[英]python: ctypes, read POINTER(c_char) in python
I have a ctypes field that is a POINTER(c_char)
(it had to be, per the documentation, c_char_p didn't work for my application:https://docs.python.org/3.7/library/ctypes.html#ctypes.c_char_p )我有一个 ctypes 字段,它是一个POINTER(c_char)
(它必须是,根据文档, c_char_p 对我的应用程序不起作用:https ://docs.python.org/3.7/library/ctypes.html#ctypes.c_char_p )
For a general character pointer that may also point to binary data, POINTER(c_char) must be used.对于也可能指向二进制数据的通用字符指针,必须使用 POINTER(c_char)。
However, this usage recommended by ctypes itself seems to have the downside that, it claims to be a pointer to a single character, however, it isn't, it's a pointer to an array of bytes.然而,ctypes 本身推荐的这种用法似乎有一个缺点,它声称是一个指向单个字符的指针,然而,它不是,它是一个指向字节数组的指针。
How can I read the array returned by the ctypes fucntion (I know the length
) in Python?如何在 Python 中读取 ctypes 函数返回的数组(我知道length
)? Trying to index it like foo[0:len]
where foo
is a POINTER(c_char)
blows up with TypeError: 'c_char' object is not subscriptable
尝试像foo[0:len]
那样索引它,其中foo
是一个POINTER(c_char)
因TypeError: 'c_char' object is not subscriptable
I can print the first character of the bytestring using either print(foo)
or print(foo[0])
我可以使用print(foo)
或print(foo[0])
字节串的第一个字符
I was thinking that ctypes.cast
might work, however I don't know how to pass it the length of the cast (as in interpret the first N bytes from address foo as a bytes
object)我在想ctypes.cast
可能会起作用,但是我不知道如何将转换的长度传递给它(如将地址 foo 的前 N 个字节解释为bytes
对象)
EDIT: some code.编辑:一些代码。
So I have a structure:所以我有一个结构:
class foo(Structure):
_fields_ = [("state", c_int),
("type", c_int),
("len", c_int),
("payload", POINTER(c_char))] # according to th following the python bytes are already unsinged https://bytes.com/topic/python/answers/695078-ctypes-unsigned-char
And I have another function that returns a POINTER(foo)
我还有另一个返回POINTER(foo)
函数
lib3 = CDLL(....so)
f = lib3.f
f.restype = POINTER(foo)
I call f
, which returns a POINTER(foo)
:我调用f
,它返回一个POINTER(foo)
:
ptrf = f(....)
And then I was trying to access ptrf.payload
.然后我试图访问ptrf.payload
。 The following code works:以下代码有效:
def get_payload(ptr_to_foo):
val = cast(ptr_to_foo.contents.payload, c_char_p).value
return val[:ptr_to_foo.contents.len]
So I do所以我做
ptrf = f(....)
get_payload(ptrf)
I was wondering whether the get_payload
function would be written more easily.我想知道get_payload
函数是否会更容易编写。
As [Python.Docs]: ctypes - A foreign function library for Python states, you must not use c_char_p with binary data.作为[Python.Docs]: ctypes - Python状态的外部函数库,您不得将c_char_p与二进制数据一起使用。
Of course that can be ignored, but then surprises (string silently truncated) may occur.当然可以忽略,但可能会出现意外(字符串被静默截断)。
Although it could be exemplified in ~5 lines of code, pasting the whole thing:虽然可以用大约 5行代码来举例说明,但粘贴整个内容:
dll.c : dll.c :
#include <stdlib.h>
#if defined(_WIN32)
# define DLL_EXPORT __declspec(dllexport)
#else
# define DLL_EXPORT
#endif
#define LEN 5
typedef struct CharPtrWrapperTag {
int len;
char *data;
} CharPtrWrapper;
DLL_EXPORT CharPtrWrapper *get() {
CharPtrWrapper *ret = malloc(sizeof(CharPtrWrapper));
ret->len = LEN;
ret->data = malloc(LEN * sizeof(char));
ret->data[0] = 'A';
ret->data[1] = 'B';
ret->data[2] = 0;
ret->data[3] = 'C';
ret->data[4] = 'D';
return ret;
}
DLL_EXPORT void release(CharPtrWrapper *pWrap) {
if (pWrap) {
free(pWrap->data);
pWrap->data = NULL;
pWrap->len = 0;
free(pWrap);
}
}
code.py :代码.py :
#!/usr/bin/env python3
import sys
import ctypes
DLL_NAME = "./dll.dll"
CharPtr = ctypes.POINTER(ctypes.c_char)
class CharPtrWrapper(ctypes.Structure):
_fields_ = [
("len", ctypes.c_int),
("data", CharPtr),
]
CharPtrWrapperPtr = ctypes.POINTER(CharPtrWrapper)
def main():
dll = ctypes.CDLL(DLL_NAME)
get = dll.get
get.restype = CharPtrWrapperPtr
release = dll.release
release.argtypes = [CharPtrWrapperPtr]
wrap_ptr = get()
wrap = wrap_ptr.contents
print("{:}\n Len: {:d}".format(wrap, wrap.len))
for idx in range(wrap.len):
print(" {:d}: {:}".format(idx, wrap.data[idx]))
s = ctypes.cast(wrap.data, ctypes.c_char_p).value[:wrap.len]
print("\nctypes.c_char_p cast: {:}".format(s))
CharArr = ctypes.c_char * wrap.len
char_arr = CharArr(*wrap.data[:wrap.len])
print("CharArr: {:}".format(char_arr.raw))
release(wrap_ptr)
print("\nDone.")
if __name__ == "__main__":
print("Python {:s} on {:s}\n".format(sys.version, sys.platform))
main()
Output :输出:
[cfati@CFATI-5510-0:e:\\Work\\Dev\\StackOverflow\\q055103298]> sopr.bat *** Set shorter prompt to better fit when pasted in StackOverflow (or other) pages *** [prompt]> "c:\\Install\\x86\\Microsoft\\Visual Studio Community\\2015\\vc\\vcvarsall.bat" x64 [prompt]> dir /b code.py dll.c [prompt]> cl /nologo /DDLL /MD dll.c /link /NOLOGO /DLL /OUT:dll.dll dll.c Creating library dll.lib and object dll.exp [prompt]> dir /b code.py dll.c dll.dll dll.exp dll.lib dll.obj [prompt]> "e:\\Work\\Dev\\VEnvs\\py_064_03.06.08_test0\\Scripts\\python.exe" code.py Python 3.6.8 (tags/v3.6.8:3c6b436a57, Dec 24 2018, 00:16:47) [MSC v.1916 64 bit (AMD64)] on win32 <__main__.CharPtrWrapper object at 0x000001279250D248> Len: 5 0: b'A' 1: b'B' 2: b'\\x00' 3: b'C' 4: b'D' ctypes.c_char_p cast: b'AB' CharArr: b'AB\\x00CD' Done.
If you truly have a POINTER(c_char)
type, it is subscriptable.如果你真的有一个POINTER(c_char)
类型,它是可下标的。 In the future provide code that reproduces your issue:将来提供重现您的问题的代码:
>>> p = cast(create_string_buffer(b'Hello, world!'),POINTER(c_char))
>>> p
<ctypes.LP_c_char object at 0x000001C2F6B58848>
>>> p[0]
b'H'
>>> p[:14]
b'Hello, world!\x00'
>>> cast(p,c_char_p).value # only if known to be nul-terminated
b'Hello, world!'
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