6.4.2 �޼�� ť - �ý�� ȣ��:msgctl()

�� α׷��Ӹ� �� ̵�

�ý�� ȣ��:msgctl() (SYSTEM CALL:msgctl())

�� wrapper �Լ�� Ǿ��, ��α׷�� ޼�� ť�� , Ȱ��ϱ� �� ϰ� �� ٹ�� ִ�. �־�� ޼�� ť�� ü�� ϱ� �� п� �� ��.

�޼�� ť�� ϱ� ��, msgctl() �ý�� ȣ�� Ѵ�.

  SYSTEM CALL: msgctl();
  PROTOTYPE: int msgctl ( int msgqid, int cmd, struct msqid_ds *buf );
    RETURNS: 0 on success
             -1 on error: errno = EACCES (No read permission and cmd is IPC_STAT)
                                  EFAULT (Address pointed to by buf is invalid with IPC_SET and
                                          IPC_STAT commands)
                                  EIDRM  (Queue was removed during retrieval)
                                  EINVAL (msgqid invalid, or msgsz less than 0)
                                  EPERM  (IPC_SET or IPC_RMID command was issued, but
                                          calling process does not have write (alter)
                                          access to the queue)
  NOTES:

�� Ŀ�� ڷ� �� ʵ�� ̸� �� ִ�. ��ེ��Ե�, ��α׷��ӵ�� Ϻο�� ӹ�� ⵿�� IPC subsystem�� Ѵٸ� ��̷� �з��Ǿ� �� ִ�. ��õ� ��ɾ� �� msgctl()�� Ͽ�, �糭�� ų ��ɼ�� ִ� �ɷ�� ´�.

IPC_STAT: ť�� msqid_ds ��ü�� ȸ�Ͽ� �� ƱԸ�Ʈ�� ּ�� Ѵ�.
IPC_SET: ť�� msqid_ds ��ü�� ipc_perm �� Ѵ�. �� ƱԸ�Ʈ�� Ѵ�.
IPC_RMID: Ŀ�η� �� ť�� Ѵ�.

�޼�� ť�� ڷ� ��(msqid_ds)�� ߴ� �� . Ŀ�� ý��ۿ� ��ϴ� �� ť�� ̷�� ü�� ϳ�� ִ�. IPC_STAT ��ɾ ��Ͽ� �˻縦 �� ü�� 纻�� ȸ�� ִ�. �� ȸ�Ͽ� �Ѱ�� ּҷ� ��ϴ� �� wrapper �Լ�� 캸��.:

int get_queue_ds( int qid, struct msgqid_ds *qbuf )
{
        if( msgctl( qid, IPC_STAT, qbuf) == -1)
        {
                return(-1);
        }
        
        return(0);
}

�� ۷� �� ٸ� ȣ�� Լ�� -1�� ȯ�ȴ�. �� Ǹ�, 0�� ȯ�ǰ� �Ѱ�� ۴� �Ѱܹ�� ť Ȯ��(queue identifier,qid)�� ǥ�Ǵ� �޼�� ť�� ڷ� �� 纻�� ´�.

ť�� ڷ� ��ü�� 纻�� , � Ư�� ְ�, �װ͵�� �� ų �� ִ°�? �ڷ� �� ȿ�� ִ� �� ipc_perm ��̴�. �̰�� (owner)�� (creator)�� , ť�� 㰡��׵� ��Ѵ�. �׷�� ִ� ipc_perm ��ü�� mode,uid,gid ��̴�. ��(owner)�� user id, ��(owner)�� group id, ť�� 㰡��(access permission)�� ٲ� �� ִ�.

