Nim & C disagree on type size

[kvinwang]

By turn on checkabi, many errors in stdlib show up.
These errors are very dangerous, that can cause undefined runtime behaviors. See #6496

$ nim c -d:checkabi test.nim
In file included from csources/c/i386/stdlib_system.c:10:0:
/home/loong/osc/nim/lib/nimbase.h:509:3: error: static assertion failed: "Nim & C disagree on type size"
   _Static_assert(sizeof(typ) == sz, "Nim & C disagree on type size")

A list of errors show up in my app (It's only a small part of whole stdlib):

 NIM_CHECK_SIZE(struct stat, 4);/*  Stat @ ["lib/system/sysio.nim", 323, 6] */
 NIM_CHECK_SIZE(dev_t, 4);/*  Dev @ ["lib/posix/posix_other.nim", 139, 2] */
 NIM_CHECK_SIZE(off_t, 8);/*  Off @ ["lib/posix/posix_other.nim", 148, 2] */
 NIM_CHECK_SIZE(struct stat, 68);/*  Stat @ ["lib/posix/posix_other.nim", 202, 2] */
 NIM_CHECK_SIZE(struct tm, 36);/*  StructTM @ ["lib/pure/times.nim", 1169, 6] */
 NIM_CHECK_SIZE(union epoll_data, 4);/*  epoll_data @ ["lib/posix/epoll.nim", 38, 2] */
 NIM_CHECK_SIZE(struct epoll_event, 8);/*  epoll_event @ ["lib/posix/epoll.nim", 47, 2] */
 NIM_CHECK_SIZE(sa_family_t, 4);/*  TSa_Family @ ["lib/posix/posix_other.nim", 407, 2] */
 NIM_CHECK_SIZE(struct sockaddr_storage, 4);/*  Sockaddr_storage @ ["lib/posix/posix_other.nim", 419, 2] */
 NIM_CHECK_SIZE(struct sockaddr, 260);/*  SockAddr @ ["lib/posix/posix_other.nim", 409, 2] */
 NIM_CHECK_SIZE(struct sockaddr_in, 12);/*  Sockaddr_in @ ["lib/posix/posix_other.nim", 467, 2] */
 NIM_CHECK_SIZE(struct sockaddr_in6, 32);/*  Sockaddr_in6 @ ["lib/posix/posix_other.nim", 477, 2] */
 NIM_CHECK_SIZE(struct sockaddr_un, 112);/*  Sockaddr_un @ ["lib/posix/posix_other.nim", 414, 2] */

sizeof(              struct stat) ==  88 != 4
sizeof(                    dev_t) ==   8 != 4
sizeof(                    off_t) ==   4 != 8
sizeof(              struct stat) ==  88 != 68
sizeof(                struct tm) ==  44 != 36
sizeof(         union epoll_data) ==   8 != 4
sizeof(       struct epoll_event) ==  12 != 8
sizeof(              sa_family_t) ==   2 != 4
sizeof(  struct sockaddr_storage) == 128 != 4
sizeof(          struct sockaddr) ==  16 != 260
sizeof(       struct sockaddr_in) ==  16 != 12
sizeof(      struct sockaddr_in6) ==  28 != 32
sizeof(       struct sockaddr_un) == 110 != 112

[kvinwang]

I think the -d:checkabi should be turned on by default. Just like this one:
typedef int Nim_and_C_compiler_disagree_on_target_architecture[sizeof(NI) == sizeof(void*) && NIM_INTBITS == sizeof(NI)*8 ? 1 : -1];

[Araq]

They are only dangerous if sizeof(T) is used. When T is an object the compiler doesn't allow that.

[kvinwang]

They are only dangerous if sizeof(T) is used. When T is an object the compiler doesn't allow that.

Not exactly. This is a simple example on macOS:

import posix
var ids: seq[Uid] = @[1, 2, 3, 4]
echo "ids:", ids   # ids:@[1, 0, 2, 0]

[krux02]

What is the test.nim that you used?

When I run the last example I get the following error:

/home/arne/.cache/nim/scratch_d/stdlib_dollars.c:52:1: error: static_assert failed "Nim & C disagree on type size"
NIM_CHECK_SIZE(TFrame, -3);

Which means that -d:checkabi is incorrect now, because -3 (szUnknownSize) means that Nim doesn't know the size.

[kvinwang]

When I run the last example I get the following error:

/home/arne/.cache/nim/scratch_d/stdlib_dollars.c:52:1: error: static_assert failed "Nim & C disagree on type size"
NIM_CHECK_SIZE(TFrame, -3);

Seems like thats a bug in -d:checkabi. It works fine with Nim 0.19.2.
I get this error:

/Users/loong/.cache/nim/hi_d/stdlib_posix.c:42:1: error: static_assert failed "Nim & C disagree on type size"
NIM_CHECK_SIZE(uid_t, 8);
^~~~~~~~~~~~~~~~~~~~~~~~

[krux02]

I just fixed checkabi (not merged yet). The problem was that the constant -3 for the size of a type is used to tag the type as: nim doesn't know the size of that type and should not even try to guess. The switch checkabi didn't know about this magic constant, because it is older than that. -3 is the correct value here. The problem is that the check is generated even though in this case it should not be generated.