Re: [PATCH glibc 1/3] glibc: Perform rseq registration at C startup and thread creation (v19)

----- On May 26, 2020, at 10:57 AM, Florian Weimer fweimer@redhat.com wrote:

* Mathieu Desnoyers:

quoted

quoted

Like the attribute, it needs to come right after the struct keyword, I
think.  (Trailing attributes can be ambiguous, but not in this case.)

Nope. _Alignas really _is_ special :-(

struct _Alignas (16) blah {
        int a;
};

p.c:1:8: error: expected ‘{’ before ‘_Alignas’
 struct _Alignas (16) blah {

Meh, yet another unnecessary C++ incompatibility.  C does not support
empty structs, so I assume they didn't see the field requirement as a
burden.

Indeed, it's weird.

quoted

One last thing I'm planning to add in sys/rseq.h to cover acessing the
rseq_cs pointers with both the UAPI headers and the glibc struct rseq
declarations:

/* The rseq_cs_ptr macro can be used to access the pointer to the current
   rseq critical section descriptor.  */
#ifdef __LP64__
# define rseq_cs_ptr(rseq) \
           ((const struct rseq_cs *) (rseq)->rseq_cs.ptr)
#else /* __LP64__ */
# define rseq_cs_ptr(rseq) \
           ((const struct rseq_cs *) (rseq)->rseq_cs.ptr.ptr32)
#endif /* __LP64__ */

Does it make sense ?

Written this way, it's an aliasing violation.  I don't think it's very
useful.

OK, I'll just remove it.

Thanks,

Mathieu


-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com