Re: Help fixing NativeMessaging host: read 32-bit message length in native byte order

[guest271314]

Thanks. Can you explain why the double quoting of assigned variables is
impotant?

On Sat, Jun 24, 2023 at 3:59 PM dan b <tekvax@hotmail.com> wrote:

> “Important safety tip #427” :)
>
> Remember to always double quote around assigned variables.
> The life you save maybe your own…
> I was just caught by this in a while done read csv file variable, because
> I failed to double quote the data=“$line” assignment, the string contents
> were getting changed in strange ways.
>
> length=“$(head -q -z --bytes=4 -| od -An -td4 -)“
>
> message=“$(head -q -z --bytes=$((length)) -)
>  sendMessage "$message"
> }“
>
>  Cheers,
> Dan
>
> --
> t e k v a x @ hotmail.com
> sent via mobile.
> +19059205138
>
> On Jun 24, 2023, at 18:32, guest271314 <guest271314@gmail.com> wrote:
>
> I finally figured this out with you folks' help.
>
> I substituted using head for read.
>
> Thank you so much!
>
> #!/bin/bash
> set -x
> #!/bin/bash
> getMessage() {
>  length=$(head -q -z --bytes=4 -| od -An -td4 -)
>  message=$(head -q -z --bytes=$((length)) -)
>  sendMessage "$message"
> }
> # https://stackoverflow.com/a/24777120
> sendMessage() {
>  message="$*"
>  # Calculate the byte size of the string.
>  # NOTE: This assumes that byte length is identical to the string length!
>  # Do not use multibyte (unicode) characters, escape them instead, e.g.
>  # message='"Some unicode character:\u1234"'
>  messagelen=${#message}
>  # Convert to an integer in native byte order.
>  # If you see an error message in Chrome's stdout with
>  # "Native Messaging host tried sending a message that is ... bytes long.",
>  # then just swap the order, i.e. messagelen1 <-> messagelen4 and
>  # messagelen2 <-> messagelen3
>  messagelen1=$(((messagelen) & 0xFF))
>  messagelen2=$(((messagelen >> 8) & 0xFF))
>  messagelen3=$(((messagelen >> 16) & 0xFF))
>  messagelen4=$(((messagelen >> 24) & 0xFF))
>  # Print the message byte length followed by the actual message.
>  printf "$(printf '\\x%x\\x%x\\x%x\\x%x' \
>    $messagelen1 $messagelen2 $messagelen3 $messagelen4)%s" "$message"
> }
>
> main() {
>  while true; do
>    getMessage
>  done
> }
>
> main
>
>
>
> On Sat, Jun 24, 2023 at 8:21 AM Greg Wooledge <greg@wooledge.org> wrote:
>
> > On Sat, Jun 24, 2023 at 08:10:39AM -0700, guest271314 wrote:
> >> So, in the end, you can use something like this.
> >>
> >>  length=$(dd iflag=fullblock bs=4 count=1 | od -An -td4)
> >>  length=$((length))      # trim leading spaces
> >>  IFS= read -rN"$length" json
> >>
> >> That still winds up showing "Invalid byte sequence in conversion input"
> >> when echo'ing the output to a file.
> >
> > Please show us the input.  Since it's binary, you'll need to represent
> > it in some way that can fit into an e-mail body.  Perhaps a hex dump of
> > the first 128 bytes or something.
> >
> > I'm rather curious what possible input could be an "invalid byte
> sequence"
> > when interpreted as 32 bits that make up an integer.  Is it having the
> > high bit set (meaning negative when read as a signed integer)?
> >
> > unicorn:~$ printf '\x00\x00\x00\x80' | od -An -td4
> > -2147483648
> >
> > Hmm, no, that looks all right to me.  Certainly didn't give that error.
> >
> > What operating system and architecture are you on, and what input are you
> > using to generate this message, and which program is actually writing it?
> >
> >
>