Hi there:
Thanks first. When I use a message queue amony severl processes, will I have to synchronize the queue? I don't think I would have to because a message queue is implemented in a link listed. Correct me If I am wrong... (0 Replies)
hi all,
I'm working on this problem for 2 days. Can somebody tell me that how to delete all message queues from the system ?
Since "ipcs -q" gives the list of all existing message queue, then there must be a system call and data stucture where from I can fetch the data about all existing... (2 Replies)
I am sending and retriving the message to the queue
the problem is
after retrieving the message can i see what is there in my message queue.
(actually in my application i am encountring some garbage value)
so i want to retieve this garbage value and also want to know its size
how... (0 Replies)
hi all...
ive been trying this program where i spawn 4 threads... and i am trying to use message queue to send msgs from 3 of the threads to the parent thread... but it doent seem to be working... ive almost pulled out my hair tryin to fix the prob :confused:
another wierd thing... (1 Reply)
Hello,
i need to write a message queue "chat server", that should work only localy.
Can anyone please help me with some ideas and peshaps code. I'm studying the UNIX IPC mechanisms right now. So far, i understand how it works but i still cannot get an idea how to write a chat programm...
... (2 Replies)
Hii can anyone pls tell how to limit the max no of message in a posix message queue. I have made changes in proc/sys/fs/mqueue/msg_max
But still whenever i try to read the value of max. message in the queue using attr.mq_curmsgs (where struct mq_attr attr) its giving the default value as 10.... (0 Replies)
Hi all,
I need help about message queues, i have a server-client program that communicates each other via msg queue, firstly server opens its msg queue and waits for msg then client opens server msg queue and its own msg queue(for receiving msg from server,clients sends msg to server msg... (7 Replies)
Hello !!!!! I have a simple question but i can't find the answer anywhere hope to meet it here.
Why it is a bad idea to pass pointers through message queues ?
Most structs i see all of their char types are arrays... Is it becase having pointers means we could possibily send wrong bytes ?
For... (2 Replies)
Hello
How can I see the created message queues in the system? (4 Replies)
Discussion started by: xyzt
4 Replies
LEARN ABOUT OPENDARWIN
semop
SEMOP(2) BSD System Calls Manual SEMOP(2)NAME
semop -- atomic array of operations on a semaphore set
LIBRARY
Standard C Library (libc, -lc)
SYNOPSIS
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/sem.h>
int
semop(int semid, struct sembuf *array, size_t nops);
DESCRIPTION
The semop() system call atomically performs the array of operations indicated by array on the semaphore set indicated by semid. The length
of array is indicated by nops. Each operation is encoded in a struct sembuf, which is defined as follows:
struct sembuf {
u_short sem_num; /* semaphore # */
short sem_op; /* semaphore operation */
short sem_flg; /* operation flags */
};
For each element in array, sem_op and sem_flg determine an operation to be performed on semaphore number sem_num in the set. The values
SEM_UNDO and IPC_NOWAIT may be OR'ed into the sem_flg member in order to modify the behavior of the given operation.
The operation performed depends as follows on the value of sem_op:
o When sem_op is positive and the process has alter permission, the semaphore's value is incremented by sem_op's value. If SEM_UNDO is
specified, the semaphore's adjust on exit value is decremented by sem_op's value. A positive value for sem_op generally corresponds to a
process releasing a resource associated with the semaphore.
o The behavior when sem_op is negative and the process has alter permission, depends on the current value of the semaphore:
o If the current value of the semaphore is greater than or equal to the absolute value of sem_op, then the value is decremented by the
absolute value of sem_op. If SEM_UNDO is specified, the semaphore's adjust on exit value is incremented by the absolute value of
sem_op.
o If the current value of the semaphore is less than the absolute value of sem_op, one of the following happens:
o If IPC_NOWAIT was specified, then semop() returns immediately with a return value of EAGAIN.
o Otherwise, the calling process is put to sleep until one of the following conditions is satisfied:
o Some other process removes the semaphore with the IPC_RMID option of semctl(2). In this case, semop() returns immediately
with a return value of EIDRM.
o The process receives a signal that is to be caught. In this case, the process will resume execution as defined by
sigaction(2).
o The semaphore's value is greater than or equal to the absolute value of sem_op. When this condition becomes true, the sema-
phore's value is decremented by the absolute value of sem_op, the semaphore's adjust on exit value is incremented by the
absolute value of sem_op.
A negative value for sem_op generally means that a process is waiting for a resource to become available.
o When sem_op is zero and the process has read permission, one of the following will occur:
o If the current value of the semaphore is equal to zero then semop() can return immediately.
o If IPC_NOWAIT was specified, then semop() returns immediately with a return value of EAGAIN.
o Otherwise, the calling process is put to sleep until one of the following conditions is satisfied:
o Some other process removes the semaphore with the IPC_RMID option of semctl(2). In this case, semop() returns immediately with a
return value of EIDRM.
o The process receives a signal that is to be caught. In this case, the process will resume execution as defined by sigaction(2).
o The semaphore's value becomes zero.
For each semaphore a process has in use, the kernel maintains an ``adjust on exit'' value, as alluded to earlier. When a process exits,
either voluntarily or involuntarily, the adjust on exit value for each semaphore is added to the semaphore's value. This can be used to
insure that a resource is released if a process terminates unexpectedly.
RETURN VALUES
The semop() function returns the value 0 if successful; otherwise the value -1 is returned and the global variable errno is set to indicate
the error.
ERRORS
The semop() system call will fail if:
[EINVAL] No semaphore set corresponds to semid, or the process would exceed the system-defined limit for the number of per-process
SEM_UNDO structures.
[EACCES] Permission denied due to mismatch between operation and mode of semaphore set.
[EAGAIN] The semaphore's value would have resulted in the process being put to sleep and IPC_NOWAIT was specified.
[E2BIG] Too many operations were specified. [SEMOPM]
[EFBIG] sem_num was not in the range of valid semaphores for the set.
[EIDRM] The semaphore set was removed from the system.
[EINTR] The semop() system call was interrupted by a signal.
[ENOSPC] The system SEM_UNDO pool [SEMMNU] is full.
[ERANGE] The requested operation would cause either the semaphore's current value [SEMVMX] or its adjust on exit value [SEMAEM] to
exceed the system-imposed limits.
SEE ALSO semctl(2), semget(2), sigaction(2)BUGS
The semop() system call may block waiting for memory even if IPC_NOWAIT was specified.
BSD September 22, 1995 BSD