ctypes Fixed-size char Array in Structure Field and Initialization

Question

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.Structure):
    _fields_ = [("a", ctypes.c_char * 32)]

a = (ctypes.c_char * 32)(*b"Hi!")
mystruct = MyStructure(a=a)

This gives me an error:

Traceback (most recent call last):
  File "...", line ..., in <module>
    mystruct = MyStructure(a=a)
TypeError: expected bytes, c_char_Array_32 found

Additional info: this is a MWE of a C++ DLL that has a structure with fixed-size character arrays that act as strings (e.g., names of things).

Mark Tolonen · Accepted Answer · 2019-01-19 06:15:49Z

2

Pass a byte string as the error message suggests. ctypes will complain if it is too long:

import ctypes

class MyStructure(ctypes.Structure):
    _fields_ = [("a", ctypes.c_char * 32)]

mystruct = MyStructure(b'Hi')
print(mystruct.a)
mystruct = MyStructure(b'Hi'*16)
print(mystruct.a)
mystruct = MyStructure(b'Hi'*17)
print(mystruct.a)

Result:

b'Hi'
b'HiHiHiHiHiHiHiHiHiHiHiHiHiHiHiHi'
Traceback (most recent call last):
  File "D:\dev\Python36\Lib\site-packages\pythonwin\pywin\framework\scriptutils.py", line 326, in RunScript
    exec(codeObject, __main__.__dict__)
  File "C:\Users\metolone\Desktop\x.py", line 10, in <module>
    mystruct = MyStructure(b'Hi'*17)
ValueError: bytes too long (34, maximum length 32)

answered Jan 19, 2019 at 6:15

Mark Tolonen

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2 Comments

Bruno Over a year ago

Funny... I thought I had to initialize variables with their respective ctypes or ctypes.wintypes constructors. For example, there's a ctypes.int field, and it works if I either initialize it as b = 123 or b = ctypes.int(123).

Mark Tolonen Over a year ago

Since type is declared in the structure (or .argtypes, for functions) Python knows the type and constructs it for you.

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