python:ctypes，argtypes会自动调用构造函数吗？

Question

我有代码，但我不知道为什么它工作。你知道吗

str2payload = lib3.str2payload
str2payload.argtypes = [POINTER(mbuf_t), POINTER(c_char)]

# sbuf = ..
val = b"asdfasdfasdhfjkasgdfjasgdfhjasgdfjhasgdfjhasgdfgjh"
rmr_str2payload(sbuf, val)

这和预期的一样。在这里，val只是python中的一个bytes对象。但是，函数签名表示它应该是POINTER(c_char)。那么什么是魔法转换呢？你知道吗

顺便说一句，我之所以使用POINTER(c_char)，是因为它在这里说：https://docs.python.org/3.7/library/ctypes.html#ctypes.c_char_p

Answer 1

从ctypes documentation：

When a foreign function is called, each actual argument is passed to the from_param() class method of the items in the argtypes tuple, this method allows adapting the actual argument to an object that the foreign function accepts. For example, a c_char_p item in the argtypes tuple will convert a unicode string passed as argument into a byte string using ctypes conversion rules.

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). This allows defining adapters that can adapt custom objects as function parameters.