NCHU System & Network Lab Lab 10 Message Queue and Shared Memory.

NCHU System & Network LabNCHU System & Network Lab

Lab 10Lab 10Message Queue and Shared MemoryMessage Queue and Shared Memory

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Message QueueMessage Queue



Message QueueMessage Queue

• provide a reasonably easy and efficient way of passing data between two unrelated processes

Process 1 process2

Message queue 1 message


Create and Access a Message Queue

#include <sys/msg.h>

int msgget(key_t key, int msgflg);key : names a particular message queue.msgflg: IPC_PRIVATE only used by process itself

IPC_CREAT create new message queue, if exist, ignore this flag

permission flags

r w x r w x r w x0 0 0

owner permission

group permission

others permission

Return queue identifier on success, -1 on failure


Send a Message to a message queue

int msgsnd(int msqid, const void *msg_ptr, size_t msg_sz, int msgflg);

msqid : message queue identifiermsg_ptr : a pointer to the message to be sent.msg_sz: size of the message pointed to by msg_ptr.

This size must not include the long int message type

msgflg: controls what happens if either the current message queue is full or the systemwide limit on queued messages has been reached

IPC_NOWAIT return -1 without sending message

If no IPC_NOWAIT, it will be suspended and waiting for space to become availablein the queue.

Return 0 on success, -1 on error.


MessageMessage

• The structure of the message must follow the following two rules.

struct my_message { long int message_type;/* The data you wish to transfer */}

1. smaller than the system limit.2. start with a long int.


retrieves messages from a message queue

int msgrcv(int msqid, void *msg_ptr, size_t msg_sz, long int msgtype, int msgflg);

msqid : message queue identifier.msg_ptr: point to the message to be received.msg_sz: size of the message pointed to by msg_ptr.msgtype: a simple form of reception priority. 0 retrieves first available message. >0 retrieves first with the same type message. <0 retrieves the same type or less than it message.

msgflg: controls what happens when no message of the appropriate type is waiting to be received IPC_NOWAIT return -1 without sending messageIf no IPC_NOWAIT, it will be suspended and waiting for an appropriate type of message arrived in queue.

return number of bytes placed in the receive buffer, -1 on error.


Control functionControl function

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

msqid: message queue identifier.command:

IPC_STAT Sets the data in the msqid_ds structure to reflect the values associated with the message queue.

IPC_SET If the process has permission to do so, this sets the values associated with the message queue to those provided in the msqid_ds data structure.

IPC_RMID Deletes the message queue.

buf: save the information of msqid.

return 0 on success, -1 on error.


msqid_dsmsqid_ds

struct msqid_ds { uid_t msg_perm.uid; uid_t msg_perm.gid; mode_t msg_perm.mode;}


Example: retrieve and showExample: retrieve and show


Example : input and sendExample : input and send


Lab ILab I• Use message queue write a program that

– Show the used message queue information• creator id and permission mode number

– Two processes can chat with each other– Just like…

uid:1 Pmode:0666P1:hello!!P2:Hi~!P2:how are you ?P1:so so.Input: _

uid:1 Pmode:0666P1:hello!!P2:Hi~!P2:how are you ?P1:so so.Input: _

Process 1 Process 2


Shared MemoryShared Memory



Shared MemoryShared Memory

• Two or multiple processes share the same memory region.

• The fastest form of IPC

Process 1 Process 2Shared memory

Two processes share the same memory region


Obtain a shared memory id.Obtain a shared memory id.

#include <sys/shm.h>

int shmget(key_t key, size_t size, int flag);

size : the size of the shared memory segment in bytes.flag :

IPC_PRIVATE only used by process itselfIPC_CREAT create new message queue, if exist, ignore this flag permission flags

return shared memory on success, -1 on error.

r w x r w x r w x0 0 0

owner permission

group permission

others permission


attachmentattachment

# include <sys/types.h># include <sys/ipc.h># include <sys/shm.h>char *shmat ( int shmid, char *shmaddr, int shmflg )

shmid: shared memory identifiershmaddr: the address at which the shared memory is to be attached to the current process. This should almost always be a null pointershmflg : SHM_RND indicates that the address specified for the second parameter

should be rounded down to a multiple of the page size. SHM_RDONLY indicates that the segment will be only read, not written.

return -1 on error, the address of the attached shared memory segment for success


detachmentdetachment# include <sys/types.h># include <sys/ipc.h># include <sys/shm.h>

int shmdt ( char *shmaddr)

return 0 on success, -1 on error.


Controlling and Deallocating Shared MControlling and Deallocating Shared Memoryemory

#include <sys/ipc.h>#include <sys/shm.h>int shmctl(int shm_id, int command, struct shmid_ds *buf);

shm_id : shared memory identifier

command:IPC_STAT copy the information about the shared memory

segment into the buffer bufIPC_SET apply the changes the user has made to the

uid, gid, or mode members of the shm_perms field.

IPC_RMID mark the segment as destroyed.

buf : save the information of shmid.Return 0 on success, -1 on error


shmid_ds structureshmid_ds structurestruct shmid_ds { struct ipc_perm shm_perm; /* operation perms */ int shm_segsz; /* size of segment(bytes) */ time_t shm_atime; /* last attach time */ time_t shm_dtime; /* last detach time */ time_t shm_ctime; /* last change time */ unsigned short shm_cpid; /* pid of creator */ unsigned short shm_lpid; /* pid of last operator */ short shm_nattch; /* no. of current attaches */ /* the following are private */ unsigned short shm_npages; /* size of segment (pages) */ unsigned long *shm_pages; struct shm_desc *attaches; /* descriptors for attaches */};


ipc_perm structureipc_perm structurestruct ipc_perm { key_t key; ushort uid; /* owner euid and egid */ ushort gid; ushort cuid; /* creator euid and egid */ ushort cgid; ushort mode; /* lower 9-bits of access modes */ ushort seq; /* sequence number */};


Example input.cExample input.c


Example output.cExample output.c


Lab ILab I

• Use shared memory write a program that– Show the information of shared memory

• pid of creator and last attach time.

– Two processes can chat with each other

pid:2 last: xxxxP1:hello!!P2:Hi~!P2:how are you ?P1:so so.

Input: _

pid:2 last: xxxxP1:hello!!P2:Hi~!P2:how are you ?P1:so so.

Input: _

Process 1 Process 2


ReferenceReference•Advanced Programming in the UNIX Environment 2nd

Author : Richard Stevens, Stephen A.Rago, Publisher : Addison-Wesley

•Beginning Linux ProgrammingAuthor : Richard Stones, Neil Matthew Publisher : Wrox

•Operating System Concepts 6th edition Author : Abraham Silberschatz, Peter Baer Galvin, Greg Gagne Publisher : John Wiley & Sons, inc.

•Google•LINUX MAN PAGES ONLINE

http://www.google.com.tw/

http://man.he.net/

NCHU System & Network Lab Lab 10 Message Queue and Shared Memory.

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