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

[dan b]

“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?
> 
>