. } Your problem is that ctypes tries to do some magic for you with char arrays, auto-converting them into NUL-terminated strings. say_hi. My problem is to access the return parame I am using a DLL with js-ctypes that is made in C. In Go you can convert a *char to a Go string using C. “clibrary” is the name we gave to our C file. To test if the C function received the strings correctly, I place one of th If you're going to workaround, wouldn't it make more sense to send your c_char_p to a C++ function that converts it to std::string and then calls HandleString immediately? In fact, there's an implicit conversion from const char* to std::string, so your workaround function can just use the pointer as the argument expression for HandleString. You can get around this magic by using the ctypes. address: The memory address where the wide character string starts. Then you can create Python side POINT structures by using the (hidden) "from_address" method in your POINT class: I am working on implementation of new fiscal device. See Return types in the ctype doc here. This is a small part of my project on my Raspberry Pi. 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. get_errno() returns the value of the ctypes private copy, and the function ctypes. _getcwd(out,30)) You could use create_string_buffer. If we try WString Access in Ctypes . python ctypes unexpected behavior on windows. c_char_p("file1. I've found I need to use python pure string and not a python unicode string. import ctypes kernel32 = ctypes. e. string_at (address, size=-1) This function returns the C string starting at memory address address as a bytes object. 23. This copies the However, if I try the same in Python, only the first character in my string is printed. Most efficient way to convert a multidimensional numpy array to ctypes array. sizeof (obj_or_type) Returns the size in bytes of a ctypes type or instance memory buffer. I suppose this is because my created byte array starts with a zero, and the . WinDLL("MyDLLName. import ctypes my_dll = ctypes. To handle an output string, it will be thread safe if you allocate the buffer in Python instead of using a global variable, or just return a wide string constant. 0. The problem can be solved by using some Ctypes data types. If size is specified, it is used as size, otherwise the string is assumed to be As already mentioned in comments, although perhaps not as explicitly as some might require, the . Along the lines of my original example, a solution would look like this: I'm familiar with python but not with ctypes. restype = ctypes. sep + dll_name pyaio = ctypes. restype correctly helps ctypes marshal parameters correctly and detect incorrectly passed parameters. c The ReadProcessMemory function of kernel32. You can then wrap the C++ code in a Python class, hiding the implementation details of ctypes. kernel32 PROCESS_QUERY_INFORMATION = 0x0400 PROCESS_VM_READ = 0x0010 pid = int(raw_input(" I wrote a Python wrapper for a C library using ctypes. c_char_p appId=ctypes. This is useful because sometimes library functions need a callback function parameter; the qsort C function is such an example. create_string_buffer(init_or_size, size=None) This function creates a mutable character buffer. kernel32 = ctypes. If I use the unicode string, the variables just get overwritten instead of being sent properly. raw >>> a b'\x00' >>> b b'\xff' This point in the documentation comments on this for the ctypes type c_char_p, but it As David Schwartz pointed out, if you set restype to c_char_p, ctypes returns a regular Python string object. value property, so I had to directly dereference the pointer to affect change in the value. 31. hello. string_at(address[, size]) This function returns the string starting at memory address address. 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 instance I don't think ctypes allows one to translate std::pair to a python tuple without much boilerplate code. I am very new to ctypes and have no experience with C at all. gp_library_version(0) x = ctypes. Checked on multiple PCs. Commented Mar 4, 2013 at 9:00. wchar_p object representing the wide character string. This function doesn't have a consistent return type, since the length of the returned array is part of the array's type. Meanwhile i found usage in C# for this function if that will help to define proper data types ` [DllImport(@"utill. foo(byref(o), s) Output before change (garbage in second string indicates accessing garbage-collected memory and therefore undefined behavior): A return type of ctypes. – Mark Tolonen. x(default encoding is ASCII), then the return shows the string like the Python 3. It returns a string via szProgSize containing the program version the dll is specific to. Unfortunately, your C code create_string_buffer(s) makes a temporary string that is garbage-collected after the function call. If you have a function