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msgque use software pdf417 creator toattach pdf417 with software itf barcode msg_perm msg_ rst msg_last msg_next msg_spot msg_next msg_spot msg_next msg_spot msg_perm msg_ rst msg_last msg_perm msg_ rst msg_last msg_next msg_spot msg_next msg_spot msg_perm msg_ rst msg_last active message headers array of msqds_id structures msgmap m_size m_addr m_size m_addr m_size m_addr m_size m_addr m_size m_addr pool of kernel memory for messages msgh msg_next msg_spot ...




msg_next msg_spot msg_next msg_spot free pool of message headers Figure 10.2 System V Message Queue Structures System V Message Queues 10.4.2 Kernel Implementation of Message Queues We ll walk through the kernel ow involved in the creation of a message queue and the sending and receiving of messages, since these represent the vast majority of message queue activities. The creation of a message on behalf of an application calling the msgget(2) system call starts with a call to the kernel ipcget() routine. An ipc_perm structure will be available for every message queue identi er (msgmni).

Once a structure has been allocated, the system initializes the structure members as speci ed by the UID and GID of the calling process and the permission mode bits passed by the calling code, then sets the IPC_ALLOC bit to signify the ipc_perm structure has been allocated. (The ipcget() code is the same as illustrated previously it s the same function! so we won t revisit it here.) If ipcget() returns successfully, the application code has a valid message queue identi er, can send and receive messages, and can run message control (msgctl(2)) operations.

A message send (msgsnd(2)) call requires the application to construct a message, setting a message type eld and creating the body of the message (e.g., a text message).

When the code path for the message send kernel support code is rst entered, the code does some general housekeeping such as incrementing the processor statistics to announce a message queue system call is being executed. The cpu_sysinfo structure maintains a msg counter that re ects the total number of message queue system calls executed. The code veri es the calling process s access permissions to the message queue and tests the message size against the msgmax tunable.

Next, the code copies the message type eld from the user address space to a designated area in the kernel.. Interprocess Communication The rest of the message send ow is best represented in pseudocode. if (message queue no longer e barcode pdf417 for None xists) return EIDRM error if (current bytes on queue + bytes in new msg > msgmax) if (IPC_NOWAIT is set) return EAGAIN else set MSGWAIT flag in msqid_ds.msg_perm.mode call cv_wait_sig() /* put the caller to sleep until space is available on the queue */ /* on wakeup, code will validate msqid, and set EDIRM if itUs been removed */ allocate space for message from resource map (msgmap) if (space not available) if (IPC_NOWAIT) return EAGAIN error else call cv_wait_sig() /* sleep waiting for space */ /* once the wakeup is issued, the necessary resources are available for putting the message on the queue */ copy message data from user space to kernel space (map area) increment msqid_ds.

qnum msqid_ds.msg_cbytes += new message size msqid_ds.mds_lspid = PID of caller msqid_ds.

msg_stime = current time update message header msg_type, msg_ts (text size) and msg_spot (map location pointer) adjust the queue pointers (msg_first, msg_next, msg_last) return success to calling program. The msgrcv support code is a little less painful, since now we re looking for a message on the queue (as opposed to putting one on the queue). Kernel resources do not need to be allocated for a msgrcv. The general ow of the kernel code path for receiving messages involves checking permissions for operation in a loop through all the messages on the queue.

If the requested message type matches a message on the queue, the code copies the message type to the user-supplied location and copies the message data to the user-supplied location. Next, the code updates the msqid_ds structure elds, subtracts the message size from msg_cbytes, sets PID in msg_lrpid, sets time in msg_rtime, frees the message resources, frees the message header (msg structure), and frees the resource map entry. If the code looped through all messages and found no matching type, it returns a No Message error.

When the sequence is completed, the application code will have the message type and data in a buffer area supplied in the msgrcv(2) system call. The only remaining callable routine for applications to use is the msgctl(2) system call. The control functions are straightforward; they typically involve either retrieving or setting values in a message queue s ipc_perm structure.

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