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

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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 SUNOS

crypt_unix

crypt_unix(5)						Standards, Environments, and Macros					     crypt_unix(5)

NAME

       crypt_unix - traditional UNIX crypt algorithm

DESCRIPTION

       The  crypt_unix algorithm is the traditional UNIX crypt algorithm. It is not considered sufficiently secure for current systems and is pro-
       vided for backwards compatibility. The crypt_sunmd5(5), crypt_bsdmd5(5), or crypt_bsdbf(5) algorithm should be used instead.

       The algorithm identifier for policy.conf(4) is __unix__. There is no entry in crypt.conf(4) for this algorithm.

       The crypt_unix algorithm is internal to libc and provides the string encoding function used by crypt(3C) when the first	character  of  the
       salt is not a "$".

       This algorithm is based on a one-way encryption algorithm with variations intended (among other things) to frustrate use of hardware imple-
       mentations of a key search.  Only the first eight characters of the key passed to crypt() are  used  with  this	algorithm;  the  rest  are
       silently  ignored.  The salt is a two-character string chosen from the set [a-zA-Z0-9./].  This string is used to perturb the hashing algo-
       rithm in one of 4096 different ways.

       The maximum password length for crypt_unix is 8 characters.

USAGE

       The return value of the crypt_unix algorithm might not be portable among standard-conforming systems. See standards(5).

ATTRIBUTES

       See attributes(5) for descriptions of the following attributes:

       +-----------------------------+-----------------------------+
       |      ATTRIBUTE TYPE	     |	    ATTRIBUTE VALUE	   |
       +-----------------------------+-----------------------------+
       |MT-Level		     |Safe			   |
       +-----------------------------+-----------------------------+

SEE ALSO

       passwd(1), crypt(3C), crypt_genhash_impl(3C), crypt_gensalt(3C), crypt_gensalt_impl(3C), getpassphrase(3C), crypt.conf(4), passwd(4),  pol-
       icy.conf(4), attributes(5), crypt_bsdbf(5), crypt_bsdmd5(5), crypt_sunmd5(5), standards(5)

SunOS 5.10							    6 Aug 2003							     crypt_unix(5)