René Scharfe <> writes:

> The name field of a tar header has a size of 100 characters.  This limit
> was extended long ago in a backward compatible way by providing the
> additional prefix field, which can hold 155 additional characters.  The
> actual path is constructed at extraction time by concatenating the prefix
> field, a slash and the name field.
> get_path_prefix() is used to determine which slash in the path is used as
> the cutting point and thus which part of it is placed into the field
> prefix and which into the field name.  It tries to cram as much into the
> prefix field as possible.  (And only if we can't fit a path into the
> provided 255 characters we use a pax extended header to store it.)
> If a path is longer than 100 but shorter than 156 characters and ends
> with a slash (i.e. is for a directory) then get_path_prefix() puts the
> whole path in the prefix field and leaves the name field empty.  GNU tar
> reconstructs the path without complaint, but the tar included with
> NetBSD 6 does not: It reports the header to be invalid.
> For compatibility with this version of tar, make sure to never leave the
> name field empty.  In order to do that, trim the trailing slash from the
> part considered as possible prefix, if it exists -- that way the last
> path component (or more, but not less) will end up in the name field.

Nicely explained; thanks.

Makes me wonder what we should do for a file inside a directory
whose name is 10 bytes long, and whose filename is 120 bytes long,

Sounds like people on NetBSD are SOL due to the 155+'/'+100 in such
a case and there is nothing we can do, I guess.

> Signed-off-by: Rene Scharfe <>
> ---
>  archive-tar.c | 2 ++
>  1 file changed, 2 insertions(+)
> diff --git a/archive-tar.c b/archive-tar.c
> index 0ba3f25..923daf5 100644
> --- a/archive-tar.c
> +++ b/archive-tar.c
> @@ -153,6 +153,8 @@ static unsigned int ustar_header_chksum(const struct 
> ustar_header *header)
>  static size_t get_path_prefix(const char *path, size_t pathlen, size_t 
> maxlen)
>  {
>       size_t i = pathlen;
> +     if (i > 1 && path[i - 1] == '/')
> +             i--;
>       if (i > maxlen)
>               i = maxlen;
>       do {
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