Lab 10 Message Queue and Shared Memory

1. Lab 10Message Queue and Shared Memory

2. Message Queue

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

4. 0 0 0 r w x r w x r w x owner permission group permission others permission 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 Return queue identifier on success, -1 on failure NCHU System & Network Lab

5. 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_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. NCHU System & Network Lab

6. 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 */ } • smaller than the system limit. • start with a long int. NCHU System & Network Lab

7. 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 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

8. 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

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

10. Example: retrieve and show NCHU System & Network Lab

11. Example : input and send NCHU System & Network Lab

12. 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 1 Process 2 NCHU System & Network Lab

13. Shared Memory

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

15. 0 0 0 r w x r w x r w x owner permission group permission others permission 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 itself IPC_CREAT create new message queue, if exist, ignore this flag permission flags return shared memory on success, -1 on error. NCHU System & Network Lab

16. attachment # include <sys/types.h> # include <sys/ipc.h> # include <sys/shm.h> 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_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 NCHU System & Network Lab

17. detachment # include <sys/types.h> # include <sys/ipc.h> # include <sys/shm.h> int shmdt ( char *shmaddr) return 0 on success, -1 on error. NCHU System & Network Lab

18. Controlling and Deallocating Shared Memory #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 buf IPC_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 NCHU System & Network Lab

19. 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

20. 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

21. Example input.c NCHU System & Network Lab

22. Example output.c NCHU System & Network Lab

23. 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 1 Process 2 NCHU System & Network Lab

24. Reference • 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 NCHU System & Network Lab