ť�� (mode)�� ٲٵ�� ȵ� wrapper �Լ�� . ��(mode)�� 迭�� Ѱ�� Ѵ� (i.e. "660").

int change_queue_mode( int qid, char *mode )
{
        struct msqid_ds tmpbuf;

	/* ���� �ڷ� ������ ���� ���纻�� ��ȸ�Ѵ� */
        get_queue_ds( qid, &tmpbuf);

	/* ������ �ⱳ�� ����Ͽ� �㰡����(permission)�� �����Ѵ� */
        sscanf(mode, "%ho", &tmpbuf.msg_perm.mode);

	/* ���� �ڷ� ������ �����Ѵ� */
        if( msgctl( qid, IPC_SET, &tmpbuf) == -1)
        {
                return(-1);
        }
        
        return(0);
}

get_queue_ds wrapper �Լ�� ȣ��Ͽ� �� ڷ� �� 纻�� ȸ�Ѵ�. �� msg_perm �� mode �� ϱ� �� sscanf()�� ȣ��Ѵ�. �� 纻�� Ʈ�ϱ� �� ƹ�� ȭ�� Ͼ�� ʴ´�. �̷�� IPC_SET ��ɾ ��Ͽ� msgctl()�� ȣ��Ͽ� �� ִ�.

��! ť�� 㰡�� ְ�, �׷�� ؼ� ��Ǿƴϰ� ��θ� ��ܽ�ų �� ִ�. ��϶�, �̷�� IPC ��ü�� ϰų� �ý�� ʴ´�. �� ipcs�� ť�� ٰ� �Ͽ� �� ʴ´ٴ� �� ǹ�� ʴ´�.

�� ϱ� ��, �ټ� ��ִ� ��ȭ�� Ұ��ϰڴ�. South Florida ��п�� н� ��θ� ��ġ�� , �� Ȳ�� �� (Blocking)�� ߴ�. ��㿡 �� ϰ�� ھ�� ׽�Ʈ�ϰ� ��ϱ� �� ߴ�. �׽�Ʈ�� ϴ� ��ȿ� �� ڵ�ȿ� �޼�� ť�� 㰡��(permission)�� ϴ� Ÿ�� ٴ� �� ޾Ҵ�. �� ޼�� ť�� , �ƹ�� ְ� �� ִ�� ׽�Ʈ �ߴ�. �׷��, �� ť�� 带 "660"�� "600"�� Ϸ�� , �� ڽ�� ť�� ڽ�� ƹ�� ʷ��ߴ�. �� ҽ� ��丮�� ޼�� ť ��۾�� ׽�Ʈ �� . �� IPC Ű�� ftok() �Լ�� ߱⿡, �� ť�� Ϸ�� 㰡��(permission)�� ߴ�. �� ִ� ��ħ�� ᱹ �� ý�� ڿ�� ޼�� ť�� װ� �� ipcrm ��ɾ ��Ѿ� �ϴ�� ѽð�� ϰ� �� ´�.

ť�κ�� ޼�� , �޼�� ŵȴ�. �տ�� ߴ� ��ó��, IPC ��ü�� ŵǰų� �ý�� ʴ� �� ý��ۿ� �� ִ´�. �׷��Ƿ�, �츮�� ޼�� ť�� Ŀ�ξȿ� ��ϰ� �� ޼�� Ŀ�� ִ�. �޼�� ť�� ֱ�(life cycle)�� ϼ��ϱ� �� IPC_RMID ��ɾ ��Ͽ� msgctl()�� ȣ�⿡ �� ŵǾ�� Ѵ�.:

int remove_queue( int qid )
{
        if( msgctl( qid, IPC_RMID, 0) == -1)
        {
                return(-1);
        }
        
        return(0);
}

�� wrapper �Լ�� ť�� ŵǸ� 0�� ȯ�ϰ� �ƴϸ� -1�� ȯ�Ѵ�. ť�� Ŵ� �ڿ�� atomic�ϰ� � �� ť��  �Ϸ�� ٵ� �� ̴�.

��:�ý�� ȣ��:msgsnd() ��:msgtool:��ȣ�ۿ� �޼�� ť ��

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LAST UPDATE Nov 18,1997
Created Nov 17,1997

�ý��� ȣ��:msgctl() (SYSTEM CALL:msgctl())

�ý�� ȣ��:msgctl() (SYSTEM CALL:msgctl())