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

[Greg Wooledge]

On Fri, Jun 23, 2023 at 10:20:33PM -0700, guest271314 wrote:
> > Chrome starts each native messaging host in a separate process and
> communicates with it using standard input (stdin) and standard output
> (stdout). The same format is used to send messages in both directions; each
> message is serialized using JSON, UTF-8 encoded and is preceded with 32-bit
> message length in native byte order.

Reading that 4-byte binary number value is the hard part, because bash
does not handle NUL bytes gracefully.  In bash, all input values are
strings, which are stored as "C strings" internally (NUL being the string
value terminator).  Bash has no means of storing binary values without
doing some kind of encoding.  Not even "declare -i" causes binary storage
to be used internally.  Everything is still a string.

The only way to read this value would be to have some external program
do it, and spit out a text encoding that bash can read.  Given that
the 4 bytes are the prefix of a larger data stream, we need to ensure
that *only* those 4 bytes are read, and that the rest of the stream
is untouched.  The tool of choice for that is dd(1).

So then, the next requirement is something which can take those 4 bytes
of binary data and turn them into a textual representation of a number.
And we don't want to use a "real" scripting language like perl or python
or tcl, because if we're doing that, we would just write the entire
project in that language, rather than in a hybrid of bash + other.

My mind goes first to od(1).

The GNU man page for od(1) is lacking some critical information, but the
POSIX od(1p) gives me what I need.

unicorn:~$ printf '\x00\x03\x01\x00' | od -An -td4
       66304

My system is amd64, which is little endian, so the 4-byte number that
I wrote out with printf is indeed 66304 in base 10.  So that od call
gives the correct value.

Putting the pieces together, you might write something like this:

length=$(dd bs=4 count=1 | od -An -td4)
length=$((length))      # trim leading spaces
IFS= read -rN"$length" json

Then of course you'll need to use something which can decode the JSON
for you.  jq(1) is popular in bash scripts.