Talk:Semaphore (programming)

Can anyone pls tell me which assembly language instruction supports construction of semaphores ?

- Computer Freak

@computer freak. There is no assembly instruction. You need an instruction to dis-/enable interrupts. Then you can make 'atomic' pieces of code.

The P is short for 'Prolaag' which means try to decrement. The V is short for Verhoog which means increment. Please check it in the original document: http://www.cs.utexas.edu/users/EWD/ewd00xx/EWD74.PDF

Lex Meuldijk

Could anyone provide supporting information for the statements in the "semaphores today" section?
I fail to understand why semaphores should be considered as bad as goto.
The only info I was able to google did not clarify it (such as: preferring the synchronize keyword for the Java language, the difficulty of the use of semaphore).
Thanks!
--Fr3d3r1c 17:50, 27 Feb 2005 (UTC)

Actually the text does not say semaphores are as bad as GOTO. The comparison between semaphores and GOTO is that both seemed to be good ideas in the beginning. A personal view is that a semaphore is just the simplest implementation which does the job and employs 'atomic' or 'atomicised' instructions. More modern alternatives will do the same job by the same underlying means, but will wrap them up differently for easier application to a programming problem. Much like a roller makes it possible to move heavy loads without sliding friction and a wheel is a better implementation of the same idea, because you don't need to keep moving rollers from behind to the front.

VL

I don't understand the example

I'm sorry to ask stupid questions, but I don't fully understand the example:

...It (thread A) then posts a DBDataRequest to both the threads(B,C) and adds a reference to the semaphore as well in the request it posted. After this it immediately does P(S) and blocks. The other two threads meanwhile take their own time to obtain the information and post the responses back and also do a V(S) on the passed semaphore. Only after both threads have done their part of V(S) will the first thread be able to continue. This is exactly what we wanted as well. A semaphore used in this way is called a "counting semaphore".

If the pseudocode in thread A is something like

read data from B
read data from C
P(S)

then won't it proceed as soon as either B or C has done V(S)? Should there be two calls to P(S)? I'm reading the example text like there is only one call to P(S)... ~Samuli

... Because there is a Problem with the Example

The call to "init(S,0)" SHOULD BE "init(S, 2)". Then the call to "P(S)" blocks, or wait, until there have been 2 calls to "V(S)". Both threads, B and C, will call "V(S)" once when they're done. So essentially the "P(S)" call will block, or wait, until both the threads are done with their work.

Hope this helps. - Jeremiah