Linux消息队列及函数详解（含示例）

消息队列的概念

消息队列就是一个消息的链表，每个消息队列都有一个队列头，用结构struct msg_queue来描述。队列头中包含了该队列的大量信息，包括消息队列的键值、用户ID、组ID、消息数目、读写进程ID等。其定义如下:

struct msg_queue
{
    struct ipc_perm q_perm;
    time_t q_stime;     // last msgsnd time
    time_t q_rtime;     // last msgrcv time
    time_t q_ctime;     // last change time
    unsigned long q_cbytes;      // current number of bytes on queue
    unsigned long q_qnum;        // number of message in queue
    unsigned long q_qbytes;      // max number of bytes on queue
    pid_t q_lspid;     // pid of last msgsnd
    pid_t q_lrpid;     // last receive pid
    struct list_head q_messages;
    struct list_head q_receives;
    struct list_head q_senders;
};

结构体msqid_ds用来设置或返回消息队列的信息，定义如下：

// 摘自所用ubuntu18.04电脑中的/usr/include/i386-linux-gnu/bits/msq.h
struct msqid_ds
{
  struct ipc_perm msg_perm;     /* structure describing operation permission */
  __time_t msg_stime;           /* time of last msgsnd command */
#ifndef __x86_64__
  unsigned long int __glibc_reserved1;
#endif
  __time_t msg_rtime;           /* time of last msgrcv command */
#ifndef __x86_64__
  unsigned long int __glibc_reserved2;
#endif
  __time_t msg_ctime;           /* time of last change */
#ifndef __x86_64__
  unsigned long int __glibc_reserved3;
#endif
  __syscall_ulong_t __msg_cbytes; /* current number of bytes on queue */
  msgqnum_t msg_qnum;           /* number of messages currently on queue */
  msglen_t msg_qbytes;          /* max number of bytes allowed on queue */
  __pid_t msg_lspid;            /* pid of last msgsnd() */
  __pid_t msg_lrpid;            /* pid of last msgrcv() */
  __syscall_ulong_t __glibc_reserved4;
  __syscall_ulong_t __glibc_reserved5;
};

消息队列的创建与打开

消息队列具有一个唯一的键值，或称引用标识符、消息队列的ID号，通过使用ftok()函数获取，函数原型：

#include 
#include 
key_t ftok(char *pathname, char proj);

获取成功返回消息队列的键值，失败返回-1。

参数pathname为一任意存在的路径名，参数proj为1~255之间的任一数字，ftok根据路径名，提取文件信息，再根据这些文件信息及proj的值合成key。

ftok()函数并不直接对消息队列操作，生成的键值用于msgget()函数使用，该函数用于创建或打开一个消息队列，其函数原型如下：

#include 
#include 
#include 
int msgget(key_t key, int msgflg);

运行成功则返回消息队列的引用标识符(ID)，失败则返回-1。

参数key是ftok()产生的键值，参数msgflg是一些标志位，可以取IPC_CREAT、IPC_EXCL、IPC_NOWAIT或三者的逻辑或结果。

在以下两种情况下，msgget()将创建一个新的消息队列：

• 如果没有消息队列与键值key相对应，且msgflg中包含了IPC_CREAT标志位
• key参数为IPC_PRIVATE

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消息队列的读写

消息队列传递的消息由两部分组成，包括消息类型和所传的数据，用结构体struct msgbuf表示：

struct msgbuf
{
    long msgtype;
    char msgtext[1024];
};

msgtype成员代表消息类型，msgtext成员为消息内容，长度不一定是1024。对于发送端，首先预置一个这样的msgbuf缓冲区并写入消息类型和内容，然后调用相应的发送函数；对于接收端，首先分配一个msgbuf缓冲区，然后把消息读入缓冲区即可。

发送数据(写)
向消息队列发送数据使用msgsnd()函数，发送的一个消息数据会被添加到队列的末尾，函数原型如下：

#include 
#include 
#include 
int msgsnd(int msqid, const void *prt, size_t nbytes, int flags);

