CODEWITHSUNDEEP
pythoncenumspython-cffi
Peque
Peque

Reputation: 14831

How to work with Python's cffi and enum arrays

I have a function implemented in C that translates a string to an array of a defined enum. The C code looks like this:

Header file (describes the enum):

/* File `asdf.h` */
enum translation {
        ONE,
        ANOTHER,
        FINISH,
};

void translate(char *origin, enum translation *destination);

The source file (implements the function that makes the translation):

/* File `asdf.c` */
#include "asdf.h"


void translate(char *origin, enum translation *destination)
{
    while (1) {
        if (*origin == 'F')
            break;
        if (*origin == 'O')
            *destination++ = ONE;
        if (*origin == 'A')
            *destination++ = ANOTHER;
        origin++;
    }
    *destination = FINISH;
}

Now I would like to be able to call this function from Python, using CFFI, and being able to check the correct values of the output array (translation). How could I do that?

I have tried to compile the module first and then create the variables and call the function this way, but did not manage to make it work:

# File `cffienum.py`
from pathlib import Path
from cffi import FFI


ffibuilder = FFI()
ffibuilder.cdef(Path('asdf.h').read_text())
ffibuilder.set_source(
    '_asdf',  # name of the output C extension
    '#include "asdf.h"',
    sources=['asdf.c'])   # includes asdf.c as additional sources
ffibuilder.compile()


if __name__ == '__main__':
    from _asdf.lib import translate

    ffi = FFI()
    ffi.cdef(Path('asdf.h').read_text())
    c = ffi.dlopen('c')
    source = ffi.new('char source[]', b'OAF')
    destination = ffi.new('enum translation destination[10]')

    translate(source, destination)
    print(source, destination)

This is the error I get:

$ pipenv run python cffienum.py 
Traceback (most recent call last):
  File "cffienum.py", line 24, in <module>
    translate(source, destination)
TypeError: initializer for ctype 'enum translation *' must be a pointer to same type, not cdata 'enum translation[10]'

First contact with CFFI and not an experienced C programmer, so I bet I am doing many things wrong here.

Update

Note that if instead of:

void translate(char *origin, enum translation *destination)

I use:

void translate(char *origin, char *destination)

Or:

void translate(char *origin, uint8_t *destination)

It works just fine. So there is something I must be missing when using enum translation *...

Upvotes: 1

Views: 504

Answers (1)

Armin Rigo
Armin Rigo

Reputation: 12955

You must use the ffi object from the _adsl module:

from _asdf import ffi

instead of making a new FFI instance and redefining an unrelated type enum translation that happens to be identical.

Upvotes: 2

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