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Re: [PATCH 2/3] iotests: Add poke_file_[bl]e functions
From: |
Max Reitz |
Subject: |
Re: [PATCH 2/3] iotests: Add poke_file_[bl]e functions |
Date: |
Tue, 10 Mar 2020 18:22:30 +0100 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.5.0 |
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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- Re: [PATCH 2/3] iotests: Add poke_file_[bl]e functions,
Max Reitz <=