IPC Message Queue Data Structure msgque IPNOUSED Two

IPC Message Queue

Data Structure msgque IP_NOUSED

Two channels of Message queue • channel between normal processes – IPC_KERNELD not set • channel between kernel and kernel daemon – kernel daemon：a process which automatically carries out loading and removing of modules. – IPC_KERNELD set

Message • kerneld_msg{ – mtype – id – version header – pid – text[1]} • msgbuf{ – mtype – mtext[1]}

sys_msgget() • allocate a new message queue, or gain access to an existing message queue. • Flags – IPC_CREAT：a new queue will be created if it is not yet present=>newque() – IPC_EXCL：If IPC_CREAT is set and such a queue exists, the function will return with the error EEXIST.

sys_msgctl() – IPC_INFO：enters special values in the msginfo structure. – IPC_STAT：return current values of msqid_ds – IPC_SET：modify values of msqid_ds – IPC_RMID：remove all associated message queue structure

sys_msgsnd() • Send message to message queue. • The process blocks if message length exceeds the value in msg_qbytes, the permitted maximum. • Blocking can be prevented by setting IPC_NOWAIT.

sys_msgrcv() • receive messages of specified type from message queue. – msgtyp = 0 => get first. – msgtyp > 0 => get first message of matching type. (MSG_EXCEPT not set) – msgtyp < 0 => get message with least type must be < abs(msgtype).

Message queue between kernel & kerneld • IPC_KERNELD set • kerneld_send() – responsible for sending messages to the kernel daemon. – KERNELD_WAIT set if we want an answer from kerneld. • kerneld_exit() – called when current process terminated.

IPC PIPE

Data Structure

do_pipe • Called by system call pipe • set up a temporary inode and allocating a page of memory to base. • Return two file descriptors, one for reading, and one for writing.

Open Pipe • pipe_read_open() • pipe_write_open() • pipe_rdwr_open()

Pipe_Select() • To check whether data can be read from pipe or written to it. • sel_type: – SEL_IN：for read – SEL_OUT：for write – SEL_EX：for exception

Pipe read operation Blocking Non-Blocking Locked Block calling EAGAIN pipe process Empty Block calling process EAGAIN if pipe if writing processes present, else return 0 present, else 0 Else Read maximum number of characters up to requested position.

Pipe write operation no reading process locked pipe atomic write possible, but not enough space in pipe buffer Blocking Non-blocking Send SIGPIPE to writing process and return EPIPE block calling process EAGAIN

Pipe/FIFO write op(cont. ) Buffer space sufficient for atomic write Else Blocking Non-blocking write requested number of bytes to buffer continue blocking until requested number of bytes has been written. as for blocking write. Write maximum Possible number of bytes.

Release pipe • pipe_read_release() • pipe_write_release() • pipe_rdwr_release()

Other operation • pipe_ioctl() – FIONREAD：write the number of bytes within the file not yet read to address arg. • pipe_lseek()：invalid.