10 More Discussions You Might Find Interesting
I am building a server. this server contains some data. Clients may modify this data or read this data.
If a client is reading the data and at the same time another client is modifying the data then at this case the reading client may read some false data (some old mixed with... (1 Reply)
Discussion started by: Omar_Mokhtar
2. UNIX for Dummies Questions & Answers
I am having problem with semaphores. I am trying to protect line where process prints so that every process with print in proper order.This is the code..
union... (3 Replies)
Discussion started by: joker40
I was doing an exercise of semaphores and shared memory, namely the barbers:
-B number of barbers
-S number of chairs
-C number of customers.
I have done already and I compiled the code, but when run I get an error segment. Can not be and it took several days.
If anyone sees the error... (2 Replies)
Discussion started by: ciudadwifi
Iam trying to implement the sleeping barber problem using semaphores and running on UNIX machine. Iam linking it to the thread libraries :
bash-2.03$ g++ sleepingBarber.cpp -lpthread -o sleeping
but when i execute it i get the following error:
Starting Program... (4 Replies)
Discussion started by: mohit.choudhary
5. UNIX for Advanced & Expert Users
Hello, first of all I apologize if this thread is not in the correct section of this forum, but this one just seemed the most appropriate.
The question I have does not concern Unix specifically, it applies to virtually any OS, however it is in Unix where I learned about this problem.
So, the... (8 Replies)
Discussion started by: Watto86
been searching around on how to use an up and down function with semaphores but i can't find an example. i looked into using: "semop" but i have no idea how to use it. I have been able to declared the semaphores using semget and initializing them using semctl. (7 Replies)
Discussion started by: ddx08
7. Shell Programming and Scripting
If i execute this command it comes back with 300 lines:
ipcs|grep cerebrus >>> i would like to clear the semaphores but ipcrm can remove one id at a time. is there a quicker way of removing semaphores maybe using awk?
Regards, (1 Reply)
Discussion started by: kekanap
Could someone please confirm which POSIX semaphore routines should be used for a multiprocess (and not multithreaded) environment?
sys/sem.h definitely works. but the routines, semget, semctl, semop are pretty unwieldy. So, I am looking for an easier way out.
From the man pages... (2 Replies)
Discussion started by: qntmteleporter
9. UNIX for Dummies Questions & Answers
I am using HP 10.20 on A 9000/785. My question is:
If I am the only person logged in as root at the moment, how many "semaphore proccesses" should I have?? Is it only one, or it is relevant to other system proccesses?
Here is what I get listing the current semaphores
# ipcs -sp... (1 Reply)
Discussion started by: guest100
I'm in a multiprocess environment working with shared mem and semaphores as mutex.. The problem is -- If one of the process hooked up with the semaphore and accessing the shared mem, terminates abruptly ( or got killed ), other process which are in want of the semaphore are... (1 Reply)
Discussion started by: joseph_shibu
SEMA(9) BSD Kernel Developer's Manual SEMA(9)
sema, sema_init, sema_destroy, sema_post, sema_wait, sema_timedwait, sema_trywait, sema_value -- kernel counting semaphore
sema_init(struct sema *sema, int value, const char *description);
sema_destroy(struct sema *sema);
sema_post(struct sema *sema);
sema_wait(struct sema *sema);
sema_timedwait(struct sema *sema, int timo);
sema_trywait(struct sema *sema);
sema_value(struct sema *sema);
Counting semaphores provide a mechanism for synchronizing access to a pool of resources. Unlike mutexes, semaphores do not have the concept
of an owner, so they can also be useful in situations where one thread needs to acquire a resource, and another thread needs to release it.
Each semaphore has an integer value associated with it. Posting (incrementing) always succeeds, but waiting (decrementing) can only success-
fully complete if the resulting value of the semaphore is greater than or equal to zero.
Semaphores should not be used where mutexes and condition variables will suffice. Semaphores are a more complex synchronization mechanism
than mutexes and condition variables, and are not as efficient.
Semaphores are created with sema_init(), where sema is a pointer to space for a struct sema, value is the initial value of the semaphore, and
description is a pointer to a null-terminated character string that describes the semaphore. Semaphores are destroyed with sema_destroy().
A semaphore is posted (incremented) with sema_post(). A semaphore is waited on (decremented) with sema_wait(), sema_timedwait(), or
sema_trywait(). The timo argument to sema_timedwait() specifies the minimum time in ticks to wait before returning with failure.
sema_value() is used to read the current value of the semaphore.
The sema_value() function returns the current value of the semaphore.
If decrementing the semaphore would result in its value being negative, sema_trywait() returns 0 to indicate failure. Otherwise, a non-zero
value is returned to indicate success.
The sema_timedwait() function returns 0 if waiting on the semaphore succeeded; otherwise a non-zero error code is returned.
The sema_timedwait() function will fail if:
[EWOULDBLOCK] Timeout expired.
condvar(9), locking(9), mtx_pool(9), mutex(9), rwlock(9), sx(9)
February 1, 2006 BSD