Re: [PATCH bpf-next v1 3/4] bpftool: Don't drop a type in the sorted C dump

On Wed, Aug 19, 2026 at 5:06 PM Ihor Solodrai <[email protected]> wrote:
>
> The C dump sorts types by default, so that generated headers are
> diffable. The sorted dump emits one type fewer than the unsorted dump
> of the same BTF.
>
> dump_btf_c() starts its loop at index 1 to skip the void type at BTF
> type ID 0. That holds for the unsorted dump, where the array index is
> the type ID, but not after qsort(): position 0 is then the lowest ranked
> type, and btf_type_rank() ranks an anonymous enum 0 while void takes the
> default rank of 10. So the enum is skipped, and void is emitted instead
> as a no-op.
>
> Skip by type ID rather than by position.
>
> Fixes: 94133cf24bb3 ("bpftool: Introduce btf c dump sorting")
> Signed-off-by: Ihor Solodrai <[email protected]>
> ---
>  tools/bpf/bpftool/btf.c | 6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)
>
> diff --git a/tools/bpf/bpftool/btf.c b/tools/bpf/bpftool/btf.c
> index c9589026da8d..345d49a9a22c 100644
> --- a/tools/bpf/bpftool/btf.c
> +++ b/tools/bpf/bpftool/btf.c
> @@ -805,9 +805,13 @@ static int dump_btf_c(const struct btf *btf,
>
>                 if (sort_dump)
>                         datums = sort_btf_c(btf);
> -               for (i = 1; i < cnt; i++) {
> +               for (i = 0; i < cnt; i++) {
>                         int idx = datums ? datums[i].index : i;
>
> +                       /* type ID 0 is void, skip it */
> +                       if (!idx)

nit: my personal preference, but when dealing with integers that do
not represent "error/success" (zero being a success), let's not use
boolean operations. this should be obvious `idx == 0`, IMO

pw-bot: cr


(but nice find!)

> +                               continue;
> +
>                         err = btf_dump__dump_type(d, idx);
>                         if (err)
>                                 goto done;
> --
> 2.55.0
>