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1. Solaris
Hi,
Can somebody please explain me what semaphores are? there purpose? and there effects?
Thanks in advance:) (0 Replies)
Discussion started by: Laxxi
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2. Solaris
Hi, I've used the following way to set ssh public key authentication and it is working fine on Solaris 10, RedHat Linux and SuSE Linux servers without any problem. But I got error 'Server refused our key' on Solaris 8 system. Solaris 8 uses SSH2 too. Why? Please help. Thanks.
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3. 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..
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/ipc.h>
#include <sys/sem.h>
#include <sys/types.h>
union... (3 Replies)
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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)
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5. Programming
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
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6. Shell Programming and Scripting
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)
Discussion started by: kekanap
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7. Programming
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)
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8. UNIX for Dummies Questions & Answers
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)
Discussion started by: guest100
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9. Programming
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)
Discussion started by: joseph_shibu
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SEMGET(2) Linux Programmer's Manual SEMGET(2)
NAME
semget - get a semaphore set identifier
SYNOPSIS
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/sem.h>
int semget(key_t key, int nsems, int semflg);
DESCRIPTION
This function returns the semaphore set identifier associated with the argument key. A new set of nsems semaphores is created if key has
the value IPC_PRIVATE or if no existing semaphore set is associated to key and IPC_CREAT is asserted in semflg (i.e. semflg & IPC_CREAT
isn't zero).
The presence in semflg of the fields IPC_CREAT and IPC_EXCL plays the same role, with respect to the existence of the semaphore set, as the
presence of O_CREAT and O_EXCL in the mode argument of the open(2) system call: i.e. the semget function fails if semflg asserts both
IPC_CREAT and IPC_EXCL and a semaphore set already exists for key.
Upon creation, the low-order 9 bits of the argument semflg define the access permissions (for owner, group and others) for the semaphore
set. These bits have the same format, and the same meaning, as the mode argument in the open(2) or creat(2) system calls (though the exe-
cute permissions are not meaningful for semaphores, and write permissions mean permission to alter semaphore values).
When creating a new semaphore set, semget initializes the semaphore set's associated data structure semid_ds as follows:
sem_perm.cuid and sem_perm.uid are set to the effective user-ID of the calling process.
sem_perm.cgid and sem_perm.gid are set to the effective group-ID of the calling process.
The low-order 9 bits of sem_perm.mode are set to the low-order 9 bits of semflg.
sem_nsems is set to the value of nsems.
sem_otime is set to 0.
sem_ctime is set to the current time.
The argument nsems can be 0 (a don't care) when a semaphore set is not being created. Otherwise nsems must be greater than 0 and less than
or equal to the maximum number of semaphores per semaphore set (SEMMSL).
If the semaphore set already exists, the access permissions are verified.
RETURN VALUE
If successful, the return value will be the semaphore set identifier (a nonnegative integer), otherwise -1 is returned, with errno indicat-
ing the error.
ERRORS
On failure errno will be set to one of the following:
EACCES A semaphore set exists for key, but the calling process does not have permission to access the set.
EEXIST A semaphore set exists for key and semflg was asserting both IPC_CREAT and IPC_EXCL.
ENOENT No semaphore set exists for key and semflg wasn't asserting IPC_CREAT.
EINVAL nsems is less than 0 or greater than the limit on the number of semaphores per semaphore set (SEMMSL), or a semaphore set corre-
sponding to key already exists, and nsems is larger than the number of semaphores in that set.
ENOMEM A semaphore set has to be created but the system has not enough memory for the new data structure.
ENOSPC A semaphore set has to be created but the system limit for the maximum number of semaphore sets (SEMMNI), or the system wide
maximum number of semaphores (SEMMNS), would be exceeded.
NOTES
IPC_PRIVATE isn't a flag field but a key_t type. If this special value is used for key, the system call ignores everything but the low-
order 9 bits of semflg and creates a new semaphore set (on success).
The followings are limits on semaphore set resources affecting a semget call:
SEMMNI System wide maximum number of semaphore sets: policy dependent.
SEMMSL Maximum number of semaphores per semid: implementation dependent (500 currently).
SEMMNS System wide maximum number of semaphores: policy dependent. Values greater than SEMMSL * SEMMNI makes it irrelevant.
BUGS
Use of IPC_PRIVATE doesn't inhibit to other processes the access to the allocated semaphore set.
There is currently no intrinsic way for a process to ensure exclusive access to a semaphore set. Asserting both IPC_CREAT and IPC_EXCL in
semflg only ensures (on success) that a new semaphore set will be created, it doesn't imply exclusive access to the semaphore set.
The data structure associated with each semaphore in the set isn't initialized by the system call. In order to initialize those data
structures, one has to execute a subsequent call to semctl(2) to perform a SETVAL or a SETALL command on the semaphore set.
CONFORMING TO
SVr4, SVID. SVr4 documents additional error conditions EFBIG, E2BIG, EAGAIN, ERANGE, EFAULT.
SEE ALSO
ftok(3), ipc(5), semctl(2), semop(2)
Linux 2.5 2002-01-04 SEMGET(2)