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Message-ID: <20251014165307.GO1827@brightrain.aerifal.cx>
Date: Tue, 14 Oct 2025 12:53:08 -0400
From: Rich Felker <dalias@...c.org>
To: "Chen, Qi" <Qi.Chen@...driver.com>
Cc: "musl@...ts.openwall.com" <musl@...ts.openwall.com>
Subject: Re: [PATCH 2/2] include/netinet/if_ether.h: guard ethhdr
definition to better work with other header files
On Thu, Oct 09, 2025 at 02:53:13AM +0000, Chen, Qi wrote:
> Hi Rich,
>
> The problem is about the re-definition of 'struct ethhdr'.
>
> For example, a header also defines 'struct ethhdr' and
> __UAPI_DEF_ETHHDR, and it's included *before* our
> <netinet/if_ether.h>, then the unconditional definition of 'struct
> ethhdr' in this header will be treated as an error.
>
> Some background info:
> I was making systemd to compile with musl. I got this re-definition
> error. I checked the headers and thought that for different headers
> to work together, the definition of this 'struct ethhdr' needs to be
> guarded by the same macro.
It's intentional that we don't support getting definitions for the
libc types from kernel headers. They may have subtle mismatches that
could break things. For example some types in the kernel are
known-wrong, like struct msghdr, and others have members that are the
same size/representation but different type (like long long vs
int64_t).
For the most part conflicting kernel headers shouldn't be used in
userspace. In cases where they should/need to be used, they should be
used either in separate translation units or included after the libc
headers.
This is documented in the original commit which added the __UAPI
macros, 04983f2272382af92eb8f8838964ff944fbb8258.
Rich
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