Re: per-thread global variables

["J.A. Magallon" <jamagallon@able.es>]

On 2002.07.12 Robert M. Hyatt wrote:
>
>
>I do this in Crafty.  The approach I chose is to create a structure
>for each thread (I also use clone() since the glibc guys broke 
>pthreads a while back) and then let each thread set its pointer to
>its own private structure (really shared, but since only one thread
>has a pointer to each structure, it is like a private but global
>group of variables).
>
>Doing it like this will guarantee it will work with any compiler,
>which might be a plus...
>

I think something is very dark for me...The hard way in pseudo-C

short id[PID_MAX];
#define setself(k) do { tid[getpid()]=k; } while(0)
#define self() (id[getpid()])

// master
  setself(-1);
// slaves
  for (i in 0..nslaves-1)
    clone(f(),CLONE_VM,i);

f(void *arg)
{
  setself((int)arg);
  print(self());
}

But how about the 32K*2 array ??? (hash table, ordered vector and binary
search...)

Quoting your answer:

>pthreads a while back) and then let each thread set its pointer to
                                                     ^^^
>its own private structure (really shared, but since only one thread

How do you define 'its' pointer ? That is like if I just defined

short id;

and tried to set 'its id' for each thread ?
Whichever method you use, at the end you always need one, the last, the
unique variable (one int, a pointer to a data area), declared as a global
variable in C that you need to be distinct for each thread. The only
thing (in my short undestanding) that can distinguish a thread from one
other is _pid_. Of course, I don't want to mess passing an info struct
to all functions in my code.

On IRIX, you have a

// Fixed address space always private for each process/thread
#define PRDA        ((struct prda *)0x00200000L)

struct prda*	thrprda;
int*	self;

f(int k)
{
  thrprda = (struct prda*)PRDA; // The magic global pointer
                                // different for each thread
  self = &(thrprda->user_area);
  *self = k;
}

If Linux supported this it will make many many things simpler.
A fixed zone of virtual address space that is never shared.

Or perhaps... Could I do in Linux:

#define PRDA        ((void*)0x00200000L) // Or get any free address space

int* self = PRDA; // will be shared over clone(), so we fix it before

for (i in 0..nslaves-1)
  clone(f(),CLONE_VM,i);

f(void *arg)
{
  // Get a chunk, mapped on a fixed address, and do not propagate the
  // map to parent or siblings
  mmap(self,1024,PROT_??,MAP_ANONYMOUS|MAP_FIXED|MAP_PRIVATE,0,0);
  *self = (int)arg;
}


??

TIA

-- 
J.A. Magallon             \   Software is like sex: It's better when it's free
mailto:jamagallon@able.es  \                    -- Linus Torvalds, FSF T-shirt
Linux werewolf 2.4.19-rc1-jam3, Mandrake Linux 8.3 (Cooker) for i586
gcc (GCC) 3.1.1 (Mandrake Linux 8.3 3.1.1-0.7mdk)