运行成功返回0，失败返回-1。参数msqid为消息队列的引用标识符(ID)，参数prt为void型指针，指向要发送到的消息，参数nbytes为发送的消息的字节长度，参数flag用于指定消息队列满时的处理方法。

对发送消息来说，有意义的flags标志为IPC_NOWAIT，在消息队列没有足够的空间容纳要发送的数据时，设置了该标志，则msgsnd()函数立刻出错返回，否则发送消息的进程被阻塞，直至消息队列有空间或队列被删除时返回。

接收数据(读)
从消息队列接收数据使用msgrcv()函数，函数原型如下：

#include 
#include 
#include 
int msgrcv(int msqid, const void *prt, size_t nbytes, long type, int flags);

运行成功返回0，失败返回-1。

参数含义与msgsnd()函数类似，参数flag用于指定消息队列满时的处理方法，取值有3种以及3种的或结果，参数type表示接收的数据类型。

消息队列属性设置

消息队列的信息基本都保存在消息队列头中，可分配一个类似于消息队列头的结构struct msqid_ds来返回消息队列的属性，同样可以设置该数据结构。属性设置使用msgctl()函数，函数原型如下：

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

运行成功返回0，失败返回-1。

函数msgctl()将对参数msqid标识的消息队列执行参数cmd所指的命令，包括3种命令：

• IPC_STAT：用于获取消息队列信息，返回的信息存贮在参数buf中
• IPC_SET：用于设置消息队列的属性，要设置的属性存储在参数buf中
• PC_RMID：删除msqid标识的消息队列

编程示例

消息队列编程步骤：

• 使用ftok()生成key
• 使用msgget()创建/获取消息队列，返回值为队列标识符
• 发送消息msgsnd()/接收消息msgrcv()
• 消息队列属性与删除msgctl()

示例1
简单使用。

发送端，msg1_snd.c:

#include 
#include 
#include 
#include 
#include 
#include 

int main()
{
    // generate key
    key_t key = ftok(".", 100);
    if(key == -1)
    {
        perror("ftoke failed");
        exit(1);
    }
    printf("key = %#x\n", key);

    // create message queue
    int msgid = msgget(key, 0666|IPC_CREAT|IPC_EXCL);
    if(msgid == -1)
    {
        perror("msgget failed");
        exit(2);
    }

    // send data
    msgsnd(msgid, "hello world!\n", 14, 0);

    printf("use Enter to destory the message queue!\n");
    getchar();
    // detele message queue
    if(msgctl(msgid, IPC_RMID, NULL) == -1)
    {
        perror("msgctl failed");
        exit(3);
    }

    return 0;
}

接收端，msg1_rcv.c:

#include 
#include 
#include 
#include 
#include 
#include 

int main()
{
    // generate key
    key_t key = ftok(".", 100);
    if(key == -1)
    {
        perror("ftoke failed");
        exit(1);
    }
    printf("key = %#x\n", key);

    // get message queue
    int msgid = msgget(key, 0);
    if(msgid == -1)
    {
        perror("msgget failed");
        exit(2);
    }

    // read from the message queue
    char buf[100] = {};
    msgrcv(msgid, buf, 100, 0, 0);
    printf("read from message queue:%s\n", buf);

    return 0;
}

在一个shell中运行消息队列发送程序：

$ ./msg1_snd
key = 0x641102ed
use Enter to destory the message queue!

在另一个shell中运行消息队列接收程序：

$ ./msg1_rcv
key = 0x641102ed
read from message queue:hello world!

示例2
发送带消息类型的数据。

发送端，msg2_send.c:

#include 
#include 
#include 
#include 
#include 
#include 
#include 

struct _msg
{
    long mtype;
    char buf[256];
}msg1,msg2;

int main()
{
    // generate key
    key_t key = ftok(".", 100);
    if(key == -1)
    {
        perror("ftoke failed");
        exit(1);
    }
    printf("key = %#x\n", key);

    // create message queue
    int msgid = msgget(key, 0666|IPC_CREAT);
    if(msgid == -1)
    {
        perror("msgget failed");
        exit(2);
    }