return a subclass of c_longdouble, it will return the ctypes wrapped field object rather than converting to a python float. restype For example, a c_char_p item in the argtypes tuple will convert a unicode string passed as argument into an byte string using ctypes conversion rules. Notes:. set_errno() changes the ctypes private copy to a new value and returns the Now let’s try to import a basic library with a single function over to our Python program using ctypes. c_char_p instead helps. c_char_p to a Python string. Efficient way to convert string to ctypes. dll (. Here's an example: C++ I need to pass a string to a C++ function and have it return the string, but when I try to do this I get an error: OSError: exception: access violation reading 0x0000000000000005 Code python 3. Hot Network Questions How *exactly* is divisibility defined? How to explain why I don't have a reference letter from my supervisor Can a hyphen be a Well, in your code you are trying to return a String (in C which is nothing but a null-terminated array of characters), but the return type of your function is char which is causing all the trouble for you. c_char_p('XXXXXXXX-XXXX-XXXX-XXXX-XXXXXXXXXXXX') challenge= myDll. Casting my pointer to ctypes. You can also create writable arrays of ctypes types with the syntax: variable_name = (ctypes_type * length)(initial_values) test. c_char_p. To do this, we need to 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. import ctypes hello = ctypes. addressof(ctypes. h file: As the ctypes documentation shows, you need to explicitly tell it how to handle the return type: def saystr(): lib. Alternatively, we just use cstring = b”Hello”, as Python bytes are also supported by C. The method return a string, but when I try access the content of the pointer firefox crashes! The following code works: Function declaration: var getStr = lib. Using ctypes. Faster way to convert ctypes array In Linux system, I want to pass the fname from Python using ctypes so I use ctypes. I'm trying to pass a string (as a pointer) from python to a C function using cTypes. 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. c_char_p): pass (derivations suppress the behavior); Note that an LP_c_char doesn't have a . Explanation: After importing and storing a value in value_1 variable we are using a variable named ptr (resembles pointer) and using the pointer method of the ctypes module to point to that value_1. Add a comment | 2 Answers Sorted by: Reset to ctypes pass string by reference: string is not updated. Although not strictly required in this case, setting . ctypes is a foreign function library Each ctypes data type has an attribute that returns a native Python object that will help you return a string from the char pointer. restype=ctypes. Does the same as the C sizeof operator. Topic detailed in [SO]: C function called from Python via ctypes returns incorrect value (@CristiFati's answer). fileopen(ctypes. When using character pointers, we aren’t exactly dealing with mutable memory. h> #ifdef _WIN32 # define API __declspec(dllexport) #else # define API #endif API int read_mesh(int *node, char *buf, int bufsize, int exitflag, int Strings are passed by converting a Python string to a bytes object (typically by calling encode()), then to a C-pointer and then to a Go-string. We will need to first explicitly define the return type of the getArray() function as a “integer pointer” using the restype attribute (part of the function signature in ctypes). text definition (as you noticed). 1. I included what I thought would be helpful magic @user595985, for an input string, use c_char_p and pass the input string. argtypes and . Documentation says: ctypes. import ctypes buffer = ctypes. value to return the original string. Ctypes Return Array. ctypes. Creating mutable memory with string buffers When using char pointers, you are not dealing with mutable 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: 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). 5 ctypes doc shows. 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 or less unusable. Configure return type of say_hi to "pointer to char" # (setting it to ctypes. It's a convenience: from ctypes import * dll = CDLL('msvcrt') dll. 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). argtypes must be a sequence of C data types (the printf function is probably not a good example here, because it takes a variable number and different types of parameters depending on the format string, on I want to pass a python list containing string data to a "c" DLL, which process the data and should return an array containing integer data. dll") myDll. (Note that the length of the string "voidFunct called!!!