Serial Comm shared library sending wrong data when used with ctypes

Question

I have a built a simple serial communication shared library which I am trying to use in python using the ctypes library. The steps that I am following are:

1. Load the .so file by calling ctypes.cdll.loadlibrary().
2. Initialize the serial communication module.
3. Send 0xFF as sync bytes.

When I do the above steps I am not getting only junk data at the other end. Funnily when I use the .so file in C and do the same operation it works absolutely fine. So My question is does the ctypes module manipulate the loaded library in any way? I am quite new to using C in Python and I am drawing blank here. Any suggestions would be very helpful.

#!/usr/bin/env python3
import ctypes
test_lib = ctypes.cdll.LoadLibrary("./demo_static.so")
string2 = "/dev/ttyS2"
# create byte objects from the strings
UART_RIGHT = string2.encode('utf-8')
baud = 500000
test_lib.serial_com_init(0, UART_RIGHT, baud)

The (relevant) C code:

int serial_com_init(char *left, char *right, int baudrate) {
    int fd_l, fd_r;
    uart_t uart_left, uart_right;
    uint8_t flags_l, flags_r;
    if (left) {
        fd_l = uart_init_linux(left, baudrate);
        uart_left->fd = fd_l;
    }
    if (right) {
        fd_r = uart_init_linux(right, baudrate);
        uart_right->fd = fd_r;
    }
    serial_com_init_cr(uart_left, uart_right, flags_l, flags_r); 
    serial_com_hello_init();
    return 0;
}

Answer 1

According to [Python.Docs]: ctypes - Specifying the required argument types (function prototypes), for a function like:

int serial_com_init(char *left, char *right, int baudrate);

you need to specify argument types (before the function call). This makes a crucial difference on 064bit Python. Check [SO]: C function called from Python via ctypes returns incorrect value (@CristiFati's answer) for more details:

test_lib.serial_com_init.argtypes = (ctypes.c_char_p, ctypes.c_char_p, ctypes.c_int)
test_lib.serial_com_init.restype = ctypes.c_int
init_result = test_lib.serial_com_init(None, UART_RIGHT, baud)

Notes:

• This is one of the possible causes

• Always check functions return codes (well, here it will always be 0, and this might hide errors in the C code)

• You can use bytes literals directly: UART_RIGHT = b"/dev/ttyS2"