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

NCHU System & Network Lab Message Queue

NCHU System & Network Lab Message Queue provide a reasonably easy and efficient way of passing data between two unrelated processes Process 1 process2 Message queue 1 message

NCHU System & Network Lab Create and Access a Message Queue #include 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 rwxrwxrwx000 owner permission group permission others permission Return queue identifier on success, -1 on failure

NCHU System & Network Lab Send a Message to a message queue int msgsnd(int msqid, const void *msg_ptr, size_t msg_sz, int msgflg); msqid : message queue identifier msg_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_NOWAITreturn -1 without sending message If no IPC_NOWAIT, it will be suspended and waiting for space to become available in the queue. Return 0 on success, -1 on error.

NCHU System & Network Lab Message 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.

NCHU System & Network Lab 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. 0retrieves first available message. >0retrieves first with the same type message. <0retrieves 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_NOWAITreturn -1 without sending message If 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.

NCHU System & Network Lab Control 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.

NCHU System & Network Lab msqid_ds struct msqid_ds { uid_t msg_perm.uid; uid_t msg_perm.gid; mode_t msg_perm.mode; }

NCHU System & Network Lab Example: retrieve and show

NCHU System & Network Lab Example : input and send

NCHU System & Network Lab Lab 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:0666 P1:hello!! P2:Hi~! P2:how are you ? P1:so so. Input: _ uid:1 Pmode:0666 P1:hello!! P2:Hi~! P2:how are you ? P1:so so. Input: _ Process 1Process 2

NCHU System & Network Lab Shared Memory

NCHU System & Network Lab Shared Memory Two or multiple processes share the same memory region. The fastest form of IPC Process 1Process 2 Shared memory Two processes share the same memory region

NCHU System & Network Lab Obtain a shared memory id. #include 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 itself IPC_CREAT create new message queue, if exist, ignore this flag permission flags return shared memory on success, -1 on error. rwxrwxrwx000 owner permission group permission others permission

NCHU System & Network Lab attachment # include char *shmat ( int shmid, char *shmaddr, int shmflg ) shmid: shared memory identifier shmaddr: the address at which the shared memory is to be attached to the current process. This should almost always be a null pointer shmflg : SHM_RNDindicates that the address specified for the second parameter should be rounded down to a multiple of the page size. SHM_RDONLYindicates that the segment will be only read, not written. return -1 on error, the address of the attached shared memory segment for success

NCHU System & Network Lab detachment # include int shmdt ( char *shmaddr) return 0 on success, -1 on error.

NCHU System & Network Lab Controlling and Deallocating Shared Memory #include int shmctl(int shm_id, int command, struct shmid_ds *buf); shm_id : shared memory identifier command: IPC_STATcopy the information about the shared memory segment into the buffer buf IPC_SET apply the changes the user has made to the uid, gid, or mode members of the shm_perms field. IPC_RMIDmark the segment as destroyed. buf : save the information of shmid. Return 0 on success, -1 on error

NCHU System & Network Lab shmid_ds structure struct 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 */ };

NCHU System & Network Lab ipc_perm structure struct 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 */ };

NCHU System & Network Lab Example input.c

NCHU System & Network Lab Example output.c

NCHU System & Network Lab Lab 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: xxxx P1:hello!! P2:Hi~! P2:how are you ? P1:so so. Input: _ pid:2 last: xxxx P1:hello!! P2:Hi~! P2:how are you ? P1:so so. Input: _ Process 1Process 2

NCHU System & Network Lab Reference Advanced Programming in the UNIX Environment 2nd Author : Richard Stevens, Stephen A.Rago, Publisher : Addison-Wesley Beginning Linux Programming Author : Richard Stones, Neil MatthewPublisher : Wrox Operating System Concepts 6th edition Author : Abraham Silberschatz, Peter Baer Galvin, Greg Gagne Publisher : John Wiley & Sons, inc. Google LINUX MAN PAGES ONLINE