System V IPC (InterProcess Communication) Messages Queue, Shared Memory, and Semaphores

Messages Four system call –msgget( ), msgsnd( ), msgrcv( ), and msgctl( ) msgqid = msgget(key, flag) –Create a new message queue (entry) or to retrieve an existing message queue –key: a name chosen by user, represent a message queue. –Flag: such as create flag bit with different permission –Return a kernel –chosen descriptor which points to the message queue

Each message queue (or IPC entry) has a permissions structure –Pointers to the first and last messages on a linked list –The number of messages and the total number of data bytes on the linked list –The process ID of the last processes to send and receive messages –Time stamps of the last msgsnd, msgrc, and msgctl operation

msgsnd (msgqid, msg, count, flag)

count = msgrcv(id, msg, maxcount, type, flag) –Id: message descriptor –Msg: the address of a user structure (message) –Maxcount: the size of the msg –Type: the message type that user wants to read –Flag: specifies what the kernel should do if no message are on the queue –Count: the number of bytes returned to the user Msgctl(id, cmd, mstatbuf) –Query the status of a message descriptor, set its status, and remove a message queue

Shared Memory Shmget: create a new region of shared memory or returns an existing one –Shmid = shmget (key, size, flag) Shmat: logically attacheds a shared memory to the virtual address space of a process –Virtaddr = shmat (id, addr, flags) Shmdt: detaches a shared memory from the virtual address –Shmdt(addr) Shmctl: manipulate various parameters associated with the shared memory –Shmctl9id, cmd, shmstatbuf)

Semaphores Id=semget (key, nsems, flag) –nsems: represent the number of the semaphore element

Oldval = Semop(id, oplist, nsops) –Oplist: point to an array of semaphore operation include sem_num, operation (positive or negative), flag –Nsops: indicate the size of operation array –Oldval: the value of the semaphore before operation The kernel change the value of a semaphore according to the value of the operation –Operation = 0, if element is 0, continue, else sleep –Operation is positive: increase the semaphore, awaken all process that are waiting the semaphore –Operation is negative: if operation + element > 0, element – operation, and continue If operation + element < 0, sleep If operation + element = 0, wakeup a process which wait the element to 0

Semctl(id, number, cmd, arg) –Number: the number of semaphore elements required to do the cmd operation –Cmd: retrieve or set control parameters (permissions and others) –Set one or all semaphore values in a set –Read the semaphore values –Arg: is interpreted based on the value of cmd. Union semunion{ int val; struct semid_ds *semstat; unsigned short *array; }arg;

The program is a.out User executes it three times In the following sequence: a.out & a.out a & a.out b &

Undo flag Dangerous situations could occur –If a process does a semaphore operation, locking some resource, and then exit without resetting the semaphore value (ex: program error, receipt a signal, sudden termination, core down) To avoid such problems –Process can set the SEM_UNDO flag in the semop call; when it exits, the kernel reverses the effect of every semaphore operation the process has done.

ptrace(cmd, pid, addr, data) Cmd: read/write data,.. Addr: a virtual address to be read/written in the traced process Data: an integer value to be written (write to addr) P42: how do system calls actually work? –Use interrupt 0x80 which provides the argument of sys_call_num and sys_call_args –If the process which PF_TRACESYS is set (means traced process), the system call sends a SIGTRAP signal to the parent process and calls the scheduler. The traced process is interrupted until the parent process reactivates it (the ptrace() system call is called again)

pid = Fork() system call –Only way for a user to create a new process –The return of the fork system call In the parent process, pid is the child process ID In the child process, pid is 0 –Kernel does the following sequence of operations for fork Allocate a slot in the process table for the new process Assign a unique ID number to the child process Make a logical copy of the context of the parent process (text region is shared between the parent and child process). Increment file and inode table counters for files associated with the process Return the ID number of the child to the parent process, and a 0 value to the child process

Process tracing Debugger forks a child process Child process invokes the ptrace system call thus kernel sets a trace bit in the child process table entry The child process execs the traced program The kernel executes the exec call –Due to the trace-bit is set, the kernel sends the “trap” signal to the parent which wake up the parent from the wait call. The parent call ptrace to do the read/write operation of the traced process The traced process wakeup and do the operation of ptrace call and continue execution until execute the system call