alignment and packed structs

[Patrick Welche <prlw1%cam.ac.uk@localhost>]

I just tried to compile if_iwn as a module. It failed with

dev/pci/if_iwn.c:2685:6: error: converting a packed 'struct iwn_fw_dump' pointer (alignment 1) to a 'uint32_t' {aka 'unsigned int'} pointer (alignment 4) may result in an unaligned pointer value [-Werror=address-of-packed-member]


I got around it with

diff -u -r1.17 if_iwnreg.h
--- if_iwnreg.h 19 Jul 2017 16:55:12 -0000      1.17
+++ if_iwnreg.h 6 Jan 2021 17:24:01 -0000
@@ -1447,7 +1447,7 @@
        uint32_t        src_line;
        uint32_t        tsf;
        uint32_t        time[2];
-} __packed;
+} __attribute__((aligned(4),packed));
 
 /* TLV firmware header. */
 struct iwn_fw_tlv_hdr {


Why isn't this necessary when building if_iwn.c as part of a kernel?
Is the above the right solution?


Cheers,

Patrick