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

[Eric Blake]

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?

> +        else
> +            chr="\0"

Why do we have to special-case 0?  printf '\x00' does the right thing.

> +        fi
> +        str+="$chr"

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

> +        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"
}

> +
> +# 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"
}

--
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org