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Top Forums Programming pthread_mutex_lock in ANSI C vs using Atomic builtins of GCC Post 302448684 by Corona688 on Thursday 26th of August 2010 04:23:43 PM
Old 08-26-2010
"volatile" has nothing to do with atomic, "volatile" just tells the compiler to never assume that variable is whatever it set it to last. (otherwise, it might assume it never changes and hardcode it in the instructions.)

No variables (except the gcc atomic extensions) are guaranteed to be atomic, at all, ever. There's other problems than being atomic, too -- on a multicore system, one core might have a different copy of the memory still in cache, causing even "volatile" to fail. You need to inform other cores you're modifying values out from under them, which is what memory barriers are for. pthreads does memory barriers for you. I don't know if gcc atomic ops do.

Last edited by Corona688; 08-26-2010 at 05:33 PM..
 

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ATOMIC_DEC(3)						   BSD Library Functions Manual 					     ATOMIC_DEC(3)

NAME
atomic_dec, atomic_dec_32, atomic_dec_uint, atomic_dec_ulong, atomic_dec_ptr, atomic_dec_64, atomic_dec_32_nv, atomic_dec_uint_nv, atomic_dec_ulong_nv, atomic_dec_ptr_nv, atomic_dec_64_nv -- atomic decrement operations SYNOPSIS
#include <sys/atomic.h> void atomic_dec_32(volatile uint32_t *ptr); void atomic_dec_uint(volatile unsigned int *ptr); void atomic_dec_ulong(volatile unsigned long *ptr); void atomic_dec_ptr(volatile void *ptr); void atomic_dec_64(volatile uint64_t *ptr); uint32_t atomic_dec_32_nv(volatile uint32_t *ptr); unsigned int atomic_dec_uint_nv(volatile unsigned int *ptr); unsigned long atomic_dec_ulong_nv(volatile unsigned long *ptr); void * atomic_dec_ptr_nv(volatile void *ptr); uint64_t atomic_dec_64_nv(volatile uint64_t *ptr); DESCRIPTION
The atomic_dec family of functions decrement (by one) the variable referenced by ptr in an atomic fashion. The *_nv() variants of these functions return the new value. The 64-bit variants of these functions are available only on platforms that can support atomic 64-bit memory access. Applications can check for the availability of 64-bit atomic memory operations by testing if the pre-processor macro __HAVE_ATOMIC64_OPS is defined. SEE ALSO
atomic_ops(3) HISTORY
The atomic_dec functions first appeared in NetBSD 5.0. BSD
April 11, 2007 BSD
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