psem

class psem : public pipc

psem is a quick and dirty encapsulation of the system V sem utility

Inheritance:

Public Methods
int add(int isem, int ival, int mode = DEFAULT_MODE): adds ival to the semaphore isem
int decr(int isem, int mode = DEFAULT_MODE): decrements the semaphore isem with mode
static int exists( int nb_sem, key_t my_key, char *filename ): returns true if it exists
int get_mode(): returns the default access mode
int get_nproc_waiting(int isem): returns the number of processes waiting for the semaphore to change
int get_sem( int isem ): returns the value of the semaphore isem
int get_sem_values(int &nb_sem, int *&values): get all semaphores, client responsible for deleting the returned array
int incr(int isem, int mode = DEFAULT_MODE): increments the semaphore isem in the set
int lock(int isem): locks the semaphore (i.e. decr with SEM_UNDO )
psem(int nb_sem, int init_val = 0, key_t key = 1, char *filename = NULL): creates a semaphore set with nb_sem semaphores and initialises it to init_val
static int remove(int id): forces the removal of the IPC with identifier id
int set_all_sem( int sem_val ): set all semaphore in the set to the value sem_val
void set_mode( int mode ): sets the default access mode
int set_sem( int isem, int sem_val ): set semaphore isem to the value sem_val
int sub(int isem, int ival, int mode = DEFAULT_MODE): subtracts ival from the semaphore isem
int unlock(int isem): unlocks the semaphore (i.e. incr with SEM_UNDO)

Private Fields
int default_mode: the default mode of the semaphore - determines how the emaphore is manipulates

Private Methods
void addToCleanupScript( int theId ): Routine to be called to add an entry for cleaning up the semaphore from the cleanup script

Inherited from pipc:

Public Methods
int get_id()
key_t get_key()
int get_size()
int get_status()
int i_created()

Protected Methods
static key_t get_existing_key( char *filename, key_t key )
static key_t get_key( char *filename, key_t key = 1, int create = -1 )
static key_t get_new_key( char *filename, key_t key )

Protected
protected data members for access from derived class

int id
identifier
int size
size
key_t key
the unique key
int status
status of the last command
time_t time_created
time when the resource was created
int created
whether the process was the one that created the resource (or just attached to it)
static char* pcIpcCleanupFile
Pointer to the cleanup file name that has to be created if we want to have a script at the end that eliminates all IPC resources

Documentation

psem is a quick and dirty encapsulation of the system V sem utility.
Usage:
 psem *sem = new psem(size) for just getting one shm of size size
           = new psem(size, my_key) if you want to get more than
                           one shm per user, you must specify
                           the key for each shm
           = new psem(size, key, project) if you want to share
                           different shm between different users
                           [project is the environment variable HOME by default]
           Note: you must test status after constructing the
                 shm for == 0 to make sure it worked!
                 [We want to avoid exception handling as long
                 as we use irix 5.3]

 delete psem : to deattach (but not delete the shm)

 static int psem::remove(int shmid) to remove the shmid
                         from the system

 
COMMENTS:

this is a crude implementation, one can add more functionality later (such as resizing, ownership, etc....)
Access modes for changing the semaphores:
psem as UNIX semaphores knows 2 modes, IPC_NOWAIT and SEM_UNDO. We encapsulate this into our own modes and move the system V stuff into the member function.
Define 3 modes:
DEFAULT_MODE (which defaults to the private data member default_mode)
UNDO_MODE
NOWAIT_MODE
Difference between incr/decr and lock/unlock: in calling lock/unlock we assume we are using the semaphore really as a lock ,i.e. we use UNDO_MODE.
MODIFICATIONS: