Thread: Sync Process A and B with Semaphore

hi

i've done some pseudo code

basically I need to get the thing to run in ABABABAB

here is what i've come up with

Code:

Initialise Semaphore
semaphore = sync

Process A
Begin
Down(sync)
doA
Up(sync)
End

Process B
Begin
Down (sync)
doB
Up (sync)
End

can you check if its correct.

Originally Posted by Ti22

basically I need to get the thing to run in ABABABAB.

This is very likely to be the output, but I don't think it is really guaranteed. If an interrupt kicks in after A leaves the critical section and another right before B can enter it's own section, A might enter again. Mutual exclusion is guaranteed, but ABAB... not. Especially when you start to add non-critical code to A or B outside sem.

You could use some variable, i.e. "next" and set it in each process' critical section to the id of the other process. It would have to be guarded and you'd have to find some way for each process to wait on next becoming it's own id (without the use of busy waiting I guess).

so if i did

Code:

Initialise Semaphore
semaphore = sync
int count = 0

Process A
Begin
if count = 0 (Down(sync))
doA
count++
if count = 1 (Up(sync)_
End

Process B
Begin
if count = 1 (Down (sync))
doB
count --
if count = 0 (Up (sync))
End

Try it, looks good to me though.

we weren't exactly taught to program in linux just writing shell scripts and he just told us about semaphores and all that,

I know C but how would I go about trying something like that?

just make it out put text in the console?

i don't even know how to apply semaphores in C like how to do Up(sync) Down(Sync)

we weren't taught all that stuff just the pseudo code.

for the project we're not supposed to write a fully working code, but if u can tell me how or point me to a URL i'll give it a shot.

Originally Posted by Ti22

so if i did

Code:

Initialise Semaphore
semaphore = sync
int count = 0

Process A
Begin
if count = 0 (Down(sync))
doA
count++
if count = 1 (Up(sync)_
End

Process B
Begin
if count = 1 (Down (sync))
doB
count --
if count = 0 (Up (sync))
End

That first line should probably be

Code:

semaphore sync = 1

for a binary semaphore. Also, does that "if count = 0..." start some kind of if-block or does it only conditionalize Down(sync)?

If so, what happens if A runs twice? The first run, sync would not block this process (but the other) because it started as 1, downed to 0, everything ok. doA was protected, count becomes 1, the semaphore is upped to 1 again. Fine.
A runs again, b is out for lunch. count is 1 so down(sync) is not even invoked. Critical section is entered unprotected, count is increased to 2. 2 is not 1 so sync is not upped and now still 1.
Now count is 2, neither process will invoke any semaphore instructions, doA and doB can be entered freely. End of world :-(

Or maybe I'm just misunderstanding that pseudo code

Well for how to implement them in c, it depends on which operating system you are using. If you are using any posix operating system, just search google for posix semaphores, it brings up a lot of hits.