Linux Message Queuing practices (2)

Message Queuing functions

#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 function

function: Get/Set Message queue information

Prototype:

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

Parameters:

MSQID: Message Queue identification code returned by the Msgget function

CMD: Is the action that will be taken (see below)

return value:

Successful return 0, failure return-1

CMD: The action to be taken (there are three desirable values), respectively, as follows:

message queue data structure

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) */< c14/>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 Data Structure

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 */};

Practice: Ipc_statint 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 in octal print    cout << "buf.__msg_cbytes =" << buf.__msg_cbytes << Endl;    cout << "buf.msg_qbytes =" << buf.msg_qbytes << Endl;    cout << "buf.msg_lspid =" << buf.msg_lspid << Endl;}

Practice: Ipc_set, generally need to get first, then set int main () {    int msgid = Msgget (0x1234, 0666);    if (MsgId = =-1)    {        err_exit ("Msgget error");    }    Gets the properties of the message queue    struct Msqid_ds buf;    if (Msgctl (msgid,ipc_stat,&buf) = =-1)    {        err_exit ("Msgctl get Error");    }    Set the properties of Message Queuing    buf.msg_perm.mode = 0644;    if (Msgctl (msgid,ipc_set,&buf) = =-1)    {        err_exit ("Msgctl SET error");    }    Get and print    if (msgctl (msgid,ipc_stat,&buf) = =-1)    {        err_exit ("Msgctl get Error");    }    printf ("Buf.msg_perm.mode =%o\n", buf.msg_perm.mode);   %o Print in Octal}

Practice: ipc_rmid, deleting Message Queuing/** Description: You can test by running several of the programs on multiple windows: Message Queuing does not use the "reference count" function!*/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;}        }    }

sending and receiving of messages

msgsnd function

Function: Add a message to the message queue

Prototype

int msgsnd (int msqid, const void *MSGP, size_t msgsz, int msgflg);

Parameters

MSGID: Message Queue identification code returned by the Msgget function

MSGP: is a pointer to a message ready to be sent,

MSGSZ: Is the length of the message that the MSGP points to, which does not contain the long int length integer that holds the message type

MSGFLG: Controls what is going to happen when the current message queue is full or the system upper limit is reached

return value:

Successful return 0, failure return-1

Msgflg=ipc_nowait indicates that the queue is full without waiting and returns a Eagain error.

The message structure is constrained in two ways. First, it must be less than the upper limit set by the system ; second, it must start with a long int, and the receiver function will use this long integer to determine the type of the message.

The message structure reference form is as follows:

struct msgbuf{    long mtype;       /* Message type, must be > 0 */    char mtext[1];    /* Message data */};

Practice/** Send Structure */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");    }    Initializes the message structure of    struct msgbuf mybuffer;    Mybuffer.mtype = 1;    strcpy (Mybuffer.mtext, "Hello xiaofang!");    Send message to Message queue    if (msgsnd (Msgid,&mybuffer,strlen (Mybuffer.mtext), ipc_nowait) = =-1)    {        Err_exit (" MSGSND error ");    }    return 0;}

MSGRCV function

Function: A message is received from a message queue

Prototype

ssize_t MSGRCV (int msqid, void *msgp, size_t msgsz, long Msgtyp, int msgflg);

 

Parameters

MSGID: Message Queue identification code returned by the Msgget function

MSGP: is a pointer to a message ready to be received,

MSGSZ: Is the length of the message that the MSGP points to, which does not contain the long int length integer that holds the message type

Msgtype: It can achieve a simple form of receiving priority

MSGFLG: Controls What happens when there are no corresponding types of messages in the queue to receive

return value:

Succeeded-Returns the number of bytes actually put into the receive buffer; failure---1

Msgtyp
Msgtyp=0	Returns the first message of the queue
Msgtyp>0	Returns a message with the first type of the queue equal to Msgtype
Msgtyp<0	Returns a message with the first type of a queue that is less than or equal to the absolute value of Msgtype, and is the smallest message type that satisfies the condition

　

Msgflg
Msgflg=ipc_nowait	The queue does not have a readable message and does not wait to return a enomsg error.
Msgflg=msg_noerror	Truncation when message size exceeds MSGSZ
Msgtyp>0 and Msgflg=msg_except	The first message that the receive type is not equal to Msgtype

Practice: Message sent int main () {    int msgid = Msgget (0x1234,0666| Ipc_creat);    if (MsgId = =-1)    {        err_exit ("Msgget error");    }    struct MSGBUF mybuffer;    for (int i = 0; i < ++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;}

Practice: Message Reception: The data is constantly taken from the head of the team int main () {    int msgid = Msgget (0x1234,0666);    if (MsgId = =-1)    {        err_exit ("Msgget error");    }    From the team's first continuous data fetching, 10    struct msgbuf mybuffer;    for (int i = 0; i < ++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 Message Queuing practices (2)