implicit declaration of function 'PyString_AS_STRING' is invalid

[Noon Chen]

Hi,

I ran into this error when I was trying to build my code, and here is the example code that can reproduce it (python 3.8 + cython 3.0a6):

```

# distutils: language = c

# cython: language_level=3

from cpython.string cimport PyString_AS_STRING

cdef void test(char* name):

    pass

test(PyString_AS_STRING(f'testString'))

```

Error:

```

test.c:1898:22: error: implicit declaration of function 'PyString_AS_STRING' is invalid in C99 [-Werror,-Wimplicit-function-declaration]

  __pyx_f_4test_test(PyString_AS_STRING(__pyx_n_u_testString));

                     ^

test.c:1898:22: warning: incompatible integer to pointer conversion passing 'int' to parameter of type 'char *' [-Wint-conversion]

  __pyx_f_4test_test(PyString_AS_STRING(__pyx_n_u_testString));

                     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

test.c:1468:52: note: passing argument to parameter '__pyx_v_name' here

static void __pyx_f_4test_test(CYTHON_UNUSED char *__pyx_v_name) {

                                                   ^

1 warning and 1 error generated.

error: command 'clang' failed with exit status 1

```

I did search for this error in the group and google, it should be related to the import issue, but I found the `PyString_AS_STRING` in `include/cpython/string.pxd`, what did I do wrong here?

[Stefan Behnel]

You are trying to use a Py2 function in Py3.

[Stefan Behnel]

Read the documentation page on strings. You may need to encode your Unicode string. Or you may have to do something entirely different, depending on what your actual problem is.

[Noon Chen]

Seems that I can rewrite the code as follow:

```

cdef void test(char* name):

    pass

test(f'testString'.encode("utf-8"))

```

And cython would convert the code to:

```

#define __Pyx_PyBytes_AsWritableString(s) ((char*) PyBytes_AS_STRING(s))

__pyx_t_2 = __Pyx_PyBytes_AsWritableString(__pyx_n_b_testString); if (unlikely((!__pyx_t_2) && PyErr_Occurred())) __PYX_ERR(0, 205, __pyx_L1_error)

__pyx_f_4test_test(__pyx_t_2);

```
Is the performance comparable to the 1st approach?

Thanks in advance!

[Noon Chen]

Hi Stefan,

Thanks for your reply, I didn't see any version info in cpython/string.pxd. 

I'm simply want to convert python strings to char*, I used to do this:

```

cdef str py_string = "xxxxx"

cdef bytes py_byte = py_string.encode("utf-8")

cdef char* cstring = py_byte

```

However, it was quite cumbersome, and I found `PyString_AS_STRING` in cython built-in package that perfectly fits my needs.

Does it mean I can only use the 1st option? Is there any py3 alternative of `PyString_AS_STRING`?

Best Regards,

NC

On Sunday, April 25, 2021 at 6:36:35 PM UTC+8 Stefan Behnel wrote:

[Stefan Behnel]

Am 25. April 2021 15:23:25 MESZ schrieb Noon Chen:
>Seems that I can rewrite the code as follow:
>
>cdef void test(char* name):
> pass
>
>test(f'testString'.encode("utf-8"))

Note that you are not testing a real world situation here. Cython can see that the result of the encoding is a constant bytes object and uses that instead.

Read the Cython documentation on string encoding. It explains what you have to do and why some things don't work.

Stefan