Read and write methods working with a timeout

[stepanoslejsek]

I recently discovered that the already implemented read and write methods do not trigger the set timeout. I went through the source code and implemented additional read and write methods with the number of bytes that need to be read or written.

Using these methods, the timeout was triggered and the exception was handled as it should be.

[mgkuhn]

You can't just redefine

function write(sp::SerialPort, nb::UInt, b::UInt8)

because the standard Base.write(io::IO, x1, xs...) Julia meaning of that method is to output the binary representation of a UInt value followed by the binary representation of a UInt8 value, and LibSerialPorts just implements that normal IO interface. Your change would cause a very nasty and undocumented API surprise to anyone trying to call e.g.

write(sp, 0x0000000000000000, 0x01)

Likewise, what does your new read method give you that e.g. Base.readbytes! doesn't?

Could you please explain first what problem you are trying to solve, with a concrete test example, because I suspect your PR is the result of a misunderstanding of how LibSerialPort extends the existing IO methods of Base, rather than tries to reimplement that interface from scratch.

[stepanoslejsek]

I am trying to solve the following problem: I have a communication between 2 devices via serial port. The 'master' sends a frame of 8 bytes and waits for a response of the 'slave' device that is 20 bytes long. However sometimes the response (all the 20 bytes) are sent but the read method (using blocking read) only read 19 and get stuck. Therefore I tried to set the read timeout which is not working.

[mgkuhn]

Doesn't read!(::IO, ::Array{UInt8}) already do exactly what you need?

Example:

using LibSerialPort
sp = LibSerialPort.open("/dev/ttyUSB0", 9600)
set_flow_control(sp)
sp_flush(sp, SP_BUF_BOTH)
timeout = 2
set_read_timeout(sp, timeout)
request = UInt8[1,2,3,4,5,6,7,8]
reply = Vector{UInt8}(undef, 20)
write(sp, request)
try
  read!(sp, reply)
catch e
    if isa(e, LibSerialPort.Timeout)
        println("Timed out after $timeout seconds")
    else
        rethrow()
    end
end
clear_read_timeout(sp)
print(reply)

However, I just realized that the docstring of read! doesn't actually describe how to use it.

If the above example solves your problem, I think improving the docstring in Base is the solution, rather than this PR.

[mgkuhn]

Your read function also causes a heap overflow: you allocate memory for one byte, and then write nb bytes into that memory. That looks totally unsafe.

[mgkuhn]

PR for improved docstring of read! in Base: JuliaLang/julia#49769

[stepanoslejsek]

I am gonna try this, thank you, I will let know if it works or not.

[stepanoslejsek]

clear_read_timeout(sp)

This exactly worked, thank you for brief explanation.

[mgkuhn]

Thanks for reporting back. I'll close this PR here, as the main improvement needed appears to be the docstring of read! in Base.