Re: [Bug-apl] cast from pointer to smaller type 'int'

Wed, 20 Sep 2017 13:42:56 -0700

Hi,

I do NOT want to access the content of the pthread, but I need some identification that allows me
to distinguish the different threads that are running in parallel (on different cores). So I dont care if pthread is
a pointer or an int as long as I can cast it to something that << can print (preferably in hex). No need to even
print the content of the pthread.

Best Regards,
Jürgen



On 09/20/2017 10:26 PM, Xiao-Yong Jin wrote:
So you have a 32bit linux?  Things got more complicated when you want memory safety unfortunately.
POSIX basically tells us pthread_t can be anything, so if you really want to print the content, perhaps the following would work without the compiler complaining.  (I think only char can be used for type punning without breaking the strict aliasing rule, but that may only be true in C?)

const unsigned char* a = reinterpret_cast<const unsigned char*>(&thread);
for (size_t i = 0; i < sizeof(thread); ++i)
  out << hex << setw(2) << static_cast<unsigned int>(a[i]);


On Sep 20, 2017, at 3:00 PM, Juergen Sauermann <[email protected]> wrote:

Hi Xiao-Yong,

with reinterpret_cast<size_t> I am getting (gcc 4.8)  this:

Thread_context.cc: In member function ‘void Thread_context::print(std::ostream&) const’:
Thread_context.cc:73:42: error: invalid cast from type ‘pthread_t {aka long unsigned int}’ to type ‘size_t {aka unsigned int}’
        << reinterpret_cast<size_t>(thread)
                                          ^
make[1]: *** [apl-Thread_context.o] Error 1

It seems a little ridiculous to me that replacing a good old C-style cast that worked fine for the
last 10 years cannot be replaced by one of the 3 members of the C++ zoo of casts in a portable way?

Maybe some intermediate cast to const void * can be done on your machine?

Best Regards,
Jürgen


On 09/19/2017 11:44 PM, Xiao-Yong Jin wrote:

Should have got back to you sooner, but static_cast is not allowed between pointer types and non-pointer types.

Thread_context.cc:73:11: error: static_cast from 'pthread_t' (aka '_opaque_pthread_t *') to 'int' is not allowed
       << static_cast<int>(thread)
          ^~~~~~~~~~~~~~~~~~~~~~~~

I need reinterpret_cast<size_t> here.  I cannot reinterpret_cast<int> either, because of the difference in size.



On Sep 11, 2017, at 3:01 PM, Juergen Sauermann <[email protected]>
 wrote:

Hi Xiao-Yong,

I see. In this particular case the pthread_t is only used to identify a thread and
to distinguish it from other threads for debugging purposes. So as long as the
compiler does not complain about the cast everything is fine. Cast to void * instead of
int would also be an option.

/// Jürgen
 

On 09/11/2017 08:38 PM, Xiao-Yong Jin wrote:


I don't think there is a portable way of printing a variable of type pthread_t.  It could be a struct, depending on the implementation.  static_cast<int> is alright, but may not be useful in the future.




On Sep 11, 2017, at 1:08 PM, Juergen Sauermann <[email protected]>

 wrote:

Hi Xiao-Yong,

thanks, maybe fixed in SVN 1011.
Problem with that error is that the casted type is not a pointer, at least on my machine.

/// Jürgen


On 09/11/2017 06:55 PM, Xiao-Yong Jin wrote:



At revision 1010

Thread_context.cc:72:65: error: cast from pointer to smaller type 'int' loses information
   out << "thread #"     << setw(2) << N << ":" << setw(16)  << int(thread)








        

        

      

      



    

    

  
























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