Assume that a finite number of resources of a single resource type must be managed. Processes may ask for a number of these resources and -once finished-will return them. As an example, many commercial software packages provide a given number of licenses, indicating the number of applications that may run concurrently. When the application is started, the license count is decremented. When the application is terminated, the license count is incremented. If all licenses are in use, requests to start the application are denied. Such requests will only be granted when an existing license holder terminates the application and a license is returned.
The following program segment is used to manage a finite number of instances of an available resource. The maximum number of resources and the number of available resources are declared as follows:
#define MAX RESOURCES 5
int available resources = MAX RESOURCES;
When a process wishes to obtain a number of resources, it invokes the decrease count() function:
/* decrease available resources by count resources */
/* return 0 if sufficient resources available, */
/* otherwise return -1 */
int decrease count(int count) {
if (available resources < count)
return -1;
else {
available resources -= count;
return 0;
}
}
When a process wants to return a number of resources, it calls the decrease count() function:
/* increase available resources by count */
int increase count(int count) {
available resources += count;
return 0;
}
The preceding program segment produces a race condition. Do the following:
a. Identify the data involved in the race condition.
b. Identify the location (or locations) in the code where the race condition occurs.
c. Using a semaphore, fix the race condition.