Lecture 20 CSCE 510 - IPC

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Qt Designer
Review
- process mask
- sigaction
InterProcess Communications II
- pipes, fifos
- locks
- message passing
- semaphores
- shared memory
- BSD sockets
- client-server model
FIFOs
fifo is a named pipe
created with system call
```
mknod(path, mode)
mkfifo(path, mode)
```
S_IFIFO (stat.h) added to mode to get fifo
after creation, stays until unlinked
after creation, opened just as if file
created with command
```
/etc/mknod name p
mkfifo name
```

ls -l flags fifo's with a `p'

$ /etc/mknod myfifo p
$ ls -l myfifo
prw-rw-r--  matthews    fac ... myfifo
$ wc myfifo &
$ ls -l > myfifo
123 32 3

Also could use mkfifo.

[Creation]

    #include stat.h
    if(mkfifo("myfifo",0666) < 0)
       perror(...)

[Opening]

    #include fcntl.h
    if((fd = open("myfifo", O_RDONLY)) < 0)
       perror(...)

[Opening]

[no delay]

    #include fcntl.h
    if((fd=open("my",O_RDONLY | O_NDELAY))<0)
       perror(...)

\end{itemize}

Lock Files
- there is a "lock file" associated with each data file
- if the lockfile exists then cannot access data file
- if the lockfile does not exist then can access data file
- Check if lockfile exists
```
fd = open("some.lock", O_WRONLY|O_CREAT|O_EXCL, 0644)
if (fd < 0 && eerno == EEXIST)  ... /*"file locked"*/
else if (fd < 0 )  ... /*file opening error*/
else{ /* I got the file */
```
- Record Locking - The Problem
  - in Unix multiple processes can have the same file open
  - this presents problems if there are multiple processes that can update
- Example: Class Roll for CSCE 590
```
The file csce 590 has the number of empty slots and the roll for the class
csci object oriented programming.
```
  What happens when there are two editor sessions open with the same file?
- The lockf system call
```
#include unistd.h

     int lockf(int fildes, int function, off_t size);
```
- IPC Facility Keys
  - IPC Keys are used by the System V IPC communication routines to uniquely identify an IPC object much in the way a pathname identifies a file.
  - key_t ``key type'' defined in stdio.h
- ftok - non-standard library function that maps path to a key
- The function Ftok
- [Header Files:] sys/types.h sys/ipc.h
- [Parameters:]
  - [] char *pathname;
  - [] char id;
- [USAGE:] keyval = ftok(path, id);
- The function Ftok
- IPC / IO Anology {\bf Notes on FTOK}
  - The id is an additional factor so that there can be more than 1 key assoc iated with a file.
  - returns (key\_t) -1 if the path does not exist
  - if one deletes and then puts back the file key values may be different
  \newpage {\bf IPC / IO Anology} IPC keys are used:
  - to create IPC objects,
  - to gain access to existing IPC objects.
- System V IPC objects:
  1. msg_queues
  2. semaphores
  3. shared memory segments
  4. maintained in system wide table
- Semaphores- Semaphores were introduced by Edgar Dijkstra to provide a mechanism for the synchronization of processes.
```
p(sem)  or wait operation

if (sem != 0)
   decrement sem by one
else
   wait until sem becomes non-zero
```
- v(sem) or signal operation
```
if (queue of waiters non-empty)
   restart first process in the queue
else
   increment sem by one
```
- Critical Sections
- Mutual Exclusion
- msgqueue calls
  - int msgget(key_t key, int msgflg);
  - int msgrcv (msqid, msgp, msgsz, msgtyp, msgflg)
  - int msgsnd (msqid, msgp, msgsz, msgflg)
  - int msgctl(int msqid, int cmd, struct msqid_ds, buf)
- Removing an ipc object
  - ipcs - "ipc ls"
  - ipcrm -q fid
  - msgctl(shmid, IPC_RMID, 0, 0)
[Readings:]
Chapter 12 for file locking
[Assignment:]

[Reminders:]
Test Nov 16

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