Thread: Re: [pgsql-hackers-win32] Threads vs Processes
Tom Lane wrote: "Merlin Moncure" <merlin.moncure@rcsonline.com> writes: >> All TLS variables *must* be static (or implicitly static >> through extern, i.e. no 'auto' variables) >I assume you mean static as in not-auto, rather than static as in >not-global. Otherwise we have a problem here. Yes, you are correct. >> and their addresses can not be >> assumed to be constant. >Surely the addresses can be assumed constant within a thread. Otherwise >we have a problem here too. Quoting from the MSDN: The address of a thread local object is not considered constant, and any expression involving such an address is not considered a constant expression. In standard C, the effect of this is to forbid the use of the address of a thread local variable as an initializer for an object or pointer. For example, the following code will be flagged as an error by the C compiler: #define Thread __declspec( thread ) Thread int tls_i; int *p = &tls_i; //This will generate an error in C. <end> (Note this does not apply to C++) >> Taking addresses of TLS variables should be considered illegal, >Sorry, no can accept that restriction. I thought not. I believe if TLS variables are detail managed through the win32 API, some of these problems can be avoided (after all, these examples are for the Microsoft compiler). Merlin
"Merlin Moncure" <merlin.moncure@rcsonline.com> writes:
> Tom Lane wrote:
>> Surely the addresses can be assumed constant within a thread.
>> Otherwise we have a problem here too.
> Quoting from the MSDN:
> The address of a thread local object is not considered constant, and any
> expression involving such an address is not considered a constant
> expression.
Ah. That's probably reasonable. Still a bit of a PITA for us, as there
are various places that do give a static variable an initializer
pointing to another static. But that could be worked around I think.
I thought you were saying that the compiler would forbid taking a TLS
variable's address even at runtime.
regards, tom lane
Tom Lane wrote: >"Merlin Moncure" <merlin.moncure@rcsonline.com> writes: > > >>Tom Lane wrote: >> >> >>>Surely the addresses can be assumed constant within a thread. >>>Otherwise we have a problem here too. >>> >>> > > > >>Quoting from the MSDN: >>The address of a thread local object is not considered constant, and any >>expression involving such an address is not considered a constant >>expression. >> >> > >Ah. That's probably reasonable. Still a bit of a PITA for us, as there >are various places that do give a static variable an initializer >pointing to another static. But that could be worked around I think. >I thought you were saying that the compiler would forbid taking a TLS >variable's address even at runtime. > > Tom, you wrote you wouldn't like the idea of a struct representing the now global variables for a thread, because this would mean that every module would need to access it, and any change of a module-local variable would affect the complete backend. This could be worked around: If that global struct is just a list of pointers to memory blocks, each block representing the opaque local data of a module, this can be avoided. This could be duplicated easily for thread creation, if zwo ints representing mem block size and size to copy for each block is included. Additional pointers for creation and cleanup functions could help for more special initializations. This would make the thing independent of fancy compiler features and platforms. Regards, Andreas