[Toybox] tar creating empty archive?

[Rob Landley]

On 4/17/25 14:28, Hong, Yifan wrote:
> Hello Rob,
> 
> Is there a way to create an empty tar archive with toybox? I tried:
> 
> $ ./toybox tar czf foo.tar.gz -T /dev/null
> tar: empty archive

So... two terminating 512 byte blocks of zeroes with no other content? 
You could do it with something like:

   dd if=/dev/zero bs=1k | gzip > foo.tar.gz

I've just never encountered this use case before?

> It seems that this comes from from
> https://github.com/gfto/toybox/blob/75ebbd1571c85a06c0f4767beb7c20a19068f0b6/toys/pending/tar.c#L763

Tar moved out of pending in something like 2018, but the test and error 
message still exist on line 1048.

> Background The reason I ask is that I want to do this:
> 
> tar czf out.tar.gz -T filelist.txt
> 
> But filelist.txt could be empty and I expect it to create an empty archive.
> If toybox can't create empty archive when -T is empty, I have to use the
> following workaround instead (untested pseudocode):
> 
> if [ -s filelist.txt ]; then
>      tar czf out.tar.gz -T filelist.txt
> else
>      gzip < /dev/null > out.tar.gz
> fi

Technically each tar stream is ended with 2 entirely zeroed 512 byte 
header blocks, hence the 1k of /dev/zero with dd above. (The gnu/dammit 
version tends to add lots of extra zero padding blocks at the end for no 
obvious reason, but back when posix had a standard for this it only 
REQUIRED two.)

> ... which works, but it is not quite gracefully handled IMO.
> 
> As a reference, GNU tar 1.35 does the following:
> 
> # This fails
> $ tar czf out.tar.gz
> tar: Cowardly refusing to create an empty archive
> 
> # This succeeds
> $ tar czf out.tar.gz -T /dev/null

I didn't know there was a distinction between them.

So you want something like:

--- a/toys/posix/tar.c
+++ b/toys/posix/tar.c
@@ -1045,7 +1045,7 @@ void tar_main(void)

    // If include file list empty, don't create empty archive
    if (FLAG(c)) {
-    if (!TT.incl) error_exit("empty archive");
+    if (!TT.incl && !FLAG(T)) error_exit("empty archive");
      TT.fd = 1;
    }

Rob