On Mon, 16 Jun 2025, Jeremy Drake via Cygwin-patches wrote:
> In order to avoid a restriction on any reinterpret_cast-like behavior in
> constinit expressions, use assembly and the linker to define symbols
> with the not-valid-address addresses.
>
> Signed-off-by: Jeremy Drake <[email protected]>
> ---
> This is gross, but I couldn't come up with a way to make this both source
> and ABI (including C++ name mangling) compatible. I'd be happy to be
> shown a cleaner way. I built libc++ without a patch to remove constinit,
> and I'm working on building gcc/libstdc++ to confirm it didn't break
> anything there.
gcc 13.4.0 rebuilt successfully too. The thing to watch out for with this
is "relocation truncated to fit" link errors. Oddly enough, I saw this
when using the absolute symbols from the C++ inside the Cygwin DLL build,
but have not seen it building either DLLs or EXEs using clang or gcc, even
when trying to recreate the scenario (comparing a pthread_mutex_t to
PTHREAD_MUTEX_INITIALIZER).
>
> winsup/cygwin/Makefile.am | 4 +++-
> winsup/cygwin/include/pthread.h | 27 ++++++++++++++++++++-------
> winsup/cygwin/lib/pthreadconst.S | 17 +++++++++++++++++
> 3 files changed, 40 insertions(+), 8 deletions(-)
> create mode 100644 winsup/cygwin/lib/pthreadconst.S
>
> diff --git a/winsup/cygwin/Makefile.am b/winsup/cygwin/Makefile.am
> index 6438a41487..31747ac98c 100644
> --- a/winsup/cygwin/Makefile.am
> +++ b/winsup/cygwin/Makefile.am
> @@ -78,7 +78,8 @@ LIB_FILES= \
> lib/premain1.c \
> lib/premain2.c \
> lib/premain3.c \
> - lib/pseudo-reloc-dummy.c
> + lib/pseudo-reloc-dummy.c \
> + lib/pthreadconst.S
>
> FHANDLER_FILES= \
> fhandler/base.cc \
> @@ -315,6 +316,7 @@ DLL_FILES= \
> ipc.cc \
> kernel32.cc \
> ldap.cc \
> + lib/pthreadconst.S \
> libstdcxx_wrapper.cc \
> loadavg.cc \
> lsearch.cc \
> diff --git a/winsup/cygwin/include/pthread.h b/winsup/cygwin/include/pthread.h
> index 8e296303d7..6910a5a886 100644
> --- a/winsup/cygwin/include/pthread.h
> +++ b/winsup/cygwin/include/pthread.h
> @@ -31,8 +31,6 @@ extern "C"
> #define PTHREAD_CANCEL_DEFERRED 0
> #define PTHREAD_CANCEL_DISABLE 1
> #define PTHREAD_CANCELED ((void *)-1)
> -/* this should be a value that can never be a valid address */
> -#define PTHREAD_COND_INITIALIZER (pthread_cond_t)21
> #define PTHREAD_CREATE_DETACHED 1
> /* the default : joinable */
> #define PTHREAD_CREATE_JOINABLE 0
> @@ -42,10 +40,6 @@ extern "C"
> #define PTHREAD_MUTEX_ERRORCHECK 1
> #define PTHREAD_MUTEX_NORMAL 2
> #define PTHREAD_MUTEX_DEFAULT PTHREAD_MUTEX_NORMAL
> -/* this should be too low to ever be a valid address */
> -#define PTHREAD_RECURSIVE_MUTEX_INITIALIZER_NP (pthread_mutex_t)18
> -#define PTHREAD_NORMAL_MUTEX_INITIALIZER_NP (pthread_mutex_t)19
