Build fails with strict-aliasing violations

[eli-schwartz]

I tried to build with the following *FLAGS to optimize the build: -flto=4 -Werror=odr -Werror=lto-type-mismatch -Werror=strict-aliasing

The -Werror=* flags are important to detect cases where the compiler can try to optimize based on assuming Undefined Behavior (UB) cannot happen, and miscompile code that has UB in it. strict-aliasing issues are always bad but LTO can make them even worse.

I got this error:

x86_64-pc-linux-gnu-gcc -pipe -ggdb -march=native -fstack-protector-all -O2 -fdiagnostics-color=always -frecord-gcc-switches -flto=4 -Werror=odr -Werror=lto-type-mismatch -Werror=strict-aliasing  -Wformat -Werror=format-security -W -Wall -pedantic -DCF_XFREE86_TEXTPROP_BUG_WORKAROUND -DHAVE_X11_BMF   -DHAS_SYSTEM_ASPRINTF=1 -DNOTION_RELEASE='"4.0.4"' -MMD -W -Wall -pedantic -DCF_XFREE86_TEXTPROP_BUG_WORKAROUND -DHAVE_X11_BMF  -I.. -I.. -I..  -DHAS_SYSTEM_ASPRINTF=1 -DNOTION_RELEASE='"4.0.4"' -D_XOPEN_SOURCE -D_XOPEN_SOURCE_EXTENDED -std=c99 -DCF_HAS_VA_COPY -MMD   -c -o rootwin.o rootwin.c
In file included from common.h:18,
                 from rootwin.c:22:
rootwin.c: In function ‘rootwin_init’:
rootwin.c:316:16: error: dereferencing type-punned pointer will break strict-aliasing rules [-Werror=strict-aliasing]
  316 |     LINK_ITEM(*(WRegion**)&ioncore_g.rootwins, (WRegion*)rootwin, p_next, p_prev);
      |                ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
../libtu/dlist.h:19:9: note: in definition of macro ‘LINK_ITEM’
   19 |     if((LIST)==NULL){                     \
      |         ^~~~
rootwin.c:316:16: error: dereferencing type-punned pointer will break strict-aliasing rules [-Werror=strict-aliasing]
  316 |     LINK_ITEM(*(WRegion**)&ioncore_g.rootwins, (WRegion*)rootwin, p_next, p_prev);
      |                ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
../libtu/dlist.h:20:10: note: in definition of macro ‘LINK_ITEM’
   20 |         (LIST)=(ITEM);                    \
      |          ^~~~
rootwin.c:316:16: error: dereferencing type-punned pointer will break strict-aliasing rules [-Werror=strict-aliasing]
  316 |     LINK_ITEM(*(WRegion**)&ioncore_g.rootwins, (WRegion*)rootwin, p_next, p_prev);
      |                ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
../libtu/dlist.h:23:23: note: in definition of macro ‘LINK_ITEM’
   23 |         (ITEM)->PREV=(LIST)->PREV;        \
      |                       ^~~~
rootwin.c:316:16: error: dereferencing type-punned pointer will break strict-aliasing rules [-Werror=strict-aliasing]
  316 |     LINK_ITEM(*(WRegion**)&ioncore_g.rootwins, (WRegion*)rootwin, p_next, p_prev);
      |                ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
../libtu/dlist.h:25:10: note: in definition of macro ‘LINK_ITEM’
   25 |         (LIST)->PREV=(ITEM);              \
      |          ^~~~
rootwin.c: In function ‘rootwin_deinit’:
rootwin.c:341:18: error: dereferencing type-punned pointer will break strict-aliasing rules [-Werror=strict-aliasing]
  341 |     UNLINK_ITEM(*(WRegion**)&ioncore_g.rootwins, (WRegion*)rw, p_next, p_prev);
      |                  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
../libtu/dlist.h:68:21: note: in definition of macro ‘UNLINK_ITEM’
   68 |         if((ITEM)==(LIST)){                  \
      |                     ^~~~
rootwin.c:341:18: error: dereferencing type-punned pointer will break strict-aliasing rules [-Werror=strict-aliasing]
  341 |     UNLINK_ITEM(*(WRegion**)&ioncore_g.rootwins, (WRegion*)rw, p_next, p_prev);
      |                  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
../libtu/dlist.h:69:14: note: in definition of macro ‘UNLINK_ITEM’
   69 |             (LIST)=(ITEM)->NEXT;             \
      |              ^~~~
rootwin.c:341:18: error: dereferencing type-punned pointer will break strict-aliasing rules [-Werror=strict-aliasing]
  341 |     UNLINK_ITEM(*(WRegion**)&ioncore_g.rootwins, (WRegion*)rw, p_next, p_prev);
      |                  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
../libtu/dlist.h:70:17: note: in definition of macro ‘UNLINK_ITEM’
   70 |             if((LIST)!=NULL)                 \
      |                 ^~~~
rootwin.c:341:18: error: dereferencing type-punned pointer will break strict-aliasing rules [-Werror=strict-aliasing]
  341 |     UNLINK_ITEM(*(WRegion**)&ioncore_g.rootwins, (WRegion*)rw, p_next, p_prev);
      |                  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
../libtu/dlist.h:71:18: note: in definition of macro ‘UNLINK_ITEM’
   71 |                 (LIST)->PREV=(ITEM)->PREV;   \
      |                  ^~~~
rootwin.c:341:18: error: dereferencing type-punned pointer will break strict-aliasing rules [-Werror=strict-aliasing]
  341 |     UNLINK_ITEM(*(WRegion**)&ioncore_g.rootwins, (WRegion*)rw, p_next, p_prev);
      |                  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
../libtu/dlist.h:74:14: note: in definition of macro ‘UNLINK_ITEM’
   74 |             (LIST)->PREV=(ITEM)->PREV;       \
      |              ^~~~

