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..
I would like first process to print something then second and so on. But i am getting all process mixed up and not in proper order.I am getting something like this...
Can anyone point me in what i am doing wrong.I think output should print...
Dear Reader,
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)
Hi all,
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)
Hi there,
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)
Hi Friends,
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)
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)
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)
Hello,
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:
bash-2.03$ sleeping
Starting Program... (4 Replies)
Hello,
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)
Hello Everybody,
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)
Hi,
Can somebody please explain me what semaphores are? there purpose? and there effects?
Thanks in advance:) (0 Replies)
Discussion started by: Laxxi
0 Replies
LEARN ABOUT NETBSD
semop
SEMOP(2) BSD System Calls Manual SEMOP(2)NAME
semop -- semaphore operations
LIBRARY
Standard C Library (libc, -lc)
SYNOPSIS
#include <sys/sem.h>
int
semop(int semid, struct sembuf *sops, size_t nsops);
DESCRIPTION
semop() provides a number of atomic operations on a set of semaphores. The semaphore set is specified by semid, sops is an array of sema-
phore operations, and nsops is the number of operations in this array. The sembuf structures in the array contain the following members:
unsigned short sem_num; /* semaphore # */
short sem_op; /* semaphore operation */
short sem_flg; /* operation flags */
Each operation (specified in sem_op) is applied to semaphore number sem_num in the set of semaphores specified by semid. The value of sem_op
determines the action taken in the following way:
o sem_op is less than 0. The current process is blocked until the value of the semaphore is greater than or equal to the absolute value of
sem_op. The absolute value of sem_op is then subtracted from the value of the semaphore, and the calling process continues. Negative
values of sem_op are thus used to enter critical regions.
o sem_op is greater than 0. Its value is added to the value of the specified semaphore. This is used to leave critical regions.
o sem_op is equal to 0. The calling process is blocked until the value of the specified semaphore reaches 0.
The behaviour of each operation is influenced by the flags set in sem_flg in the following way:
IPC_NOWAIT In the case where the calling process would normally block, waiting for a semaphore to reach a certain value, IPC_NOWAIT makes
the call return immediately, returning a value of -1 and setting errno to EAGAIN.
SEM_UNDO Keep track of the changes that this call makes to the value of a semaphore, so that they can be undone when the calling process
terminates. This is useful to prevent other processes waiting on a semaphore to block forever, should the process that has the
semaphore locked terminate in a critical section.
RETURN VALUES
Upon successful completion, a value of 0 is returned. Otherwise, -1 is returned and the global variable errno is set to indicate the error.
ERRORS
semop() will fail if:
[EINVAL] There is no semaphore associated with semid.
[EIDRM] The semaphore set was removed while the process was waiting for one of its semaphores to reach a certain value.
[EACCES] The calling process has no permission to access the specified semaphore set.
[E2BIG] The value of nsops is too big. The maximum is defined as MAX_SOPS in <sys/sem.h>.
[EFBIG] sem_num in one of the sem_buf structures is less than 0, or greater than the actual number of semaphores in the set speci-
fied by semid.
[ENOSPC] SEM_UNDO was requested, and there is not enough space left in the kernel to store the undo information.
[EAGAIN] The requested operation can not immediately be performed, and IPC_NOWAIT was set in sem_flg.
[EFAULT] sops points to an illegal address.
SEE ALSO semctl(2), semget(2)STANDARDS
The semop system call conforms to X/Open System Interfaces and Headers Issue 5 (``XSH5'').
HISTORY
Semaphores appeared in the first release of AT&T System V UNIX.
BSD November 3, 2005 BSD