    //send data
    msg1.mtype = 2;
    strcpy(msg1.buf, "hello2");
    msgsnd(msgid, &msg1, sizeof(msg1.buf), 0);

    msg2.mtype = 1;
    strcpy(msg2.buf, "hello1");
    msgsnd(msgid, &msg2, sizeof(msg2.buf), 0);

    printf("use Enter to destory the message queue!\n");
    getchar();
    // destroy the messsage queue
    if(msgctl(msgid, IPC_RMID, NULL) == -1)
    {
        perror("msgctl failed");
        exit(3);
    }

    return 0;
}

接收端，msg2_rcv.c:

#include 
#include 
#include 
#include 
#include 
#include 
#include 

struct _msg
{
    long mtype;
    char buf[256];
}msg1,msg2;

int main()
{
    // generate key
    key_t key = ftok(".", 100);
    if(key == -1)
    {
        perror("ftoke failed");
        exit(1);
    }
    printf("key = %#x\n", key);

    // get message queue
    int msgid = msgget(key, 0);
    if(msgid == -1)
    {
        perror("msgget failed");
        exit(2);
    }

    // read from the message queue
    int res = msgrcv(msgid, &msg1, sizeof(msg1)-4, 0, 0);
    while(res != 1)
    {
        printf("Message:%s, Type:%ld\n", msg1.buf, msg1.mtype);
        res = msgrcv(msgid, &msg1, sizeof(msg1)-4, 0, 0);
    }

    return 0;
}

在一个shell中运行消息队列发送程序：

$ ./msg2_snd
key = 0x651102ed
use Enter to destory the message queue!

在另一个shell中运行消息队列接收程序：

$ ./msg2_rcv
key = 0x651102ed
Message:hello2, Type:2
Message:hello1, Type:1

示例3
消息队列的综合编程使用举例，msg_app.c:

#include 
#include 
#include 
#include 
#include 
#include 

void msg_stat(int, struct msqid_ds);

int main(void)
{
    int gflags, sflags, rflags;
    key_t key;
    int msgid;
    int reval;

    struct msgsbuf
    {
        int mtype;
        char mtext[10];
    }msg_sbuf;// send

    struct msgmbuf
    {
        int mtype;
        char mtext[10];
    }msg_rbuf;// receivr

    struct msqid_ds msg_ginfo, msg_sinfo;

    // create key
    key = ftok(".", 30);
    if(key == -1)
    {
        printf("ftok failed!");
        return -1;
    }
    printf("key = %#x\n", key);

    // create message queue
    gflags = IPC_CREAT|IPC_EXCL;
    msgid = msgget(key, 0666|gflags);
    if(msgid == -1)
    {
        printf("msg create error\n");
        return -1;
    }
    else
        printf("msg create ok\n");

    // after create the message queue, show it's property, use msg_stat 1st
    printf("\n msg_stat1:");
    msg_stat(msgid, msg_ginfo);

    // send message
    sflags = IPC_NOWAIT;
    msg_sbuf.mtype = 8;
    msg_sbuf.mtext[0] = 'a';
    msg_sbuf.mtext[1] = 'b';
    msg_sbuf.mtext[2] = 'c';
    reval = msgsnd(msgid, &msg_sbuf, sizeof(msg_sbuf.mtext), sflags);
    if(reval == -1)
        printf("message send error\n");
    else
        printf("message send ok\n");

    // after send a message, shoe it's property, use msg_stat 2st
    printf("\n msg_stat2:");
    msg_stat(msgid, msg_ginfo);

    // receive message
    rflags = IPC_NOWAIT|MSG_NOERROR;
    reval = msgrcv(msgid, &msg_rbuf, 3, 0, rflags);
    if(reval == -1)
        printf("msg read error\n");// ===
    else
    {
        printf("read from msg queue %d bytes\n", reval);
        printf("type:%d, message:%s\n", msg_rbuf.mtype, msg_rbuf.mtext);
    }

    // use msg_stat 3st
    printf("\n msg_stat3:");
    msg_stat(msgid, msg_ginfo);

