SEMAPHORE(2) System Calls SEMAPHORE(2)
scount, screate, sdelete, ssignal, swait - semaphore operations
int scount (int sem);
int screate (int count);
int sdelete (int sem);
int ssignal (int sem);
int swait (int sem);
screate is used to allocate a semaphore from the kernel semapore man-
ager. Semaphores are the most basic form of interprocess communication
(IPC), and these routines provide the power necessary to solve a large
number of synchronization and communication problems. (See an Operat-
ing Systems text).
The initial count determines how many times swait can be called before
processes are blocked. count must non-negative, and is usually set to
1. screate returns a semaphore ID number as an integer. This ID must
be used in all the other semaphore calls.
sdelete releases the specified semaphore, and returns to a ready state
all processes that were blocked on that semaphore.
swait decrements the value of the semaphore (initially specified by
count) by one. If the semaphore count is less than zero, the process
is blocked and queued for release by ssignal. This is what is tradi-
tionally referred to as a semaphore-down operation.
ssignal increments the semaphore count by one. If the semaphore count
is less than zero, ssignal unblocks a process blocked on the semaphore.
The selection of the process to be unblocked is arbitrary; FIFO opera-
tion is not guaranteed.
scount retuns the current value of the semaphore referred to by sem.
Note that depending on this value for synchronization can lead to race
On success, screate returns the semaphore identifier, scount returns
the semaphore value, and sdelete, ssignal, and swait return zero. All
functions return -1 and set errno on failure.
There is currently no mechanism for deallocating semaphores that are
orphaned by abnormal process termination.
These semaphore routines were designed for XINU, written by Douglas
GNO 16 January 1997 SEMAPHORE(2)
Man(1) output converted with