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To: musl@lists.openwall.com
From: "J. Hanne" <musl@jf.hanne.name>
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Date: Fri,  3 Sep 2021 12:13:52 +0200 (CEST)
Subject: [musl] CMSG_LEN macro

Hi,

can somebody enlighten me on the purpose of "CMSG_ALIGN (sizeof (struct cmsghdr))" in

#define CMSG_LEN(len) (CMSG_ALIGN (sizeof (struct cmsghdr)) + (len))

of https://git.musl-libc.org/cgit/musl/tree/include/sys/socket.h?

CMSG_ALIGN seems to round up to a multiple of sizeof(size_t) - e.g. to a multiple of 4 on x86/arm and to a multiple of 8 on x86_64/aarch64?

Given struct cmsghdr, which has a size of 16 bytes on all 4 mentioned archs, I already wonder if this has an effect on any real-world architecture.

But more importantly, I wonder *what* exactly is supposed to being aligned here:
- Shall it put some padding *before* struct cmsghdr? That doesn't seem to make sense as the result of CMSG_LEN() goes into the cmsg_len member of struct cmsg, so it would seem strange to me to include bytes preceding the struct in its length field.
- Shall it put some padding *after* struct cmsghdr? In this case, CMSG_DATA would be wrong as it puts the data directly behind struct cmsghdr without any padding between.

Regards,
  Johann