[SunHELP] What is the purpose of SEMAPHORES???

[Christopher Smiga]

Scott,

Thank you for the information.

Christopher

-----Original Message-----
From: sunhelp-admin at sunhelp.org [mailto:sunhelp-admin at sunhelp.org]On Behalf
Of Scott Jonathan
Sent: Thursday, December 20, 2001 10:55 PM
To: sunhelp at sunhelp.org
Subject: Re: [SunHELP] What is the purpose of SEMAPHORES???


Semaphores are a type of IPC (InterProcess Communication). They help to get
rid of race-conditions. They introduce a kind of hierarchy to locking
resources for use. A simple example would be:

Process A wants to lock resource X and Y.
Process B wants to lock resource X and Y, as well.
Both resources are needed to get the job done, by A and B.
Process A locks X
Process B locks Y (someone decided to lock Y first, instead of X, the
opposite of A)
Process A tries to lock Y, but gets blocked on the io call, and gets put to
sleep holding the lock on X.
Process B tries to lock X, and ends up in the same condition; i.e. blocked
on the io call for X and put to sleep.

Neither process will now wake up, and resources X and Y are no longer
available. Semaphores force programs to lock resources in an order, such
that the above cannot happen.

There are other situations you can use it in, but I cant think them up.;P
Try looking up books on OS internals. There should be good info out there.
I like the book "Linux Kernel Internals". My copy is quite old, but
explains the basics well. There should be an updated version, to 2.2, I
believe.

Good luck!

JS


>From: "Christopher Smiga" <csmiga at yahoo.com>
>Reply-To: sunhelp at sunhelp.org
>To: <sunhelp at sunhelp.org>
>Subject: [SunHELP] What is the purpose of SEMAPHORES???
>Date: Thu, 20 Dec 2001 22:43:40 -0500
>
>I was recently asked what a semaphore was in Solaris and I could not
answer
>this question. Does anyone know what the purpose of this is. I believe
they
>can be set up in "/etc/system", but that's all I know. Any information
would
>be appreciated.
>
>Thank you,
>Christopher Smiga
>e-Mail: csmiga at yahoo.com
>
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