Originally reported downstream at: https://bugs.gentoo.org/965850
Full build log: build.log

[nunotexbsd]

4.0.4: fails on FreeBSD:

<snip>
notionflux.c:96:8: error: fields must have a constant size: 'variable length array in structure' extension will never be supported
   96 |                 char buf[CMSG_SPACE(sizeof(fd))];
      |                      ^
notionflux.c:106:19: warning: call to undeclared function 'CMSG_LEN'; ISO C99 and later do not support implicit function declarations [-Wimplicit-function-declaration]
  106 |         cmsg->cmsg_len = CMSG_LEN(sizeof(fd));
      |                          ^
mod_notionflux.c:309:28: warning: a function declaration without a prototype is deprecated in all versions of C [-Wstrict-prototypes]
  309 | static bool start_listening()
      |                            ^
      |                             void
mod_notionflux.c:378:23: warning: a function declaration without a prototype is deprecated in all versions of C [-Wstrict-prototypes]
  378 | void close_connections()
      |                       ^
      |                        void
mod_notionflux.c:405:25: warning: a function declaration without a prototype is deprecated in all versions of C [-Wstrict-prototypes]
  405 | bool mod_notionflux_init()
      |                         ^
      |                          void
mod_notionflux.c:437:27: warning: a function declaration without a prototype is deprecated in all versions of C [-Wstrict-prototypes]
  437 | void mod_notionflux_deinit()
      |                           ^
      |                            void
2 warnings and 1 error generated.

[nunotexbsd]

FreeBSD specific and with some AI help on the error:

"Because the compiler can no longer resolve CMSG_SPACE as a compile-time constant, it treats the array buf as a Variable Length Array (VLA). Since VLAs are forbidden inside structures/certain contexts in standard C, the Clang compiler throws: error: fields must have a constant size.

Adding -D__BSD_VISIBLE restores access to these BSD-specific definitions in <sys/socket.h>. This allows the compiler to correctly calculate the buffer size at compile-time, treating it as a fixed-size array and resolving the error."

It fixes build but I don't know if this is a correct way for a fix.

[raboof]

Right, so the spec says <sys/socket.h> should contain it but not that it needs to be a compile-time constant. Adding -D__BSD_VISIBLE seems a bit of a weird workaround but I'm not opposed to it - just document well why it's there :).

(I also found that including sys/cdefs.h might also trigger this, but that seems like a similarly-weird workaround ;) )

[nunotexbsd]

@raboof, I also found this way, but I didn't tested yet:

notionflux.c

#ifndef __BSD_VISIBLE
#define __BSD_VISIBLE 1
#endif
#include <sys/socket.h>

or

--- notionflux.c.orig
+++ notionflux.c
@@ -93,8 +93,11 @@
 {
     struct msghdr msg = {0};
     struct cmsghdr *cmsg;
-    char buf[CMSG_SPACE(sizeof(fd))];
+    union {
+        char buf[CMSG_SPACE(sizeof(int))];
+        struct cmsghdr align;
+    } u;
 
     memset(&msg, 0, sizeof(msg));
-    msg.msg_control = buf;
-    msg.msg_controllen = sizeof(buf);
+    msg.msg_control = u.buf;
+    msg.msg_controllen = sizeof(u.buf);

For what I understand this problem relates to portability to other systems beside Linux.

Thanks

[wilhelmy]

I guess the CMSG stuff should be adjusted to match the FreeBSD code example here in CMSG_DATA(3), and then compatibility should be tested on Linux. I took the example from Linux's manpage, didn't get around to testing on FreeBSD. Too bad that this isn't portable as it is.

[nunotexbsd]

I've applied OpenBSD fix and notion 4.0.5 builds fine on FreeBSD:

--- system-autodetect.mk.orig   2026-04-21 20:44:17 UTC
+++ system-autodetect.mk
@@ -195,11 +196,11 @@ EXPORT_DYNAMIC=-Xlinker --export-dynamic

 #C89_SOURCE=-ansi

-POSIX_SOURCE?=-D_POSIX_C_SOURCE=200112L
+#POSIX_SOURCE?=-D_POSIX_C_SOURCE=200112L
 BSD_SOURCE?=-D_BSD_SOURCE

 # Most systems
-XOPEN_SOURCE=-D_XOPEN_SOURCE -D_XOPEN_SOURCE_EXTENDED
+#XOPEN_SOURCE=-D_XOPEN_SOURCE -D_XOPEN_SOURCE_EXTENDED
 # SunOS, (Irix)
 #XOPEN_SOURCE=-D__EXTENSIONS__

Could system-autodetect.mk be updated to match *BSD?

Thanks