Using a C library with Python

I'm writing a Python program under linux to interact with files written in an old format (the 'squishDB' format, where messages from the old Fidonet network were stored in the years from 1980 to 2000).

To interact with these files, I have so far used programs in C and in C++ they use the libsmapi.so library and with these languages the library works very well.

Now, as part of a larger project all written in Python, I find myself forced to use the same library helping me with cstruct and ctypes, which I had never used before.

I tried to write this simple program, which opens the library and gets a pointer, to then be able to call the individual functions.

import ctypes
import cstruct


class minf(cstruct.MemCStruct):
    __def__ = """
        struct _minf
        {
            short req_version;
            short def_zone;
            short haveshare;  /* filled in by msgapi routines - no need to set this */
        
            /* Version 2 Information */
            short smapi_version;
            short smapi_subversion;
        };
    """


def main():
    smapi = ctypes.CDLL('libsmapi.so')
    m = minf()

    m.def_zone = 2
    m.req_version = 0

    if smapi.MsgOpenApi(m):
        print("Error opening library")
        exit(1)


if __name__ == '__main__':
    main()

The problem is that, despite having defined the minf() structure with cstruct by copying it verbatim from the C file, this strange error appears when I pass it to the library call.

Traceback (most recent call last):
  File "/home/neo/progetti/PycharmProjects/smapitest/main.py", line 33, in <module>
    main()
  File "/home/neo/progetti/PycharmProjects/smapitest/main.py", line 27, in main
    if smapi.MsgOpenApi(m):
       ^^^^^^^^^^^^^^^^^^^
ctypes.ArgumentError: argument 1: KeyError: '_as_parameter_'

Process finished with exit code 1

Anyone can help me?
Thanks.

Regards

cstruct isn't compatible with ctypes. Use ctypes.Structure instead:

import ctypes as ct


class minf(ct.Structure):
    _fields_ = (('req_version', ct.c_short),
                ('def_zone', ct.c_short),
                ('haveshare', ct.c_short),
                ('smapi_version', ct.c_short),
                ('smapi_subversion', ct.c_short))

    # Provides a way for a class to display itself (optional)
    def __repr__(self):
        return (f'minf(req_version={self.req_version}, '
                f'def_zone={self.def_zone}, '
                f'haveshare={self.haveshare}, '
                f'smapi_version={self.smapi_version}, '
                f'smapi_subversion={self.smapi_subversion})')

smapi = ct.CDLL('libsmapi.so')
# Recommend defining argument types and return type.
# Don't assume ctypes knows how to marshal your parameters.
# Defining this allows ctypes to check the types of parameters passed are compatible.
#
# This assumes a C prototype of "int MsgOpenApi(struct minf*)"
smapi.MsgOpenApi.argtypes = ct.POINTER(minf),
smapi.MsgOpenApi.restype = ct.c_int

# Alternative way to initialize.
# minf() would initialize all parameters to zero.
# Provided parameters are assigned in the same order as _fields_.
m = minf(0, 2)
print(m)  # Calls m.__repr__() if m.__str__ isn't defined.
# Output:
# minf(req_version=0, def_zone=2, haveshare=0, smapi_version=0, smapi_subversion=0)

if smapi.MsgOpenApi(ct.byref(m)):  # Pass by reference
    print("Error opening library")