如何将 python 字符串写入 python 中的 C 结构 (char *) 成员？

Question

我有一个从 C 传递到 python 的 C 结构，我想更改 python 中的 (char *) 成员的内容。

little.c

#include <Python.h>
typedef struct little littleStruct;
struct little{

        char *memberStr;
        PyObject *(*callFunc)(littleStruct *s, char *funcName, PyObject *paraList);
};

littleStruct *createLittle();
void init_little(littleStruct *s);
PyObject *little_callFunc(littleStruct *s, char *funcName, PyObject *paraList);

littleStruct *createLittle(){
    littleStruct *little = malloc(sizeof(littleStruct));
    init_little(little);
    return little;
}
void init_little(littleStruct *s){
    s->memberStr = malloc(sizeof(char)*128);
    memset(s->memberStr, 0, 128);

    s->callFunc = little_callFunc;
}

PyObject *little_callFunc(littleStruct *s, char *funcName, PyObject *paraList){
    PyGILState_STATE st = PyGILState_Ensure();

    PyObject *module, *moduleDict, *moduleFunc, *moduleFuncArgs, *moduleFuncRet;

    // skipped error checking in this little example

    module = PyImport_ImportModule(funcName);
    moduleDict = PyModule_GetDict(module);
    moduleFunc = PyDict_GetItemString(moduleDict, funcName);

    // set arguments for littleOperation.littleOperation
    moduleFuncArgs = PyTuple_New(2);
    PyTuple_SetItem(moduleFuncArgs, 0, PyLong_FromVoidPtr(s));
    PyTuple_SetItem(moduleFuncArgs, 1, paraList);

    printf("==== In C, before call func: %s, address(memberStr): %p, memberStr: %s\n", funcName, s->memberStr, s->memberStr);
    moduleFuncRet = PyObject_CallObject(moduleFunc, moduleFuncArgs);
    printf("==== In C, after call func: %s, address(memberStr): %p, memberStr: %s\n", funcName, s->memberStr, s->memberStr);
    
    PyGILState_Release(st);
    return moduleFuncRet;
}

编译成.so文件

gcc -g -fPIC -c little.c -I/usr/include/python2.7 -L/usr/lib/python2.7 -lpython2.7
gcc -shared little.o -o little.so
mv little.so /usr/lib

创建结构并在little.py

中调用结构成员函数callFunc

little.py

from ctypes import *

class littleStruct(Structure):
        pass
littleStruct._fields_ = [
            ("memberStr", c_char_p),
            ("callFunc", CFUNCTYPE(py_object, POINTER(littleStruct), c_char_p, py_object))
        ]

def main():
    try:
        littleDll = PyDLL("little.so")
        littleDll.createLittle.restype = POINTER(littleStruct)
        little = littleDll.createLittle()

        paraList = {"para0": 0, "para1": "test"}
        ret = little.contents.callFunc(little, "littleOperation", paraList)
    except Exception, e:
        print("%s"%str(e))

if __name__ == "__main__":
    main()

littleOperation.py

from little import littleStruct
from ctypes import memset
def littleOperation(little_pointer, paraList):
    try:
        little = littleStruct.from_address(little_pointer)

        # set little.memberStr
        # little.memberStr = paraList["para1"]
        for i in range(0, len(paraList["para1"])):
            memset(id(little.memberStr)+i, ord(paraList["para1"][i]), 1)
        print("After set little.memberStr in littleOperation.py")
        return {"status": 0}
    except Exception, e:
        print("%s"%str(e))
        return {"status": -1}

如果我直接将python字符串赋值给C struct(char*)成员，little.memberStr = paraList["para1"]，地址将会改变。

==== In C, before call func: littleOperation, address(memberStr): 0x226a200, memberStr:
After set little.memberStr in littleOperation.py
==== In C, after call func: littleOperation, address(memberStr): 0x7fb9db196ec4, memberStr: test

我试了ctypes.memset，地址还是一样，但是(char *)成员一点都没变。

==== In C, before call func: littleOperation, address(memberStr): 0x1d97200, memberStr:
After set little.memberStr in littleOperation.py
==== In C, after call func: littleOperation, address(memberStr): 0x1d97200, memberStr:

为什么 ctypes.memset 不起作用？

Answer 1

由于 ctypes 对 c_char_p 类型的特殊处理，它 returns 一个 Python 访问时的字节字符串对象。您的 memset/id 代码正在修改表示该不可变字节字符串对象的 PyObject 结构的前几个字节。

相反，将 littleStruct._fields_ 中的 memberStr 类型更改为 POINTER(c_char) 以保留对 C 指针的访问。

然后，在 littleOperation.py 中，您可以将 POINTER(c_char) 转换为 POINTER(c_char * <size>，其中 <size> 是所需的写入大小或缓冲区的已知大小。然后你可以通过 .contents 取消引用指针来访问一个固定大小的数组，该数组可以直接通过 .value:

分配一个字节字符串

p = cast(little.memberStr,POINTER(c_char * 128))
p.contents.value = paraList['para1']

请参阅 another answer of mine，其中有一个更小的独立示例。

如何将 python 字符串写入 python 中的 C 结构 (char *) 成员？

How to write a python string to a C struct's (char *) member in python?

c

python-2.7

ctypes

python-c-api