why the implementatoin of Bakery algorithm in ANSI C does not work in ANSI C Post: 302448223

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Top Forums Programming why the implementatoin of Bakery algorithm in ANSI C does not work in ANSI C Post 302448223 by Corona688 on Wednesday 25th of August 2010 01:28:16 PM

08-25-2010

Registered User

A few problems with writing it in C.

1) You're assuming variables are atomic. Sometimes they're not.

2) You're assuming different threads have access to the same memory/cache. In multicore systems, this isn't always true, "memory barriers" are needed to force cores to be consistent with each other when necessary.

3) The compiler makes certain assumptions about memory. Unless you tell it otherwise, it will assume a variable is not "magic" and won't change mysteriously when its not looking (which is precisely what you're doing with threads). You have to make a variable volatile(i.e. "volatile int i") if you're going to have threads competing over it this way.

So, writing the algorithm in raw ANSI C isn't recommended. You might be able to make this work in raw C on a single-core system, maybe, if you make the shared variables volatile. I think there's also some extensions in GNU gcc for explicit atomic operations.

Last edited by Corona688; 08-25-2010 at 02:37 PM..

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LEARN ABOUT NETBSD

atomic_dec_64_nv

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