os.notify: fix struct epoll_event alignment

[kbkpbot]

Original code force struct epoll_event size = 12 under ARM64/Linux, this is not true, as it's size = 16.
Fix issue #25778

[spytheman]

@kbkpbot this essentially reverts 6fcab01 2022-09-29, which solved the following problem with tcc:

#0 05:56:24 ^ master ~/v>jed a.v
#0 05:56:46 ^ master ~/v>v -cc tcc run a.v
[a.v:3] sizeof(C.epoll_event): 12
#0 05:56:48 ^ master ~/v>v -cc clang-18 run a.v
[a.v:3] sizeof(C.epoll_event): 12
#0 05:56:54 ^ master ~/v>v -cc gcc-11 run a.v
[a.v:3] sizeof(C.epoll_event): 12
#0 05:56:59 ^ master ~/v>
#0 05:57:00 ^ master ~/v>
#0 05:57:00 ^ master ~/v>git switch -
Switched to branch 'fix-struct-epoll-event-align'
#0 05:57:05 ^ fix-struct-epoll-event-align ~/v>
#0 05:57:06 ^ fix-struct-epoll-event-align ~/v>
#0 05:57:06 ^ fix-struct-epoll-event-align ~/v>v -cc gcc-11 run a.v
[a.v:3] sizeof(C.epoll_event): 12
#0 05:57:10 ^ fix-struct-epoll-event-align ~/v>v -cc clang-18 run a.v
[a.v:3] sizeof(C.epoll_event): 12
#0 05:57:13 ^ fix-struct-epoll-event-align ~/v>v -cc tcc run a.v
[a.v:3] sizeof(C.epoll_event): 16
#0 05:57:16 ^ fix-struct-epoll-event-align ~/v>
#0 05:57:17 ^ fix-struct-epoll-event-align ~/v>
#0 05:57:17 ^ fix-struct-epoll-event-align ~/v>cat a.v
import os.notify as _

dump(sizeof(C.epoll_event))
#0 05:57:23 ^ fix-struct-epoll-event-align ~/v>

Note, how here, with tcc, the size of the same structure, is reported to be 16 with tcc, while it is 12 with everything else...

[spytheman]

I would like a solution, where the reported size is still the same with all compilers, so imho keep the custom .h file, and add some conditional logic there, to only do the packing for tcc/non arm?

[kbkpbot]

epoll_event

I found some bugs here.... checking....

[kbkpbot]

I found the root cause of this bug,
on my x86-64 box:

./x86_64-linux-gnu/bits/epoll.h:#define __EPOLL_PACKED __attribute__ ((__packed__))

on my aarch64 box, has not define __EPOLL_PACKED

./aarch64-linux-gnu/bits/epool.h

So it depend on the define of __EPOLL_PACKED :(

[spytheman]

Can we just do something like this? :

#if defined(__TINYC__) && defined(__V_amd64)
#pragma pack(push, 1)
#endif

#include <sys/epoll.h>

#if defined(__TINYC__) && defined(__V_amd64)
#pragma pack(pop)
#endif

[spytheman]

So it depend on the define of __EPOLL_PACKED :(

that is indeed unfortunate - we can not check that before including the header, and after that it is already too late, because the C compiler would have registered the struct definitions

[spytheman]

Ideally tcc should be extended to support __attribute__ ((__packed__)), but that is out of scope of this PR.

[kbkpbot]

Ideally tcc should be extended to support __attribute__ ((__packed__)), but that is out of scope of this PR.

This should be a tcc bug, as I think tcc already support this feature long ago.

[kbkpbot]

BTW, I checked RISC-V/Ubuntu, it did not define __EPOLL_PACKED , so the sizeof(struct epoll_event) = 16