__sync_fetch

[Hei Chan]

Hi,

After searching more for info, it seems like even though on a
 64-bit machine, reading a long (i.e. 8 bytes) is one operation, it 
might not give the "correct" value:
http://gcc.gnu.org/ml/gcc/2008-03/msg00201.html

And so, we have to use __sync_fetch_and_add(&x, 0) to read?

Could
 someone elaborate a situation that reading a long variable won't get 
the correct value given that all writes in the application use 
__sync_fetch_*()?

Thanks in advance.


Cheers,
Hei

________________________________
From: Hei Chan <structurechart@yahoo.com>
To: "gcc-help@gcc.gnu.org" <gcc-help@gcc.gnu.org> 
Sent: Friday, November 16, 2012 10:34 PM
Subject: __sync_fetch



Hi,

I am using GCC 4.1.2, and so no __atomic*().

I am looking at http://gcc.gnu.org/onlinedocs/gcc-4.1.1/gcc/Atomic-Builtins.html

I see __sync_fetch_and_*(), but I don't see __sync_fetch().  Is it because the built-in routines support integral scalar or pointer type that is up to 8 bytes in length, and so the read is automatically atomic anyway?

Thanks in advance.


Cheers,
Hei