获取和设置消息队列的属性msgctl，删除消息队列

消息队列的属性保存在系统维护的数据结构msqid_ds中，用户可以通过函数msgctl获取或设置消息队列的属性。

int msgctl(int msqid, int cmd, struct msqid_ds *buf);

msgctl：系统调用对msgqid标识的消息队列执行cmd操作，系统定义了3种cmd操作：

IPC_STAT：该命令用来获取消息队列对应的msqid_ds数据结构，并将其保存到buf指向的地址空间

IPC_SET：该命令用来设置消息队列的属性，要设置的属性存储在buf中，可设置的属性包括：

msg_perm.uid 、 msg_perm.gid、msg_perm.mode以及msg_qbytes

IPC_RMID：从内核中删除msgqid标识的消息队列

struct msqid_ds {
struct ipc_perm msg_perm; /* Ownership and permissions */
time_t msg_stime; /* Time of last msgsnd(2) */
time_t msg_rtime; /* Time of last msgrcv(2) */
time_t msg_ctime; /* Time of last change */
unsigned long __msg_cbytes; /* Current number of bytes in
queue (nonstandard) */
msgqnum_t msg_qnum; /* Current number of messages
in queue */
msglen_t msg_qbytes; /* Maximum number of bytes
allowed in queue */
pid_t msg_lspid; /* PID of last msgsnd(2) */
pid_t msg_lrpid; /* PID of last msgrcv(2) */
};

struct ipc_perm {
key_t __key; /* Key supplied to msgget(2) */
uid_t uid; /* Effective UID of owner */
gid_t gid; /* Effective GID of owner */
uid_t cuid; /* Effective UID of creator */
gid_t cgid; /* Effective GID of creator */
unsigned short mode; /* Permissions */
unsigned short __seq; /* Sequence number */
};

代码示例：

#include <stdio.h>
#include <sys/types.h>
#include <sys/ipc.h>
#include <stdlib.h>
#include <string.h>
#include <sys/msg.h>
#include <time.h>

//用户自定义消息缓冲
struct mymsgbuf{
long msgtype;
char buf[];
};

void showmsgattr(int qid, struct msqid_ds buf)
{
    if(msgctl(qid, IPC_STAT, &buf) == -)
    {
        perror("msgctl error:");
        return;
    }

    printf("***information of message queue%d****\n", qid);
    printf("msg_stime:%s\n", ctime(&(buf.msg_stime)));
    printf("msg_rtime:%s\n", ctime(&(buf.msg_rtime)));
    printf("last change msg time is:%s\n", ctime(&(buf.msg_ctime)));
    printf("number of message in queue  is:%lu\n", buf.msg_qnum);
    printf("msg uid  is:%lu\n", buf.msg_perm.uid);
    printf("***information end*******************\n");

}

int main()
{
    struct mymsgbuf mymsgbuffer;
    int msglen = ;
    int i = ;
    int msgkey = ;
    struct msqid_ds msgattr;

    int qid = ;//消息队列标识符

    //获取键值
    msgkey = ftok(".", );

    qid = msgget(msgkey, IPC_CREAT|);
    printf("msgget return %d\n", qid);

    //输出消息队列的属性
    showmsgattr(qid, msgattr);

    //填充消息结构，发送到消息队列
    msglen = sizeof(struct mymsgbuf) - ;
    strcpy(mymsgbuffer.buf , "manman");
    mymsgbuffer.msgtype = ;
    if (msgsnd(qid, &mymsgbuffer, msglen, ) == -)
    {
        perror("msgsnd error\n");
        exit();
    }
    //消息发送后输出消息队列的属性
    showmsgattr(qid, msgattr);

    //设置消息队列的属性
    msgattr.msg_perm.uid = ;
    msgctl(qid, IPC_SET, &msgattr);
    //设置属性后输出消息队列的属性
    showmsgattr(qid, msgattr);

    //删除后再输出消息队列的属性
    msgctl(qid, IPC_RMID, NULL);
    showmsgattr(qid, msgattr);

    return ;
}

执行结果：

msgget return 32768
***information of message queue32768****
msg_stime:Thu Jan 1 08:00:00 1970

msg_rtime:Thu Jan 1 08:00:00 1970

last change msg time is:Thu Apr 20 13:29:25 2017

number of message in queue is:0
msg uid is:0
***information end*******************
***information of message queue32768****
msg_stime:Thu Apr 20 13:29:25 2017

msg_rtime:Thu Jan 1 08:00:00 1970

last change msg time is:Thu Apr 20 13:29:25 2017

number of message in queue is:1
msg uid is:0
***information end*******************
***information of message queue32768****
msg_stime:Thu Apr 20 13:29:25 2017

msg_rtime:Thu Jan 1 08:00:00 1970

last change msg time is:Thu Apr 20 13:29:25 2017

number of message in queue is:1
msg uid is:33
***information end*******************
msgctl error:: Invalid argument

1、发送消息后消息队列的属性会改变

2、修改了属性后，消息队列的属性会改变。

----------------------------------------------------

关于消息队列的删除：

msgctl(qid, IPC_RMID, NULL);

删除前ipcs指令查看：

------ Message Queues --------
key　　　　 msqid 　　 owner 　　 perms　　 used-bytes 　　 messages
0x0b014424 　　0 　　　　 root 　　 660 　　　　 256 　　　　 1

删除后：

------ Message Queues --------
key 　　 msqid　　 owner　　 perms　　 used-bytes 　　 messages