Re: [PATCH 2/3] iotests: Add poke_file_[bl]e functions

[Max Reitz]

On 27.02.20 19:46, Eric Blake wrote:
> On 2/27/20 11:02 AM, Max Reitz wrote:
>> Similarly to peek_file_[bl]e, we may want to write binary integers into
>> a file.  Currently, this often means messing around with poke_file and
>> raw binary strings.  I hope these functions make it a bit more
>> comfortable.
>>
>> Signed-off-by: Max Reitz <address@hidden>
>> ---
>>   tests/qemu-iotests/common.rc | 37 ++++++++++++++++++++++++++++++++++++
>>   1 file changed, 37 insertions(+)
>>
>> diff --git a/tests/qemu-iotests/common.rc b/tests/qemu-iotests/common.rc
>> index 4c246c0450..604f837668 100644
>> --- a/tests/qemu-iotests/common.rc
>> +++ b/tests/qemu-iotests/common.rc
>> @@ -53,6 +53,43 @@ poke_file()
>>       printf "$3" | dd "of=$1" bs=1 "seek=$2" conv=notrunc &>/dev/null
>>   }
>>   +# poke_file_le 'test.img' 512 2 65534
>> +poke_file_le()
>> +{
> 
> I like the interface.  However, the implementation is a bit bloated (but
> then again, that's why you cc'd me for review ;)
> 
>> +    local img=$1 ofs=$2 len=$3 val=$4 str=''
>> +
>> +    for i in $(seq 0 $((len - 1))); do
> 
> No need to fork seq, when we can let bash do the iteration for us:
> 
> while ((len--)); do
> 
>> +        byte=$((val & 0xff))
>> +        if [ $byte != 0 ]; then
>> +            chr="$(printf "\x$(printf %x $byte)")"
> 
> Why are we doing two printf command substitutions instead of 1?

Because I had no idea that $() would evaluate '\x*' escape sequences.
Interesting.

>> +        else
>> +            chr="\0"
> 
> Why do we have to special-case 0?  printf '\x00' does the right thing.

The non-special-cased version didn’t seem to work for NUL.

>> +        fi
>> +        str+="$chr"
> 
> I'd go with the faster str+=$(printf '\\x%02x' $((val & 0xff))),
> completely skipping the byte and chr variables.

Sure!  That’s much better.

>> +        val=$((val >> 8))
>> +    done
>> +
>> +    poke_file "$img" "$ofs" "$str"
>> +}
> 
> So my version:
> 
> poke_file_le()
> {
>     local img=$1 ofs=$2 len=$3 val=$4 str=
>     while ((len--)); do
>         str+=$(printf '\\x%02x' $((val & 0xff)))
>         val=$((val >> 8))
>     done
>     poke_file "$img" "$ofs" "$str"
> }

Much better indeed.

>> +
>> +# poke_file_be 'test.img' 512 2 65279
>> +poke_file_be()
>> +{
>> +    local img=$1 ofs=$2 len=$3 val=$4 str=''
> 
> And this one's even easier: we get big-endian for free from printf
> output, with a sed post-processing to add \x:
> 
> poke_file_be()
> {
>     local str="$(printf "%0$(($3 * 2))x\n" $4 | sed 's/\(..\)/\\x\1/g')"
>     poke_file "$1" "$2" "$str"
> }

Thanks.  I knew I could count on you. :)

Max

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