Bus Station Simulation with Multiple Processes

A program simulating a bus station with multiple and different types of processes using semaphores and shared memory.
The main goal of this program implementation was to practice with fork, exec, semaphores and shared memory segment.

Here's how the bus station looks like:

Compilation

$ make clean

$ make

Usage

Command Line Flag l is required.

./mystation -l configfile

Configuration File

In order to execute Mystation, a configuration file is needed in the following form:

• Number of bays (must be 3 with current implementation, as noted in piazza)
• Capacity of each bay
• Maximum number of passengers in each bus
• Maximum period of parking for each bus
• Number of buses created by Mystation
• Maximum time of inactivity for Station-Manager to finish execution

Executables

• bus
• station-manager
• comptroller
• mystation

Implementation

• Header file with structs used: data-structs.h
• Main function for Bus: bus.c
• Main function for Station-Manager: station-manager.c
• Main function for Comptroller: comptroller.c
• Main function for Mystation: mystation.c

Communication Protocol / Execution Path

Mystation starts.
Mystation initializes shared memory and fork/exec* all other necessary processes (Station-Manager, Comptroller and many Buses).
Station-Manager, Comptroller are executed once by Mystation.
One bus every short period of time is executed by Mystation.

Buses fill in requests (only one at a time with semaphore 'requestAccess') and waits for a response.
Station-Manager waits for a request with semaphore 'requestNotif', creating a queue of buses wanting to fill in requests.
Station-Manager responds to the request with semaphore 'responseNotif'.

There are 4 types of requests by Buses:

1. Request for entering
2. Request for leaving
3. Request to inform manoeuvre is over (update from 'About to park' to 'Parked')
4. Request to inform manoeuvre is over (update from 'About to leave' to 'Left')

There are 2 types of responses by Station-Manager:

0. Rejected request
1. Accepted request

If Station-Manager cannot find a park spot for a Bus, then Station-Manager rejects its request (request 1).
Then, the Bus is placed in a queue, waiting for a parking spot to be emptied with semaphore spotEmptiedNotif.
When a parking spot is emptied, then these waiting Buses get notified to request for entering again.

If Station-Manager finds a park spot for a bus, then Station-Manager assigns that spot to the bus (request 1).
The Bus waits for previous incoming buses manoeuvring with semaphore enterMan.
When there is no other incoming Bus manoeuvring, the Bus goes in and manoeuvres.
When the Bus finishes manoeuvring, it informs the Station-Manager that it has parked (request 3) and it sleeps the maximum park period.
When the Bus wakes up, it informs the Station-Manager that it wants to leave (request 2).
The Bus waits for previous outgoing buses manoeuvring with semaphore leaveMan.
When there is no other outgoing Bus manoeuvring, the bus goes out and manoeuvres.
When the Bus finishes manoeuvring, it informs the Station-Manager that it has left (request 4).
Then, the Bus finishes execution and Station-Manager frees park spot for other buses.

The ledger in shared memory can be accessed by only one process with semaphore ledgerAccess.

The stats in shared memory can be accessed by only one process with semaphore statsAccess.

Comptroller prints the current snapshot of the ledger every specific period of time.
Comptroller also prints some stats every another specific period of time.

When Mystation stops creating new Buses and all Buses leave, then Station-Manager will eventually get blocked by semaphore requestNotif.
The semaphore requestNotif is called with sem_timedwait and as parameter the maximum time of inactivity for Station-Manager from configuration file.
When that maximum time of inactivity exceeds, then Station-Manager sends a SIGUSR2 signal to Mystation.
That signal informs Mystation that all Buses have finished execution and Station-Manager is heading towards the end of execution.
Then, Mystation sends the same SIGUSR2 signal to Comptroller to stop printing and finish execution.

All executables finish execution.

Important events are being logged to a log file.