C How to share informations between processes? - c

Question

I would need some help with some C code.
Basically I have n processes which execute some code. Once they're almost done, I'd like the "Manager Process" (which is the main function) to send to each of the n processes an int variable, which may be different for every process.
My idea was to signal(handler_function, SIGALRM) once all processes started. When process is almost done, it uses kill(getpid(), SIGSTOP) in order to wait for the Manager Process.
After SIM_TIME seconds passed, handler_function sends int variable on a Message Queue then uses kill(process_pid, SIGCONT) in order to wake up waiting processes. Those processes, after being woken up should receive that int variable from Message Queue, print it and simply terminate, letting Manager Process take control again.
Here's some code:
/**
* Child Process creation using fork() system call
* Parent Process allocates and initializes necessary variables in shared memory
* Child Process executes Student Process code defined in childProcess function
*/
pid_t runChild(int index, int (*func)(int index))
{
pid_t pid;
pid = fork();
if (pid == -1)
{
printf(RED "Fork ERROR!\n" RESET);
exit(EXIT_FAILURE);
}
else if (pid == 0)
{
int res = func(index);
return getpid();
}
else
{
/*INSIGNIFICANT CODE*/
currentStudent = createStudent(pid);
currentStudent->status = FREE;
students[index] = *currentStudent;
currentGroup = createGroup(index);
addMember(currentStudent, currentGroup);
currentGroup->closed = FALSE;
groups[index] = *currentGroup;
return pid;
}
}
Code executed by each Process
/**
* Student Process Code
* Each Student executes this code
*/
int childProcess(int index)
{
/*NOTICE: showing only relevant part of code*/
printf("Process Index %d has almost done, waiting for manager!\n", index);
/* PROGRAM GETS STUCK HERE!*/
kill(getpid(), SIGSTOP);
/* mex variable is already defines, it's a struct implementing Message Queue message struct*/
receiveMessage(mexId, mex, getpid());
printf(GREEN "Student %d has received variable %d\n" RESET, getpid(), mex->variable);
}
Handler Function:
* Handler function
* Will be launched when SIM_TIME is reached
*/
void end_handler(int sig)
{
if (sig == SIGALRM)
{
usleep(150000);
printf(RED "Time's UP!\n" RESET);
printGroups();
for(int i = 0; i < POP_SIZE; i++){
mex->mtype = childPids[i];
mex->variable = generateInt(18, 30);
sendMessage(mexId, mex);
//childPids is an array containing PIDs of all previously launched processes
kill(childPids[i], SIGCONT);
}
}
I hope my code is understandable.
I have an issue though, Using provided code the entire program gets stuck at kill(getpid(), SIGSTOP) system call.
I also tried to launch ps in terminal and no active processes are detected.
I think handler_function doesn't send kill(childPids[i], SIGCONT) system call for some reason.
Any idea how to solve this problem?
Thank you

Answer 1

You might want to start by reading the manual page for mq_overview (man mq_overview). It provides a portable and flexible communication mechanism between processes which permits sync and async mechanisms to communicate.
In your approach, there is a general problem of “how does one process know if another is waiting”. If the process hasn’t stopped itself, the SIGCONT is ignored, and when it subsequently suspends itself, nobody will continue it.
In contrast, message-based communication between the two can be viewed as a little language. For simple exchanges (such as yours), the completeness of the grammar can be readily hand checked. For more complex ones, state machines or even nested state machines can be constructed to analyze their behaviour.