How can I replace "cat" and "echo" read/write command with python/c program?

Question

I have trouble replacing device in/output commands like:

echo 100 > /dev/rtmotor_raw_l0   # output 100hz frequency

cat /dev/rtswitch0 # read switch state

output problem(python)

I tried replacing that command with python.

file = open('/dev/rtmotor_raw_l0','w')
file.write('100\n')            # I want output in this timing
file.close()                   # output reflected after closing file

The problem is that the output appears after closing the file. Does this mean I have to open and close this device each time I want to change its value? Also, changing 'w' to 'a' did not work.

input problem(python)

Almost the same problem happens in input observation.

file = open('/dev/rtswitch','r')
file.read()  # works
file.read()  # after first read it does't work anymore
file.close() # need to reopen the file to get newer value

I could only read 1 input in each opening files.

So, currently I have to reopen the devices each time I want to write/read new values. Are there any way to avoid this problem?

Thank you.

After the first read, the file pointer is at the end of the file; assuming '/dev/rtswitch is seekable, you need to rewind the file pointer to the beginning of the file before you can read the contents again: put file.seek(0) between the two reads. — chepner
– chepner, Commented Feb 1, 2019 at 14:53

Matej · Accepted Answer · 2019-02-01 12:44:48Z

1

Try call file.flush() after write() call. It will flush buffer to file and you can read data.

answered Feb 1, 2019 at 12:44

Matej

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