> -#define PTHREAD_ERRORCHECK_MUTEX_INITIALIZER_NP (pthread_mutex_t)20
> #define PTHREAD_MUTEX_INITIALIZER PTHREAD_NORMAL_MUTEX_INITIALIZER_NP
> #define PTHREAD_ONCE_INIT { PTHREAD_MUTEX_INITIALIZER, 0 }
> #if defined(_POSIX_THREAD_PRIO_INHERIT) && _POSIX_THREAD_PRIO_INHERIT >= 0
> @@ -55,12 +49,31 @@ extern "C"
> #endif
> #define PTHREAD_PROCESS_SHARED 1
> #define PTHREAD_PROCESS_PRIVATE 0
> -#define PTHREAD_RWLOCK_INITIALIZER (pthread_rwlock_t)22
> /* process is the default */
> #define PTHREAD_SCOPE_PROCESS 0
> #define PTHREAD_SCOPE_SYSTEM 1
> #define PTHREAD_BARRIER_SERIAL_THREAD (-1)
>
> +#if !defined(__INSIDE_CYGWIN__) || !defined(__cplusplus)
> +/* Constants for initializer macros */
> +extern struct __pthread_mutex_t __pthread_recursive_mutex_initializer_np;
> +extern struct __pthread_mutex_t __pthread_normal_mutex_initializer_np;
> +extern struct __pthread_mutex_t __pthread_errorcheck_mutex_initializer_np;
> +extern struct __pthread_cond_t __pthread_cond_initializer;
> +extern struct __pthread_rwlock_t __pthread_rwlock_initializer;
> +#define PTHREAD_COND_INITIALIZER (&__pthread_cond_initializer)
> +#define PTHREAD_RECURSIVE_MUTEX_INITIALIZER_NP
> (&__pthread_recursive_mutex_initializer_np)
> +#define PTHREAD_NORMAL_MUTEX_INITIALIZER_NP
> (&__pthread_normal_mutex_initializer_np)
> +#define PTHREAD_ERRORCHECK_MUTEX_INITIALIZER_NP
> (&__pthread_errorcheck_mutex_initializer_np)
> +#define PTHREAD_RWLOCK_INITIALIZER (&__pthread_rwlock_initializer)
> +#else
> +#define PTHREAD_COND_INITIALIZER (pthread_cond_t)21
> +#define PTHREAD_RECURSIVE_MUTEX_INITIALIZER_NP (pthread_mutex_t)18
> +#define PTHREAD_NORMAL_MUTEX_INITIALIZER_NP (pthread_mutex_t)19
> +#define PTHREAD_ERRORCHECK_MUTEX_INITIALIZER_NP (pthread_mutex_t)20
> +#define PTHREAD_RWLOCK_INITIALIZER (pthread_rwlock_t)22
> +#endif
> +
> /* Register Fork Handlers */
> int pthread_atfork (void (*)(void), void (*)(void), void (*)(void));
>
> diff --git a/winsup/cygwin/lib/pthreadconst.S
> b/winsup/cygwin/lib/pthreadconst.S
> new file mode 100644
> index 0000000000..6e55a832a4
> --- /dev/null
> +++ b/winsup/cygwin/lib/pthreadconst.S
> @@ -0,0 +1,17 @@
> +#if defined(__i386__)
> +# define SYM(x) _##x
> +#else
> +# define SYM(x) x
> +#endif
> +
> +/* these should all be too low to ever be valid addresses */
> +.globl SYM(__pthread_recursive_mutex_initializer_np)
> +.set __pthread_recursive_mutex_initializer_np, 18
> +.globl SYM(__pthread_normal_mutex_initializer_np)
> +.set __pthread_normal_mutex_initializer_np, 19
> +.globl SYM(__pthread_errorcheck_mutex_initializer_np)
> +.set __pthread_errorcheck_mutex_initializer_np, 20
> +.globl SYM(__pthread_cond_initializer)
> +.set __pthread_cond_initializer, 21
> +.globl SYM(__pthread_rwlock_initializer)
> +.set __pthread_rwlock_initializer, 22
>