\n" is 20, not 17. so') name = "Frank" c_name = ctypes. I tried to do the task as below. int32_t ); Function call: Afterwards, the buffer property of my test_struc is just an empty b'' string. c_char_p in argtypes makes Python expect a bytes object and convert it to a C *char. c_char_p is automatically converted to Python bytes. Here's a simple example calling _strupr() from the Windows MSVCRT library: >>> import ctypes >>> create_string_buffer can be initialized with a string or a length. c: I was curious, whether using the Windows API HeapAlloc (via ctypes. Define some pointer types: c_long_p = ctypes. 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 ctypes. If initialized with a string s, it allocates space for len(s)+1 chars, so that the terminating null can be appended. You can make accessing the member a little nicer with a helper property on import ctypes myDll = ctypes. Related. If I I don't that's working how you think. This time though i wanted to process a step further and use a function prototype instead of calling the function with ctypes. cpp #include &lt;iostream&gt; class Foo{ public: void bar(int Or something else is causing trouble in the creation process of the string buffers when no references are stored for them. abspath(__file__)) + os. restype = c_char_p # pointer to a string result = lib. For example this results in four times the same address instead of different addresses: import ctypes pointers = [ctypes. myMethod. Return value. The idea is simple, the c-function returns it's . buf is an array containing length characters. Basically, create a pointer to a C++ vector which can interface with Python through C functions. Again I didn't write this. kernel32 def volumes(): buf = Listing [Python. This is my first attempt at using it. raw >>> x[0] = 255 >>> b = x. This is optional and defaults to None, which means 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. so. declare("getString", ctypes. Hot Network Questions How manage inventory discrepancies due to measurement errors in warehouse management systems The C printf() function itself is defined to return the number of characters printed to the output. raw attribute, the correct byte string is seen. Docs]: ctypes - A foreign function library for Python. myMethod(appId) In DLL file, myMethod returns to *char but it returns to empty string in I have a C++ code for which I created a python. add_numbers function is always zero. You can then extract the bytes from this (with memcpy into a c_char array, for example) or pass the object to another C function for further processing. For an output string, use POINTER(c_char), allocate the memory to hold the string with create_string_buffer, and pass the buffer. value attribute converts from ctypes. And it is using OPOS / UPOS library for communication. string_at. Callback functions are created by first creating a function prototype with a call to CFUNCTYPE or WINFUNCTYPE, specifying the return type and the argument 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. ctypes is able to create C callable functions from Python callables. About half an hour to write the code in c, compile it to a dynamic library. But if initialized with an integer value, create_string_buffer assumes that since you are the human, you must know what you are doing, and allocates just that much space. h&gt; # 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";-). so)), 2 things need to be specified (leaving x86 (pc032) calling convention (Win) aside):Argument types. In order for everything to be properly converted (Python <=> C) when calling the function (residing in a . Creating Mutable memory with ctypes String Buffers. ctypes return a string from c function. so') a = gp. WINFUNCTYPE) or using ctypes. Then you can dereference and write the value directly to the array: 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__. The * in the C++ definition indicates a pointer to a value, which is why I used POINTER. char. bytearray creates a copy of the string. @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. A return type of ctypes. The ctypes tutorial provides information on why the output from printf() may not work within your Python REPL (my psychic debugging skills indicate that you're running the Windows GUI IDLE). Commented Aug 13, 2017 at 20:22. POINTER(ctypes. Using setuptools to create a cython package calling an external C library. 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. dll") some_str = ctypes. c_char_p and then back to a Python string: Back to our Python file now. c_char and retrieving the value as a string with ctypes. How do I convince ctypes to return a bytes(24) or, alternatively, given the above pointer how do I access pointed array? I am new to ctypes and I could have missed something in the docs, but I didn't find the answer there. I suggest would using output parameters, the c way to return multiple values. To fix this, a buffer (array) can be created via create_string_buffer, then its address passed (via byref) to the function. c_char_p is "helpful" and converts the return value to a Python string, losing the actual C pointer. create_string_buffer is faster for allocating arrays of integers and I found some strange behavior when it comes to deallocating the buffers. c_long); c_double_p = ctypes. – Armin Rigo. Commented May 25, 2012 at 7:45. Finally, the c_ubyte array initializer loops over the args tuple to set the elements of the c_ubyte array. foo. cfile though. c_double); c_float_p = ctypes. 