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Client/Server Testing - The Acquire function
<< Client/Server Testing - Synchronizing threads with semaphores | Client/Server Testing - Reporting distributed results >>
User's Guide
371
21 I
MPLEMENTING
C
LIENT
/S
ERVER
T
ESTING
Using 4Test's parallel processing features
The following code examples show legal and illegal manipulations of a
variable of type
SEMAPHORE
:
SEMAPHORE semA = 10
// Legal
semA = 20
// Illegal - existing semaphore
// cannot be reinitialized
if (semA == [SEMAPHORE]2)...
// Legal - note the typecast
Print ("SemA has {semA} resources left.")
// Legal
SEMAPHORE semB = 0
// Illegal - must be greater than 0
Note To compare an integer to a semaphore variable, you must
typecast from integer to semaphore using [SEMAPHORE]. But note
that you cannot cast a semaphore to an integer.
To use a semaphore, you first declare and initialize a variable of type
SEMAPHORE
. Thereafter, 4Test controls the value of the semaphore variable.
You can acquire the semaphore if it has a value greater than zero. When you
have completed your semaphore-protected work, you release the semaphore.
The Acquire function decrements the value of the semaphore by one and the
Release function increments it by one. Thus, if you initialize the semaphore
to 5, five threads can simultaneously execute semaphore-protected operations
while a sixth thread has to wait until one of the five invokes the Release
function for that semaphore.
The Acquire function either blocks the calling thread because the specified
semaphore is zero, or "acquires" the semaphore by decrementing its value by
one. Release checks for any threads blocked by the specified semaphore and
unblocks the first blocked thread in the list. If no thread is blocked, Release
"releases" the semaphore by incrementing its value by one so that the next
invocation of Acquire succeeds (does not block).
A call to Acquire has the form:
void Acquire (SEMAPHORE
semA)
where semA is the semaphore variable to acquire.
A call to Release has the form:
void Release (SEMAPHORE
semA)
where semA is the semaphore variable to release.
If more than one thread was suspended by a call to Acquire, the threads are
released in the order in which they were suspended.
A semaphore that is assigned an initial value of 1 is called a binary
semaphore, because it can only take on the values 0 or 1. A semaphore that is
assigned an initial value of greater than one is called a counting semaphore
because it is used to count a number of protected resources.