Linux IPC Message Queuing

1. Posix Message Queuing

/ * Mq_open-open a message queue * /
#include <fcntl.h>/*For O_* Constants*/#include<sys/stat.h>/*For Mode Constants*/#include<mqueue.h>mqd_t Mq_open (Const Char*name,intoflag); mqd_t Mq_open (Const Char*name,intOflag, mode_t mode,structMq_attr *attr);
/ * mq_send, mq_timedsend-send a message to a message queue */#include<mqueue.h>intMq_send (mqd_t mqdes,Const Char*msg_ptr, size_t msg_len, unsigned msg_prio); #include<time.h>#include<mqueue.h>intMq_timedsend (mqd_t mqdes,Const Char*msg_ptr, size_t msg_len, unsigned msg_prio,Const structTimespec *abs_timeout);
/ * mq_receive, mq_timedreceive-receive a message from a message queue */#include<mqueue.h>ssize_t mq_receive (mqd_t mqdes,Char*msg_ptr, size_t msg_len, unsigned*Msg_prio); #include<time.h>#include<mqueue.h>ssize_t mq_timedreceive (mqd_t mqdes,Char*msg_ptr, size_t msg_len, unsigned*Msg_prio,Const structTimespec *abs_timeout);
/* mq_getattr, Mq_setattr-get/set message Queue attributes */
#include <mqueue.h>intMq_getattr (mqd_t mqdes,structMq_attr *attr);intMq_setattr (mqd_t mqdes,structMq_attr *Newattr,structMq_attr *oldattr);structmq_attr {LongMq_flags;/*flags:0 or O_nonblock*/               Longmq_maxmsg;/*Max. # of messages on queue*/               LongMq_msgsize;/*Max. Message size (bytes)*/               LongMQ_CURMSGS;/*# of messages currently in queue*/ };
/* Mq_notify-register for notification if a message is available */
intMq_notify (mqd_t mqdes,Const structSigevent *SEVP);

/* Mq_unlink-remove a message Queue */
int mq_unlink (const char *name); Link with-lrt.

POSIX Message Queuing considerations

1. Mq_setattr can only modify mq_flags to be blocked or non-blocking, other parameters can only be specified by Mq_open when the message queue is established

2. Mq_receive returns the oldest message of the highest priority in the queue each time

3. Mq_notify supports registering a signal or thread, sending information when a message is added to an empty queue , or activating a thread

2. System V Message Queuing

/*msgget-get a System V message queue identifier*/#include<sys/types.h>#include<sys/ipc.h>#include<sys/msg.h>intMsgget (key_t Key,intMSGFLG);/*Ftok-convert a pathname and a project identifier to a System V IPC key*/#include<sys/types.h>#include<sys/ipc.h>key_t Ftok (Const Char*pathname,intproj_id);/*MSGRCV, Msgsnd-system V message Queue operations*/#include<sys/types.h>#include<sys/ipc.h>#include<sys/msg.h>intMSGSND (intMsqid,Const void*MSGP, size_t Msgsz,intmsgflg); ssize_t MSGRCV (intMsqid,void*MSGP, size_t Msgsz,LongMsgtyp,intMSGFLG);/*msgctl-system V Message control operations*/#include<sys/types.h>#include<sys/ipc.h>#include<sys/msg.h>intMsgctl (intMsqid,intCmdstructMsqid_ds *buf);structMsqid_ds {structIpc_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*/unsignedLong__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)*/};

System v Message Queuing considerations

1. The flag parameter of the msgsnd can be specified as ipc_nowait. The IPC_NOWAIT flag makes this msgsnd operation non-blocking: if there is no storage space for new messages, the function immediately returns a Eagin error, as in the case of the following:

A. There are already too many bytes in the message queue (corresponding to msg_qbytes in the struct Msqid_ds)

B. There are too many messages in the system scope

If one of these two conditions occurs, and msgsnd does not specify the IPC_NOWAIT flag, the calling thread is put to sleep until:

A. Have space to store new messages

B. The specified message queue is eliminated (in this case msgsnd returns a ermid error)

C. The calling thread is interrupted by a captured signal (in which case MSGSND returns a eintr error)

2. The flag parameter of the MSGRCV can also be specified as ipc_nowait, and there is another flag msg_noerror: When the received message data portion is greater than the length parameter of MSGRCV, if Msg_ is set NoError, the data is truncated, the MSGRCV function is not faulted, and if the flag is not set, the MSGRCV function returns a e2big error.

3. The msgctl cmd can be evaluated as follows:

Ipc_rmid: Delete the specified queue

Ipc_set: Supports setting msg_prem.uid, Msg_prem.gid, Msg_prem.mode and msg_qbytes in the MSGID_DS structure of the specified queue

Ipc_stat: Returns the MSGID_DS structure of the specified queue

4. Limitations of system V Message Queuing

You can view the Msgmax Msgmni under the proc/sys/kernel/folder MSGMNB

You can also view it through the ipcs-l command:

Ubuntu:ipcs-L------Shared Memory Limits--------Max Number of segments=4096Max seg Size (Kbytes)=32768Max Total Shared memory (Kbytes)=8388608min seg size (bytes)=1------Semaphore Limits--------Max Number of arrays= -max semaphores per array= -Max semaphores system wide=32000Max Ops per semop call= +semaphore max Value=32767------Messages Limits--------max queues System wide=1717 System-wide maximum number of message queues: Msgmnimax size of message (bytes)=8192 maximum number of bytes per message: MsgmaxdefaultMax size of queue (bytes) =16384 Maximum number of bytes per message queue: MSGMNB

Linux IPC Message Queuing