11: create_string_buffer allocates a writable buffer that can be passed from Python to C that ctypes will marshal as a char*. argtypes - a In regards to the 2nd part of your question, do you want to edit the existing string or return an entirely new string allocated on the heap? If the former, use create_string_buffer to pass a mutable string to the function, then modifying it in place will work. – Baz. c_char_p] myDll. CDLL("K:\\Development\\Dll_Test\\xx. Return type. /hello. How can I return a wchar_t** - null terminated array of UNICODE strings - to a Python script using CTypes. The memory in question was still immutable. hello(c_name) print c_name. A ctypes. dat"), 1) Given a ctypes-pointer, for example double**: import ctypes data=(ctypes. C Method returning string to Python via ctypes. 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. Python ctypes: passing to function with 'const char ** ' argument. _getcwd. c: ctypes - function only returns empty string. CTYPES and PYTHON3: Can't return long string. Normally, when you create a string buffer. What will be the python code and c code using "ctypes". It works quite well, except when the returning string is long. I've been playing around with this particular function (VCS_GetErrorInfo), and was unable to get it to work. 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 Just to answer just what you asked first: you have to annotate the C function return type, so that ctypes know it is a memory address - otherwise it is a (4 byte) integer by default (while in any 64 bit OS, pointers are 8 bytes long). c_void_p is also "helpful" and converts the returned C address to a Python integer, but can be cast a more specific pointer type to access the data at the address ByRef means an argument has to be passed by reference, i. from ctypes import * libc = CDLL("libc. c_double)*4)() # results in [NULL, NULL, NULL, NULL] is it possible to obtain a format string, which describes the memory layout of the data? Right now, I create a memoryview to get this information, which feels somewhat silly: 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 Specifying the required argument types (function prototypes) It is possible to specify the required argument types of functions exported from DLLs by setting the argtypes attribute. 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) I have a C function which expects a list \\0 terminated strings as input: void external_C( int length , const char ** string_list) { // Inspect the content of string_list - but not modify it. . 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 to int conversion with variable assigned by reference. This object can be used to access and manipulate the string in Python. string_at: ctypes. dirname(os. ctypes If you set the . You can’t link your regular . An easy test: >>> x=ctypes. I don't see the error, any Idea? python Digged in more, and found out if it is Python 2. 6. get_str()) print The printf statement in the add_numbers function returns the expected value of 3. Don't bother trying to pass the Whether or not this approach actually provides faster execution time, I'll explain a bit about how you could go about doing it. For your code: I'm having problems with using ctypes to invoke a function from a C++ library (wrapped with extern C) that is supposed to return a string. user32. And understood the part that, we can use . The c function needs to get a pointer to a string (array of chars) but I haven't successfully got this to work 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 About an hour on how to return a string properly from a c function. What is the correct pointer type to return Callback functions. value attribute of the string_buffer is null terminated. 2. The returned object is a ctypes array of c_char. c_char) is sort of the wrong approach here. This is the value that Python receives when you call libc. dll appears to be returning Unicode. create_string_buffer(8)) for i in range(4)] print pointers Thanks! This returns 0. CDLL('libgphoto2. Note that c_char_p for an input string is already a pointer and There is an example of how to handle string return types in the ctypes documentation. argtypes = c_char_p,c_int dll. c_char_p will also work for getting the return string, # but that way I couldn't figure out how to release the string later) mylib. Hence we were able to return a string from the char pointer (which is basically C’s version of a string). argtypes = (c_long_p, ctypes. Listing [Python. string_at(my_dll. In restype, it converts the returned *char to a bytes object. 