[转] python关于ctypes使用char指针与bytes相互转换的问题
最近研究人脸识别，需要用python调用so动态库，涉及到c/c++中的指针字符串转Python的bytes对象的问题。
按照ctypes的文档，直观方式是先创建对应的类型数组，再将指针取地址一一赋值：
from ctypes import *
p=(c_char * 10)()
for i in range(10):
p[i] = i
b=bytes(bytearray(p))
print(b) |
from ctypes import * p=(c_char * 10)() for i in range(10): p[i] = i b=bytes(bytearray(p)) print(b)
搜寻了各种资料，都未能找到更好的。。。直到ctypes.string_at
_string_at = PYFUNCTYPE(py_object, c_void_p, c_int)(_string_at_addr)
def string_at(ptr, size=-1):
"""string_at(addr[, size]) -> string
Return the string at addr."""
return _string_at(ptr, size) |
_string_at = PYFUNCTYPE(py_object, c_void_p, c_int)(_string_at_addr) def string_at(ptr, size=-1): """string_at(addr[, size]) -> string Return the string at addr.""" return _string_at(ptr, size)
于是char*转bytes可以直接用string_at方法，传入指针地址，以及字符串长度即可。
同样的问题，bytes对象需要传给c/c++代码。。。
直观方式同样是创建char数组array，拷贝bytes之后，再用cast强制转换成c_char_p
from ctypes import *
p=(c_char * 10)()
for i in range(10):
p[i] = i
m=cast(p, c_char_p)
print(m) |
from ctypes import * p=(c_char * 10)() for i in range(10): p[i] = i m=cast(p, c_char_p) print(m)
比较奇葩的是cast得到的对象，如果我们直接用bytes对象cast。。。
from ctypes import * b=b‘0123456789‘ m=cast(p, c_char_p) print(m) |
from ctypes import * b=b‘0123456789‘ m=cast(p, c_char_p) print(m)
吼吼，奇迹出现了，bytes对象cast成了char*指针。。。用string_at转换看看
string_at(m) |
string_at(m)
总结一下：
1、bytes基于Buffer Protocol，查看其c实现https://hg.python.org/cpython/file/3.4/Objects/bytesobject.c
2、string_as的c代码https://hg.python.org/cpython/file/3717b1481d1b/Modules/_ctypes/_ctypes.c
static PyObject *
string_at(const char *ptr, int size)
{
if (size == -1)
return PyString_FromString(ptr);
return PyString_FromStringAndSize(ptr, size);
} |
static PyObject * string_at(const char *ptr, int size) { if (size == -1) return PyString_FromString(ptr); return PyString_FromStringAndSize(ptr, size); }
3、cast的c代码同样在_ctypes.c(https://hg.python.org/cpython/file/3717b1481d1b/Modules/_ctypes/_ctypes.c)
static PyObject *
cast(void *ptr, PyObject *src, PyObject *ctype)
{
CDataObject *result;
if (0 == cast_check_pointertype(ctype))
return NULL;
result = (CDataObject *)PyObject_CallFunctionObjArgs(ctype, NULL);
if (result == NULL)
return NULL;
/*
The casted objects ‘_objects‘ member:
It must certainly contain the source objects one.
It must contain the source object itself.
*/
if (CDataObject_Check(src)) {
CDataObject *obj = (CDataObject *)src;
/* CData_GetContainer will initialize src.b_objects, we need
this so it can be shared */
CData_GetContainer(obj);
/* But we need a dictionary! */
if (obj->b_objects == Py_None) {
Py_DECREF(Py_None);
obj->b_objects = PyDict_New();
if (obj->b_objects == NULL)
goto failed;
}
Py_XINCREF(obj->b_objects);
result->b_objects = obj->b_objects;
if (result->b_objects && PyDict_Check(result->b_objects)) {
PyObject *index;
int rc;
index = PyLong_FromVoidPtr((void *)src);
if (index == NULL)
goto failed;
rc = PyDict_SetItem(result->b_objects, index, src);
Py_DECREF(index);
if (rc == -1)
goto failed;
}
}
/* Should we assert that result is a pointer type? */
memcpy(result->b_ptr, &ptr, sizeof(void *));
return (PyObject *)result;
failed:
Py_DECREF(result);
return NULL;
} |
static PyObject * cast(void *ptr, PyObject *src, PyObject *ctype) { CDataObject *result; if (0 == cast_check_pointertype(ctype)) return NULL; result = (CDataObject *)PyObject_CallFunctionObjArgs(ctype, NULL); if (result == NULL) return NULL; /* The casted objects ‘_objects‘ member: It must certainly contain the source objects one. It must contain the source object itself. */ if (CDataObject_Check(src)) { CDataObject *obj = (CDataObject *)src; /* CData_GetContainer will initialize src.b_objects, we need this so it can be shared */ CData_GetContainer(obj); /* But we need a dictionary! */ if (obj->b_objects == Py_None) { Py_DECREF(Py_None); obj->b_objects = PyDict_New(); if (obj->b_objects == NULL) goto failed; } Py_XINCREF(obj->b_objects); result->b_objects = obj->b_objects; if (result->b_objects && PyDict_Check(result->b_objects)) { PyObject *index; int rc; index = PyLong_FromVoidPtr((void *)src); if (index == NULL) goto failed; rc = PyDict_SetItem(result->b_objects, index, src); Py_DECREF(index); if (rc == -1) goto failed; } } /* Should we assert that result is a pointer type? */ memcpy(result->b_ptr, &ptr, sizeof(void *)); return (PyObject *)result; failed: Py_DECREF(result); return NULL; }