Python reads empty bytearray from character device

Question

I got a character device for some gpio on an industrial PC running under Debian.

Reading in C works pretty fine

#include <stdio.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <unistd.h>

int main (int argc, char **argv) {
    int fd;
    fd = open("/dev/bsw_gpio", O_RDWR);
    if (fd == -1) {
        printf("could not open device");
        return 1;
    }
    unsigned char val;
    int ret;
    ret = read(fd, &val, sizeof(val));
    if (ret == 0)
        printf("Value : %d\n", val);
    else
        printf("No val read\n");

    if(close(fd) != 0) {
        printf("Could not close file");
    }
    
    return 0;
}

Edit

I forgot; this gets me the status of the two io pins as value between 0 and 3 and works well.. But I need to do this in Python.

Whereas Python reacts this way

>>> import os
>>> fp = os.open("/dev/bsw_gpio", os.O_RDWR)
>>> os.read(fp, 1)
b''

or, using normal open:

>>> with open('/dev/bsw_gpio', 'r+b', buffering=0) as fp:
...     fp.read()
... 
b''

How can I fix this?

Answer 1

i found out that I need to read into a fixed size bytearray. Since I needed to convert to int after, I just used an array of 2 bytes, reversed it and have the correct value now.

class DigitalIO():
   
    def __init__(self):
        '''
        öffne die Datei
        '''
            
        self.file = open(DigitalIO.FILENAME, 'r+b', 0)
    
    def read_value(self):
        '''
        lies einen Wert
        '''
        val = bytearray(2)
        self.file.readinto(val)
        val.reverse()
        return int(val.hex(), 16)

    def write_value(self, value):
        '''
        schreibe einen Wert
        '''
        self.file.write(value.to_bytes(1, 'little'))