Trouble with semaphore implementation - c++

Question

I am meant to write a program that takes 2 different command line options and uses semaphore to alternate the output from "First" to "Next". More detail -
• Program runs twice with different command line options
progName randNumber fRunSeed "F" & progName randNumber nRunSeed "N" &
• F = First run, N = Next run
• Each run is meant to generate randNum amount of random numbers, but couts a line after each random number.
• Use semaphores to alternate couts between the F run and the N run. (F being first)
The output should include the run (F or N), the process ID, and the random number.
So far I have the header file written, and some of the actual code processing. But I do not know where to actually implement the semaphore... I do understand the purpose of a semaphore, and that this program does not really utilize it well but it is what I am meant to use.
Could someone please look at my code and either guide me in the right direction of where to insert the semaphore, or tell me how to do so?
Thank you for all of the help -- Code below
My header:
union semun {
int val;
struct semid_ds *buf;
ushort *array;
};
class Semaphore {
private:
const unsigned int SemCount;
int SemID;
public:
Semaphore (key_t key, int howManySemaphoresToCreate);
void Init (int SemaphoreNumber, int Value);
int ReadValue (int SemaphoreNumber);
// Decrease the value of a semaphore, semaphoreNumber, by 1
void Wait (int SemaphoreNumber);
// Increase the value of a semaphore, semaphoreNumber, by 1.
void Signal (int SemaphoreNumber);
// Remove the semaphore group.
void Destroy( );
};
And the code that I wrote to handle the data processing. I do not know where to insert semaphore, so right now the actual code is a rough sketch.
int main(int argc, char *argv[]){
int firstSeed = atoi(argv[2]);
int nextSeed = atoi(argv[2]);
int numRandNums = atoi(argv[1]);
if(argv[3] == "F"){
srand(firstSeed);
for(int i = 0; i < numRandNums; i++){
cout << "F: " << "ID: " << getpid() << " Num: " << rand() << endl;
}
}
else if(argv[3] == "N"){
srand(nextSeed);
for(int j = 0; j < numRandNums; j++){
cout << "N: " << "ID: " << getpid() << " Num: " << rand() << endl;
}
}
}
I know this website isn't for people to finish your work for you, but at this point I do not understand the actual implementation of semaphores, and although I have read a lot about them I cannot find examples similar enough to what I need to do to actually see the similarities. Also, I do not think getpid() is the proper way to get the process ID for semaphores, but as I said, just a rough draft for now.
Thank you to anyone that can help me with this.

Answer 1

Here is what I think you can do to add semaphores in your code-
int main(int argc, char *argv[]){
//two semaphores to check if it is time to ru==do first run or next run
Semaphore firstRun=0,nextRun=0;
int firstSeed = atoi(argv[2]);
int nextSeed = atoi(argv[2]);
int numRandNums = atoi(argv[1]);
//Now the program is ready for first run
firstRun.signal();
//the following code is only executed when we have signal in firstRun so the wait function is used
firstRun.wait();
if(argv[3] == "F"){
srand(firstSeed);
for(int i = 0; i < numRandNums; i++){
cout << "F: " << "ID: " << getpid() << " Num: " << rand() << endl;
}
//First Run has been done so generate signal for second
nextRun.signal();
}
//similarly a wait on nextRun to ensure it executes after firt run is finished
nextRun.wait();
else if(argv[3] == "N"){
srand(nextSeed);
for(int j = 0; j < numRandNums; j++){
cout << "N: " << "ID: " << getpid() << " Num: " << rand() << endl;
}
}
}
Or you could only one semaphore nextRun as I think the use of firstRun was trivial in my code.