Operating-Systems-Assignment 2 Processes and Threads Solved

1           Assignment description
1.1         Process and Threads
In this assignment you have to create Linux processes and threads and synchronize their executions. Beside synchronizing their executions, your processes and threads will only display few messages and then terminate. The required synchronization will affect the order the messages will be displayed.

However, in order to be able to test automatically your solution, your processes and threads should call in appropriate places and order several functions we provide to you in the “a2  helper.h” and “a2 helper.c” files. In this sense, they should comply the following requirements:

1.    The main process (the one created when you launch your program and illustrated in Figure 1 as P1) must start by calling the “init()” function, whose role is to prepare your program to interact with our tester.

2.    All processes, i.e. the main one and those created explicitly by your program, must call the “info()” function at their beginning and before their end respectively, with some predefined arguments, as illustrated in the box below, where you can see how the code of the main process and one of the processes it creates should look like.

Sample Process Code

int main(int argc, char **argv)

{

// tester initialization

// only one time in the main process init();

 // inform the tester about (main) process’ start info(BEGIN, process_no, thread_no);

 // other process’ actions ...

// create a new process if (fork() == 0) {

// inform the tester about process’ start info(BEGIN, process_no, thread_no);

// other process’ actions ...

// inform the tester about process’ termination info(END, process_no, thread_no);

}

// inform the tester about (main) process’ termination info(END, process_no, thread_no);

}
3. Each thread must call the “info()” function at its beginning and before its end respectively, with some predefined arguments, as illustrated in the box below.

Sample Thread Code

void thread_function(void* arg) {

// possible thread’ actions ...

// inform the tester about thread start info(BEGIN, process_no, thread_no);

// other possible thread’ actions ...

// inform the tester about thread termination info(END, process_no, thread_no);

}
The “info()” function basically (at least from your point of view) displays a message. Its arguments have the following meaning:

1.    the first one corresponds to one of the constant values defined in the “a2 helper.h” file, the only two supported being “BEGIN” and “END”;

2.    the second argument is an integer value, corresponding to a unique identifier of the calling process, as explained below;

3.    the third argument is an integer value, corresponding to a unique identifier of the calling thread, as explained below.

1.2         Process and Thread Naming Convention
You program must create some number of processes as described below in Section 2. It must also associate that processes unique identifiers, independent of the unique process identifiers the operating system generates. Such, when required to generated N processes, the associated identifiers must be 1, 2, ...,

N. The way you associate these identifiers to created processes is described below. We will refer to processes in this document using labels derived from their associated identifiers, such P1, P2, ..., PN for processes being associated identifiers 1, 2, ..., and N respectively.

Each process could, in its turn, be required to create some number of threads. When speaking about threads created by a process, you must pay attention that the implicit process’ thread, i.e. the one implicitly started at process creation (so, not created explicitly with the new thread creation function) is not considered. Thus, if a process is said to create N threads, that means that it calls the new thread creation function N times.

Similar to processes, each process’ threads are also associated unique thread identifiers (again, different by those associated by the operating system). Thus, for N created threads, the associated identifiers will be 1, 2, ..., N. The thread identifiers will be associated in a strict monotonically increasing manner, based on their creation order: first thread being associated the identifier 1, second thread being associated the identifier 2 and so on. The main thread is considered to be associated the thread identifier 0 (zero). Note that thread identifiers are unique only for threads of the same process, but not for two different processes.

In our requirements we will refer to a process’ threads using labels similar to those for processes. However, while thread identifier ranges for different processes are not disjoint, a thread will be uniquely identified by the unique pair formed by its process’ identifier and its own thread identifier. This can be noted in the way we specified arguments to the “info()” function. Correspondingly thread labels we use include both process and thread identifier, using the following naming convention: thread TN.M is the M’s thread created by process N, where M’ range is between 1 and the maximum number of threads created by that process.

Based on described identification and naming convention, the skeleton code of processes P1 (the main one) and P2 (one created process), considered to be run by their main threads (having identifier 0, i.e. threads T1.0 and T2.0 respectively) should look now like below.

 if (fork() == 0) {

// other process’ actions ...

// inform the tester about process’ termination info(END, 2, 0);

}

// inform the tester about (main) process’ termination info(END, 1, 0);

}
Similarly, the skeleton code of one of P2’s threads (e.g. T2.5) should look like:

Sample Code for Thread T2.5

void thread_function(void* arg) {

// possible thread’ actions ...

// inform the tester about thread start info(BEGIN, 2, 5);

// other possible thread’ actions ...

// inform the tester about thread termination info(END, 2, 5);

}
1.3         Synchronization Mechanisms and Strategy
You are free to use any Linux synchronization mechanisms you want, like:

•    System V semaphores,

•    POSIX semaphores, or

•    POSIX locks and condition variables.

You are even free to use a combination of the mentioned synchronization mechanisms, though we do not recommend something like this.

