Re: C11 threads ABI - mtx_t and cnd_t types

[pinskia at gmail dot com]

> On Aug 30, 2014, at 7:52 PM, Rich Felker <dalias@libc.org> wrote:
> 
> Another issue I have on the ABI for C11 threads pertains to the types
> for mtx_t and cnd_t. My understanding, and I agree with this, is that
> it was already decided to use the same underlying sizes/alignment, and
> for now representations, as the corresponding POSIX types. However,
> it's impossible to use the existing types directly due to namespace
> issues. If the following were used:
> 
> typedef pthread_mutex_t mtx_t;
> typedef pthread_cond_t cnd_t;
> 
> this would require pthread_mutex_t and pthread_cond_t to be visible
> when threads.h is included.
> 
> It's possible to consider adding union tags (the underlying types of
> pthread_mutex_t and pthread_cond_t are both unions without tags) in a
> protected namespace, so that you could have:
> 
> union __mutex { ... };
> typedef union __mutex pthread_mutex_t;
> typedef union __mutex mtx_t;

Why not do:
union __mutex {....}
Struct pthread_mutex_t {__mutex d; };
typedef union __mutex mtx_t;

That won't change the name mangling for pthread_mutex_t and allow for mtx_t not be a tagged one. 

Thanks,
Andrew


> 
> but this is not acceptable because it would change the C++ ABI for the
> pthread types. Since they currently do not have tags, the effective
> tag name (for C++ ABI/manging purposes) for the union defining
> pthread_mutex_t comes from the typedef name, pthread_mutex_t. Adding
> an explicit tag would change this (unless the explicit tag were still
> pthread_mutex_t, but then it's not in the namespace that can be used
> in threads.h).
> 
> This issue has been discussed in detail (I might say, to exhaustion)
> on the musl libc mailing list, and the conclusion we reached is that
> pthread_mutex_t and mtx_t (and likewise pthread_cond_t and cnd_t)
> should be defined with identical types (we happen to have a structure
> rather than a union for the outermost layer in musl, but that's an
> irrelevant difference), both without tags. This will give effective
> C++ tag names (for name mangling ABI) of mtx_t and cnd_t for the C11
> types.
> 
> The rationale for this conclusion is to allow a shared implementation.
> If pthread_mutex_t and mtx_t are not compatible types, then it's an
> aliasing violation for pthread_mutex_* functions to use the ->
> operator to access the members of mtx_t as if it were an object of
> type pthread_mutex_t -- this is what would happen when an application
> declares an object of type mtx_t and passes it to mtx_*(), which in
> turn calls pthread_mutex_*() and dereferences. By having both be
> tagless with the same definitions, we make the instance of either type
> in a given translation unit a compatible type with the instance of
> either one in a separate translation unit (but neither is compatible
> with the other within the same translation unit).
> 
> If anyone wants to check this reasoning, the relevant portions of C11
> seem to be 6.2.7p1 and 6.5p7.
> 
> TL;DR:
> 
> mtx_t and cnd_t should be defined without tags and with definitions
> identical to the corresponding pthread types. Other choices of
> definition make it difficult to implement the C11 functions as pure
> wrappers around the pthread functions without risking aliasing
> violations which may result in actual breakage at some point in the
> future with LTO enabled.
> 
> Rich