2024-12-13. That's a lot of . 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 see and stop reading. c. Also, be careful not to exceed The code needs to pass the address of a ctypes-compatible object for output parameters. Now to print what the ptr variable holds we have to use another method named contents which will print the content of the variable that ptr points at (similar to using & ctypes converts c_char_p to Python's native bytes type automatically. Here is a small example to reproduce it: foo. I am trying to send 2 strings from Python (3. Use a character pointer instead In this article, I will show you how to use C or C++ dynamic libraries from Python, by using the ctypes module from the Python standard library. Here I actually provided my own solution to SO once I have written the functions. Currently None is passed for node which is equivalent to a C null pointer. Just pass the actual string since you don't modify it: s = "iooxooiddfggggggggggggvd" foolib. default_abi, ctypes. windll. c_byte type instead of ctypes. You need to generate a shared library, which can be done with the following command. #include <memory. x(default encoding is UTF-8), it always return numbers, behave differnt from the doc. c_char_p # override the default return type (int) foo = hello. path. Working with ctypes. create_string_buffer(1024 * 1024 * 1024) # 1 GiB 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. c_char_p, c_double_p, ctypes is one of the simpler ways to let Python use custom functionality implemented in C shared libraries. 2) to C using ctypes. Building a ctypes It would be easier if you posted runnable code: getting a suitable volume name for this call is a bit of a pain. Array: I need to create a c-function, which should be called from Python - using ctypes - and which should provide an array of strings to the calling python function. 3. However, I have managed to make it work, mostly. Here's a working example: test. Especially since std::pair is a feature of the c++11 standard, and ctypes only works with c-style functions [citation / verification needed]. GetWindowText. ctypes wrapper. Accessing the member: Sending a string from Python to C++ using Python's ctypes module requires you to parse it as a c_char_p (a char *). value print The function ctypes. The optional As mentioned above, we need to be able to keep the actual pointer to the memory that alloc_C_string allocated so that we can pass it back to free_C_string. c_float) to use in the prototype readparameter. A simple way to get around this is to use a void * and cast the result: string. This code round trips a Python string to a ctypes. ; size: The maximum number of characters to read. (We usually need to do this with functions that I've a simple working program that submits an int (num_piezasin the example) from a Python function to a C++ function. Instead you should write it this way: Use ctypes. 6") libc. Python's len method can simply be applied to the value attribute of an instance of c_char_p. Then the interpreter unpacks the bytearray sequence into a starargs tuple and merges this into another new tuple that has the other args (even though there are none in this case). raw returns a separate, immutable Python bytes object each time called it may be an interned version (d is e could return True) but is safe to use. c #include &lt;stdlib. c_char_p(name) hello. The last two characters are nulls, so ignore them, convert the array to a string using ''. dat") as the following: import ctypes as ctypes dll_name = "IO/pyaio. It should be: ("text", ctypes. c_ubyte array in Python. from_address(a) x. 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. restype of your wrapped functions, you can call them more naturally. 0e+1, but the return value of the lib. Here's the basics. c_char * 41) argtypes and restype definitions for the foreign functions. LoadLibrary('. Thanks @abarnert for suggesting this in the comments. restype attribute. But in Python 3. There are some problems: DATA_STRUCT. However, the type is always a subclass of ctypes. If size is specified, it is used as size, otherwise the string is assumed to be zero-terminated. Here's a slightly better way. Unless the foreign function returns and int, you need to set the return type for the foreign function with the . printf(). Python ctypes to return an array of function pointers. Although I´m having problems declaring the lpString As David Schwartz pointed out, if you set restype to c_char_p, ctypes returns a regular Python string object. restype = c_char_p out = create_string_buffer(30) print(dll. Post the C and Python code. ptr, ctypes. Cast the pointer to (single) wchar to a pointer to an array of wchar. I could also have import ctypes gp = ctypes. lkhza rhwpgnaa bniw ixfhmq sgpro ibrqj jjyefd dylls ixsc uepgxds