    // change message property
    msg_sinfo = msg_ginfo;
    msg_sinfo.msg_perm.uid = 8; // user ID
    msg_sinfo.msg_perm.gid = 8; // group ID
    msg_sinfo.msg_qbytes = 16388; // queue bytes 16384->16388
    reval = msgctl(msgid, IPC_SET, &msg_sinfo);
    if(reval == -1)
    {
        printf("msg set info error\n");// ===
    }

    // use msg_stat 4st
    printf("\n msg_stat4:");
    msg_stat(msgid, msg_ginfo);

    // delete the message queue
    reval = msgctl(msgid, IPC_RMID, NULL);
    if(reval == -1)
    {
        printf("unlink msg queue error\n");
    }

    return 0;
}

void msg_stat(int msgid, struct msqid_ds msg_info)
{
    int reval;
    sleep(1);
    // get message property
    reval = msgctl(msgid, IPC_STAT, &msg_info);
    if(reval == -1)
    {
        printf("get msg info error\n");
        return;
    }

    printf("\n");
    printf("current number of bytes on queue is %ld\n", msg_info.msg_cbytes);
    printf("number of message in queue is %ld\n", msg_info.msg_qnum);
    printf("max number of bytes on queue is %ld\n", msg_info.msg_qbytes);
    //
    printf("pid of last msgsnd is %d\n", msg_info.msg_lspid); // last send opera                                                       te process's ID
    printf("pid of last msgrcv is %d\n", msg_info.msg_lrpid); // last receive op                                                       erate process's ID
    printf("last msgsnd time is %s", ctime(&(msg_info.msg_stime))); // last send                                                        time
    printf("last msgrcv time is %s", ctime(&(msg_info.msg_rtime))); // last rece                                                       ive time
    printf("last change time is %s", ctime(&(msg_info.msg_ctime))); // last chan                                                       ge time
    printf("msg uid is %d\n", msg_info.msg_perm.uid); // message queue user ID
    printf("msg gid is %d\n", msg_info.msg_perm.gid); // message queue group ID
}

编译后执行，需要sudo权限，否则change message property 步骤会失败：

$ sudo ./msg_app
[sudo] password for deeplearning:
key = 0x1e11060e
msg create ok

 msg_stat1:
current number of bytes on queue is 0
number of message in queue is 0
max number of bytes on queue is 16384
pid of last msgsnd is 0
pid of last msgrcv is 0
last msgsnd time is Thu Jan  1 08:00:00 1970
last msgrcv time is Thu Jan  1 08:00:00 1970
last change time is Wed Nov 27 10:40:41 2019
msg uid is 0
msg gid is 0

 message send ok

 msg_stat2:
current number of bytes on queue is 10
number of message in queue is 1
max number of bytes on queue is 16384
pid of last msgsnd is 3912
pid of last msgrcv is 0
last msgsnd time is Wed Nov 27 10:40:42 2019
last msgrcv time is Thu Jan  1 08:00:00 1970
last change time is Wed Nov 27 10:40:41 2019
msg uid is 0
msg gid is 0

 read from msg queue 3 bytes
 type:8, message:abc

 msg_stat3:
current number of bytes on queue is 0
number of message in queue is 0
max number of bytes on queue is 16384
pid of last msgsnd is 3912
pid of last msgrcv is 3912
last msgsnd time is Wed Nov 27 10:40:42 2019
last msgrcv time is Wed Nov 27 10:40:43 2019
last change time is Wed Nov 27 10:40:41 2019
msg uid is 0
msg gid is 0

 msg set ok

 msg_stat4:
current number of bytes on queue is 0
number of message in queue is 0
max number of bytes on queue is 16388
pid of last msgsnd is 3912
pid of last msgrcv is 3912
last msgsnd time is Wed Nov 27 10:40:42 2019
last msgrcv time is Wed Nov 27 10:40:43 2019
last change time is Wed Nov 27 10:40:44 2019
msg uid is 8
msg gid is 8

 unlink msg queue ok

程序首先生成key并创建消息队列，第1次输出消息队列的属性信息，接着发送数据，第2次输出属性信息，然后接收数据，第3次输出属性信息，再然后修改属性，第4次输出属性信息，最后删除消息队列。

注意观察结果中的时间变化以及各种ID数值的变化