07 - Message Queues

Question 1

Explain the concept of message queues (MQ).

Answer 1

• Link between producer and consumer is indirect
  • Producer writes messages on the queue without selecting the consumer
• The consumer retrieves a message from the queue, without selecting a producer
• Writings are unblocking as long as resources are available
• Reading is blocking when the queue is empty
• A Message Queue is a linked list of message structures stored inside the kernel’s memory space and accessible by multiple processes
• Synchronization is provided automatically by the kernel
• Preallocated message buffers
• Messages with priority. A message with a higher priority is always received first.
• Send and receive functions are synchronous by default. Possibility to set a wait timeout to avoid nondeterminism.
• Support asynchronous delivery notifications.

Question 2

What are the programming steps for MQ?

Answer 2

• Define message structure
• Create a message queue
• Connect to queue
• Read/ Write messages
• Close queue
• Destroy queue

Question 3

What are some concerns necessary to consider when defining the message structure?

Answer 3

• Multiple processes should agree on the structure/size
  • Application level
  • Message queues handle different sizes
• The programmer should define a structure
  • With pre-defined size
  • Able to accommodate multiple sub-types

Question 4

Give an example of an System V (SV) MQ message structure.

Answer 4

struct mymsg{
    long msg_type;
		char mytext[512]; /* rest of the message*/
		int something_else;
}

• msg_type used in reception

Question 5

Explain the process of creation, opening, write, read and destruction for an SV MQ.

Answer 5

Creation

• msgget- get a SV MQ identifier - int msgget(key_t key, int msg flg);
• Create private key - IPC_PRIVATE. Hinerited by child processes.
• Create public key - not in use (ipcs). msgflag - IPC_CREAT. Verify if it exists: msgflag - O_CREAT| O_EXCL

Opening

• Open a public MQ - key - already creates MQ - msgflag is NULL

Write

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

• Writes a message to the queue
• Parameters
  
  • msqid - queue id (returned from msgget)
  • Message + size
  • msgflag - IPC_NOWAIT

Read

ssize_t msgrcv(int msqid, const void *msgp, size_t msgsz, long msgtyp, int msgflg);

• Reads a message from the queue
• Parameters
  
  • msqid - queue id (returned from msgget)
  • Pointer to buffer + max size
  • Type of message
  • msgflag - IPC_NOWAIT, MSG_COPY(does not remove message)

Destruction

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

• msqid
• cmd - IPC_RMID
• msqid_ds- NULL

Question 6

Describe the POSIX MQ message structure.

Answer 6

• Array of bytes
• Priority/message selection
  
  • API
• Each message has an associated priority
• Messages are always delivered to the receiving process with the highest priority first
• Message priorities range
  
  • From 0 (low) to sysconf(_SC_MQ_PRIO_MAX) - 1 (high)
  • On linux, the max is 32768 - 1
  • POSIX.1 requires a range from 0 to 31

Question 7

Explain the process of creation, opening, write and read for a POSIX MQ.

Answer 7

Creation

• mq_open - open a message queue

mqd_t mq_open(const char * name, int oflag, mode_t mode, struct mq_attr * attr);

• name - identifier
• oflags - O_CREAT| O_RDONLY | O_WRONLY | O_RDWR
• mode - file access modes
• attr- NULL, struct mq_attr queue _attr;

Opening

• Default settings: name- identifier, oflags - O_RDONLY | O_WRONLY | O_RDWR
• Creates - O_CREAT - message queue is assigned to a file queue in /dev/msgque, where the file name is used by other processess
• mq_unlink - removes a message queue and deletes the file
• mq_close - closes a message queue descriptor and process can no longer use the queue

Write

int mq_send(mqd_t mqdes, const char *msg_ptr, size_t msg_len, unsigned int msg_prio);

• Writes a message to the queue
• Parameters
  
  • mqdes - queue id (returnedf from mq_open)
  • Message + size
  • msg_priority - used in mq_receive

Read

ssize_t mq_receive(mqd_t mqdes, char *msg_ptr, size_t msg_len, unsigned int *msg_prio);

• Reads a message from the queue
  
  • mqdes - queue id (returned from mq_open)
  • Message + buffer size
  • msg_priority - used in mq_receive
  • Messages are always delivered to the receiving process highest priority first
  • msg_priority - NULL or stores the priority types of the received message

Question 8

What will happen in read/write for empty and full queues?

Answer 8

Empty queue

• Receive call blocks
• mq_timedreceive- block some time

Full queue

• Send blocks
• mq_timedsend - blocks some time

Will lead to errno ETIMEDOUT if timeout is reached.

Question 9

What are the three ways to limit POSIX MQ?

Answer 9

On the use program - queue_attr.mq_maxmsg and queue_attr.mq_msgsize

Change directly on /etc/security/limits.conf

Values delimited by the OS - /proc/sys/fs/mqueue

Question 10

Present the characteristics of message queues.

Answer 10

• Implementation - Kernel/syscall
• Scope - local
• No duples
• Time-uncoupling
• Space-uncoupling
• Explicit
• Synchronization - Yes (reads) no (writes)
• Process relation - unrelated
• Identification - string
• API - specific API