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Linux消息队列实践(2)
消息队列函数
#include <sys/types.h> #include <sys/ipc.h> #include <sys/msg.h> int msgget(key_t key, int msgflg); int msgctl(int msqid, int cmd, struct msqid_ds *buf); int msgsnd(int msqid, const void *msgp, size_t msgsz, int msgflg); ssize_t msgrcv(int msqid, void *msgp, size_t msgsz, long msgtyp, int msgflg);
msgctl函数
功能:获取/设置消息队列的信息
原型:
int msgctl(int msqid, int cmd, struct msqid_ds *buf);
参数:
msqid: 由msgget函数返回的消息队列标识码
cmd:是将要采取的动作(见下)
返回值:
成功返回0,失败返回-1
cmd:将要采取的动作(有三个可取值),分别如下:
消息队列数据结构
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) */ };
Ipc_perm数据结构
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 */ };
//实践:IPC_STAT int main() { int msgid = msgget(0x1234, 0666); if (msgid == -1) { err_exit("msgget error"); } struct msqid_ds buf; if (msgctl(msgid,IPC_STAT,&buf) == -1) { err_exit("msgctl error"); } printf("buf.msg_perm.mode = %o\n",buf.msg_perm.mode); //%o以八进制打印 cout << "buf.__msg_cbytes = " << buf.__msg_cbytes << endl; cout << "buf.msg_qbytes = " << buf.msg_qbytes << endl; cout << "buf.msg_lspid = " << buf.msg_lspid << endl; }
//实践:IPC_SET,一般需要先获取,然后再设置 int main() { int msgid = msgget(0x1234, 0666); if (msgid == -1) { err_exit("msgget error"); } //获取消息队列的属性 struct msqid_ds buf; if (msgctl(msgid,IPC_STAT,&buf) == -1) { err_exit("msgctl get error"); } //设置消息队列的属性 buf.msg_perm.mode = 0644; if (msgctl(msgid,IPC_SET,&buf) == -1) { err_exit("msgctl set error"); } //获取并打印 if (msgctl(msgid,IPC_STAT,&buf) == -1) { err_exit("msgctl get error"); } printf("buf.msg_perm.mode = %o\n",buf.msg_perm.mode); //%o以八进制打印 }
//实践:IPC_RMID,删除消息队列 /**说明:可以通过在多个窗口上运行几个该程序,测试出: 消息队列并没有运用”引用计数”的功能! */ int main() { int msgid = msgget(0x1234, 0666); if (msgid == -1) { err_exit("msgget error"); } int choice = 0; cout << "Please input Your choice: 0-delete, other-continue: "; cin >> choice; if (!choice) { //delete msg if (msgctl(msgid,IPC_RMID,NULL) == -1) { err_exit("msgctl IPC_RMID error"); } else { cout << "msgid = " << msgid << ", IPC_RMID OK!" << endl; } } }
消息的发送和接收
msgsnd函数
功能:把一条消息添加到消息队列中
原型
int msgsnd(int msqid, const void *msgp, size_t msgsz, int msgflg);
参数
msgid: 由msgget函数返回的消息队列标识码
msgp:是一个指针,指针指向准备发送的消息,
msgsz:是msgp指向的消息长度,这个长度不含保存消息类型的那个long int长整型
msgflg:控制着当前消息队列满或到达系统上限时将要发生的事情
返回值:
成功返回0;失败返回-1
msgflg=IPC_NOWAIT表示队列满不等待,返回EAGAIN错误。
消息结构在两方面受到制约。首先,它必须小于系统规定的上限值;其次,它必须以一个long int长整数开始,接收者函数将利用这个长整数确定消息的类型
消息结构参考形式如下:
struct msgbuf { long mtype; /* message type, must be > 0 */ char mtext[1]; /* message data */ };
//实践 /**发送结构*/ struct msgBuf { long mtype; /* message type, must be > 0 */ char mtext[1024]; /* message data */ }; int main() { int msgid = msgget(0x1234,0666|IPC_CREAT); if (msgid == -1) { err_exit("msgget error"); } //初始化消息结构 struct msgBuf myBuffer; myBuffer.mtype = 1; strcpy(myBuffer.mtext,"Hello XiaoFang!"); //向消息队列发送消息 if (msgsnd(msgid,&myBuffer,strlen(myBuffer.mtext),IPC_NOWAIT) == -1) { err_exit("msgsnd error"); } return 0; }
msgrcv函数
功能:是从一个消息队列接收消息
原型
ssize_t msgrcv(int msqid, void *msgp, size_t msgsz, long msgtyp, int msgflg);
参数
msgid: 由msgget函数返回的消息队列标识码
msgp:是一个指针,指针指向准备接收的消息,
msgsz:是msgp指向的消息长度,这个长度不含保存消息类型的那个long int长整型
msgtype:它可以实现接收优先级的简单形式
msgflg:控制着队列中没有相应类型的消息可供接收时将要发生的事
返回值:
成功->返回实际放到接收缓冲区里去的字节数;失败->返回-1
msgtyp | |
msgtyp=0 | 返回队列第一条信息 |
msgtyp>0 | 返回队列第一条类型等于msgtype的消息 |
msgtyp<0 | 返回队列第一条类型小于等于msgtype绝对值的消息,并且是满足条件的消息类型最小的消息 |
msgflg | |
msgflg=IPC_NOWAIT | 队列没有可读消息不等待,返回ENOMSG错误。 |
msgflg=MSG_NOERROR | 消息大小超过msgsz时被截断 |
msgtyp>0且msgflg=MSG_EXCEPT | 接收类型不等于msgtype的第一条消息 |
//实践:消息发送 int main() { int msgid = msgget(0x1234,0666|IPC_CREAT); if (msgid == -1) { err_exit("msgget error"); } struct msgBuf myBuffer; for (int i = 0; i < 128; ++i) { myBuffer.mtype = i+1; sprintf(myBuffer.mtext,"Hello, My number is %d",i+1); if (msgsnd(msgid,&myBuffer,strlen(myBuffer.mtext),IPC_NOWAIT) == -1) { err_exit("msgsnd error"); } } return 0; }
//实践:消息接收:从队首不断的取数据 int main() { int msgid = msgget(0x1234,0666); if (msgid == -1) { err_exit("msgget error"); } //从队首不断的取数据,连续取10个 struct msgBuf myBuffer; for (int i = 0; i < 10; ++i) { int recvBytes = 0; if ((recvBytes = msgrcv(msgid,&myBuffer,sizeof(myBuffer.mtext),0,IPC_NOWAIT)) == -1) { err_exit("msgrcv error"); } else { cout << "receive recvBytes = " << recvBytes << endl; cout << "myBuffer.mtype = " << myBuffer.mtype << endl; cout << "\t" << myBuffer.mtext << endl; } } cout << "strlen(myBuffer.mtext) = " << strlen(myBuffer.mtext) << endl; return 0; }
Linux消息队列实践(2)