PyArg_ParseTupleAndKeywords 抛出警告：ISO C++ 禁止将字符串常量转换为‘char*’ [-Wwrite-strings]

Posted 2023-02-17

【中文标题】PyArg_ParseTupleAndKeywords 抛出警告：ISO C++ 禁止将字符串常量转换为‘char*’ [-Wwrite-strings]

【英文标题】：PyArg_ParseTupleAndKeywords throws warning: ISO C++ forbids converting a string constant to ‘char*’ [-Wwrite-strings]

【发布时间】：2019-06-10 19:07:48

【问题描述】：

搜索有关如何使用PyArg_ParseTupleAndKeywords 的示例我发现了这些问题：

How can one use PyArg_ParseTupleAndKeywords to parse a tuple with optional arguments as well as keywords? How does PyArg_ParseTupleAndKeywords work?

他们都使用static char* kwlist[] = "a", "b", NULL之类的东西

static int PyClass_init(PyClass* self, PyObject* args, PyObject* kwargs) 
    char* path;
    char* regex;
    static char* kwlist[] = "", "", NULL;

    if( !PyArg_ParseTupleAndKeywords( args, kwargs, "s|s", kwlist, &path, &regex ) ) 
        return -1;
    

    // other code ...
    return 0;

在setup.py 上使用language = "c++" 编译它并使用-std=c++11 构建它会引发此警告：

x86_64-linux-gnu-gcc -pthread -DNDEBUG -g -fwrapv -O0 -Wall -g -fstack-protector-strong -Wformat -Werror=format-security -Wdate-time -D_FORTIFY_SOURCE=2 -fPIC -Isource -I/usr/include/python3.6m -c source/test.cpp -o build/temp.linux-x86_64-3.6/source/test.o -O0 -g -ggdb -std=c++11 -fstack-protector-all
source/test.cpp: In function ‘int PyClass_init(PyClass*, PyObject*, PyObject*)’:
source/test.cpp:41:42: warning: ISO C++ forbids converting a string constant to ‘char*’ [-Wwrite-strings]
   static char* kwlist[] = "a", "b", NULL;
                                          ^

搜索此错误时，我发现此问题 Why is conversion from string constant to 'char*' valid in C but invalid in C++ 正在修复，但将修复应用为 static char* const kwlist[] = "a", "b", NULL; 会保留/升级引入错误的警告：

source/test.cpp:41:50: warning: ISO C++ forbids converting a string constant to ‘char*’ [-Wwrite-strings]
     static char* const kwlist[] = "a", "b", NULL;
                                                  ^
source/test.cpp:41:50: warning: ISO C++ forbids converting a string constant to ‘char*’ [-Wwrite-strings]
source/test.cpp:43:89: error: invalid conversion from ‘char* const*’ to ‘char**’ [-fpermissive]
     if( !PyArg_ParseTupleAndKeywords( args, kwargs, "s|s", kwlist, &path, &regex ) ) 
                                                                                         ^
In file included from /usr/include/python3.6m/Python.h:117:0,
                 from source/test.cpp:3:
/usr/include/python3.6m/modsupport.h:17:41: note:   initializing argument 4 of ‘int _PyArg_ParseTupleAndKeywords_SizeT(PyObject*, PyObject*, const char*, char**, ...)’
 #define PyArg_ParseTupleAndKeywords     _PyArg_ParseTupleAndKeywords_SizeT
                                         ^
/usr/include/python3.6m/modsupport.h:17:41: note: in definition of macro ‘PyArg_ParseTupleAndKeywords’
 #define PyArg_ParseTupleAndKeywords     _PyArg_ParseTupleAndKeywords_SizeT
                                         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
error: command 'x86_64-linux-gnu-gcc' failed with exit status 1

如何在与 Python C API 要求兼容的同时，使用与 static char* kwlist[] = "a", "b", NULL 和 C++ 11 等效的构造来消除警告？

---

更新

建议后我尝试了static const char* kwlist[] = "a", "b", NULL，但PyArg_ParseTupleAndKeywords 不接受：

source/test.cpp:43:89: error: invalid conversion from ‘const char**’ to ‘char**’ [-fpermissive]
     if( !PyArg_ParseTupleAndKeywords( args, kwargs, "s|s", kwlist, &filepath, &rawregex ) ) 
                                                                                         ^
In file included from /usr/include/python3.6m/Python.h:117:0,
                 from source/test.cpp:3:
/usr/include/python3.6m/modsupport.h:17:41: note:   initializing argument 4 of ‘int _PyArg_ParseTupleAndKeywords_SizeT(PyObject*, PyObject*, const char*, char**, ...)’
 #define PyArg_ParseTupleAndKeywords     _PyArg_ParseTupleAndKeywords_SizeT
                                         ^
/usr/include/python3.6m/modsupport.h:17:41: note: in definition of macro ‘PyArg_ParseTupleAndKeywords’
 #define PyArg_ParseTupleAndKeywords     _PyArg_ParseTupleAndKeywords_SizeT
                                         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
error: command 'x86_64-linux-gnu-gcc' failed with exit status 1

【问题讨论】：

err，你试过static const char* kwlist[] = "", "", NULL; 吗？

是的。它抛出error: invalid conversion from ‘const char**’ to ‘char**’ 因为PyArg_ParseTupleAndKeywords 需要char **

好的，看我更新的答案。

【参考方案1】：

您从中复制的示例代码可能是 C，其中没有认真考虑警告（gcc 报告此类代码没有错误，g++ 即使没有警告选项也会弹出警告）。

这里的解决方法是像这样添加const 关键字：

static const char* kwlist[] = "a", "b", NULL;

您尝试的 const 关键字不是应用于值，而是应用于指针。

然后让函数接受char ** 使用const_cast：

const_cast<char **>(kwlist)

像这样：

PyArg_ParseTupleAndKeywords( args, kwargs, "s|s", const_cast<char **>(kwlist), &path, &regex ) ) 

函数不会通过隐式“契约”来改变值（因为它是 python API，它不会骗你），所以在这种情况下，转换为 const 就可以了）

PS：this other answer 给出的替代方案也可以。请注意，使用空的可变字符串并不比强制转换为常量更安全，就好像被调用函数决定写入空字符串一样，它也会调用未定义的行为。

【参考方案2】：

使用可变空字符串：

char empty[] = "";
static char* kwlist[] = empty, empty, NULL;

【讨论】：

这会导致很难重现的非常奇怪的错误。下面试试。函数 foo 是 c++ 函数，它接受两个 python 关键字参数并且什么都不做。 import hello def call_foo(): hello.foo(a=1, b=2) hello.foo(a=1, b=2) # Success call_foo() # Fail