Message Queue for multi-thread synchronization in Linux

A message queue is a linked list of messages stored in the kernel with the message queue identifier.

Msgget is used to create a new queue or open an existing queue. Msgsnd adds a new message to the Message Queue. Each message includes a long type and a message cache. msgrcv is used to retrieve messages from the queue. It is intelligent to retrieve messages, not necessarily FIFO.

① Msgget: Create a New queue or open an existing queue

# Include

Int msgget (key_t key, int flag );

// Message Queue ID is returned for success;-1 is returned for Error

② Msgsnd: Send a message

# Include

Int msgsnd (int msgid, const void * ptr, size_t nbytes, int flag)

// 0 is returned for success and-1 is returned for Error

A: flag can be specified as IPC_NOWAIT. If the message queue is full, msgsnd returns EABAIN immediately after an error occurs;

If IPC_NOWAIT is not specified, msgsnd is blocked until the message queue has space.

③ Msgrcv: read the message:

Ssize_t msgrcv (int msgid, void * ptr, size_t nbytes, long type, int flag );

A. type = 0; returns the first message in the message queue.

B. type> 0 return the first message of tpye type in the message queue.

C. type <0 returns the data of type <= | type | in the message queue. If there are several types of messages, messages with the smallest type value are obtained.

To create a message queue, follow these steps:

# Define MSG_FILE "."

Struct msgtype {

Long mtype;

Char buffer [BUFFER + 1];

};

If (key = ftok (MSG_FILE, 'A') =-1)

{

Fprintf (stderr, "Creat Key Error: % s \ n", strerror (errno ));

Exit (1 );

}

If (msgid = msgget (key, IPC_CREAT | 0666/* PERM */) =-1)

{

Fprintf (stderr, "Creat Message Error: % s \ n", strerror (errno ));

Exit (1 );

}

Msg. mtype = 1;

Strncpy (msg. buffer, argv [1], BUFFER );

Msgsnd (msgid, & msg, sizeof (struct msgtype), 0 );

Msgrcv (msgid, & msg, sizeof (struct msgtype), 1, 0 );

Sample Code:

# Include

# Include

# Include

# Include

# Include

# Include

# Include

# Include

# Include

# Define MSG_FILE "."

# Define BUFFER 255.

# Define PERM S_IRUSR | S_IWUSR

# Define IPCKEY 0x111

Struct msgtype {

Long mtype;

Char buffer [BUFFER + 1];

};

Void * thr_test (void * arg ){

Struct msgtype msg;

Int msgid;

Msgid = * (int *) arg );

Printf ("msqid = % d IPC_NOWAIT = % d \ n", msgid, IPC_NOWAIT );

Time_t tt = time (0) + 8;

// While (time (0) <= tt)

//{

Msgrcv (msgid, & msg, sizeof (struct msgtype), 1, 0 );

Fprintf (stderr, "Server Receive: % s \ n", msg. buffer );

Msg. mtype = 2;

Msgsnd (msgid, & msg, sizeof (struct msgtype), 0 );

//}

Pthread_exit (void *) 2 );

}

Int main (int argc, char ** argv)

{

Struct msgtype msg;

Key_t key;

Int msgid;

Pthread_t tid;

If (argc! = 2)

{

Fprintf (stderr, "Usage: % s string \ n", argv [0]);

Exit (1 );

}

/*

Char path [256];

Sprintf (path, "% s/", (char *) getenv ("HOME "));

Printf ("path is % s \ n", path );

Msgid = ftok (path, IPCKEY );

*/

If (key = ftok (MSG_FILE, 'A') =-1)

{

Fprintf (stderr, "Creat Key Error: % s \ n", strerror (errno ));

Exit (1 );

}

If (msgid = msgget (key, IPC_CREAT | 0666/* PERM */) =-1)

{

Fprintf (stderr, "Creat Message Error: % s \ n", strerror (errno ));

Exit (1 );

}

Pthread_create (& tid, NULL, thr_test, & msgid );

Fprintf (stderr, "msid is: % d \ n", msgid );

Msg. mtype = 1;

Strncpy (msg. buffer, argv [1], BUFFER );

Msgsnd (msgid, & msg, sizeof (struct msgtype), 0 );

Exit (0 );

}