Regarding thread synchronization requirements described below, they are mainly expressed in terms of specific ordering criteria applied to the beginning and ending messages. You can control such ordering using the needed synchronization mechanisms before the corresponding calls to the “info()” function.

Other times the synchronization requirements are described in terms of a maximum number of threads that must be running in a certain process. Normally, this means that the main thread of that process should control the thread creation process, by using synchronization mechanisms. However, since our tester is only called by the “info()” function, you could also understand this requirement as the maximum number of threads running simultaneously in their function between the BEGIN and END messages. In such a case, the created threads themselves could control how many of them are running (inside the thread function) in the same time, by using synchronization mechanisms. This kind of synchronization should be done before the threads call their

“info(BEGIN, ...)” function.

The mentioned types of synchronization requirement should lead to the process and its thread skeleton functions looking like in the boxes below.

Sample Code for Processes P1 and P2 With Synchronization

int main(int argc, char **argv)

{

// tester initialization

// only one time in the main process init();

// inform the tester about (main) process’ start info(BEGIN, 1, 0);

// other process’ actions ...

// create a new process if (fork() == 0) {

// inform the tester about process’ start info(BEGIN, 2, 0);

// other process’ actions ...

// calls to synchronization mechanisms ...

// new thread creation pthread_create(...);

// inform the tester about process’ termination info(END, 2, 0);

}

// calls to synchronization mechanisms ...

// new thread creation pthread_create(...);

// inform the tester about (main) process’ termination info(END, 1, 0);

}
Sample Code for Thread T2.5 With Synchronization

void thread_function(void* arg) {

// possible thread’ actions ...

// calls to synchronization mechanisms ...

// inform the tester about thread start info(BEGIN, 2, 5);

// other possible thread’ actions ...

// calls to synchronization mechanisms ...

// inform the tester about thread termination info(END, 2, 5);

}
You can note that the same process could create both other new processes and threads, as it is the case of process P1 in our sample code above. While at process creation everything in a parent process is copied in a new child process it creates, this also includes the possible threads that process started before. Though, in the child process only the parent’s thread calling the “fork()” function will be active, the other ones not being scheduled at all. Still, because of the many complications that could occur, we recommend you to write the code of one process that is required to create both processes and threads such that it creates firstly the requested processes and only than the requested threads. This is actually what we tried to suggests you in the given sample code.

2           Assignment’s requirements
Your are required to write a C program named “a2.c” by extending the provided stub, where the following requirements will be implemented.

2.1         Compilation and Running
Your C program must be compiled with no error in order to be accepted. A sample compilation command should look like the following:

Compilation Command

gcc -Wall a2.c a2_helper.c -o a2 -lpthread
Warnings in the compilation process will trigger a 10% penalty to your final score.

 

Figure 1: Required Process Hierarchy

When run, your resulted executable (we name it “a2”) must provide the minimum required functionality and expected results, in order to be accepted. What such minimum means, will be defined below. The following box illustrates the way your program could be run.

Running Command ./a2
2.2         The process hierarchy
Your solution must generate the process hierarchy illustrated in Figure 1.

Each process having children must not terminate before its children. In terms of messages displayed by the “info()” function, the END message of a parent should never appear before any END message of its children.

2.3         Synchronizing threads from the same process
Process P7 must create 5 threads: T7.1, T7.2, ..., T7.5. You must impose on the execution of the process P7’s threads the following synchronization conditions:

•    Process’ main thread, i.e. T7.0 must not terminate before the other 5 threads. This means that its “info(END, ...)” message must not be called before the similar calls of the other threads. In terms of messages, its END message must never be displayed before any END message of the other threads.

•    Thread T7.3 must start before T7.1 starts and terminate after T7.1 terminates.

2.4         Threads barrier
Process P6 must create 37 threads: T6.1, T6.2, ..., T6.37. You must impose on the execution of the process P6’s threads the following synchronization conditions:

•    Process’ main thread, i.e. T6.0 must not terminate before the other 37 threads.

•    At any time, at most 5 threads of process P6 could be running simultaneously, not counting the main thread.

•    Thread T6.15 can only end while 5 threads (including itself) are running.

2.5         Synchronizing threads from different processes
Process P3 must create 6 threads: T3.1, T3.2, ..., T3.6. You must impose on the execution of the processes P3’s and P7’s threads the following synchronization conditions:

•    Process’ main thread, i.e. T3.0 must not terminate before the other 6 threads.

•    The thread T3.4 must terminate before T7.4 starts, but T3.6 must start only after T7.4 terminates.

3           User Manual
3.1         Self Assessment
In order to generate and run tests for your solution you could simply run the “tester.py” script provided with your assignment requirements. The script requires Python 3.x, not Python 2.x.

Sample Command for Tests Generation

python3 tester.py
Even if the script could be run in MS Windows OS (and other OSes), we highly recommend running it in Linux, while this is how we run it by ourselves, when evaluating your solutions.

The script will compile and run your program several times and assess your score based on the messages received from the program.