Creating a process pool C Linux - c

Question

I have an assignment and I am not quite sure how to go about it. Basically I have to create a coordinator process which creates 5 working processes which are waiting to be awakened. The coordinator passes a marker(integer) to the first process, then that process increments the marker by 1 and passes it to the next process. The coordinator process awakens the next process which does the same and so on. The so called marker should go through all the processes 10 times and in the end its value should be printed by the coordinator. Signals should be used as well as shared memory for the marker.
So I created 5 processes and I am thinking that on every iteration there should be a signal and a handler should be passed which will basically do all the work with the marker.
This is my first time working with processes. This is what I have so far:
#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <sys/shm.h>
#include <signal.h>
#define numOfProcesses 5
pid_t procIDs[5];
void handler(int signum){
//marker and all work here
}
void createProcesses(){
int i;
for(i = 0; i < numOfProcesses; i++){
procIDs[i] = fork();
if(procIDs[i] < 0 ){
perror("Fork error!");
}else if(procIDs == 0){
pause();
}
}
}
int main(){
createProcesses();
int i;
for(i = 0; i < numOfProcesses; i++){
pkill(SIGUSR1, handler);
}
return 0;
}
I honestly don't know how to go about this. I would really appreciate a piece of advice. Thanks in advance!

Answer 1

This is what I have come up with. Sorry for answering, I couldn't find out how to format code in a comment. Anyway:
It should be 10 times each process. I am using shared memory so I guess I don't need a global variable for the marker? This is what I have come up with:
#include<stdio.h>
#include<unistd.h>
#include<stdlib.h>
#include<sys/types.h>
#include<sys/wait.h>
#include<sys/shm.h>
#include<signal.h>
#include<sys/ipc.h>
#define numOfProcesses 5
#define numOfLoops 10
pid_t* procIDs[5];
void createProcesses(){
int i;
for(i = 0; i < numOfProcesses; i++){
procIDs[i] = fork();
if(procIDs[i] < 0 ){
perror("Fork error!");
}
else if(procIDs == 0){
pause();
}
}
}
void init(){//init marker = 0
key_t mykey = ftok(".", 0);
int shID = shmget(mykey, sizeof(int), 0666 | IPC_CREAT);
int *data;
data = (int*) shmat(shID, 0, 0);
*data = 0;
}
int* getValue(){//get value of marker
key_t mykey = ftok(".", 0);
int shID = shmget(mykey, sizeof(int), 0666 | IPC_CREAT);
int *data = shmat(shID, 0, 0);
return data;
}
void increment(int sig){//increment + 1
if(sig == SIGUSR1){
int temp;
int* data;
data = getValue();
temp = *data;
temp++;
*data = temp;
}
}
void yourFunc(int count, pid_t* mypid, int mysig){
if(count == 0){
return;
}else{
printf("Signal sent :: to PID : %d\n", mypid);
kill(*mypid, SIGUSR1);
yourFunc(count -1, ++mypid, SIGUSR1);
}
}
int main(){
signal(SIGUSR1, increment);
init();
int i,j;
createProcesses();
for(j = 0; j < numOfLoops; j++){//loop every pid 10 times
pid_t* currProcess = procIDs[0];
yourFunc(numOfProcesses, currProcess, SIGUSR1);
}
int* data = getValue();
printf("Marker: %d\n", *data);
return 0;
}

Answer 2

I tried your problem, but I am really baffled by the structure of your question, its really unclear what your problem statement is.
10 times each(10 times per process or a total of 10 times(2 times per process)
You say the processes are waiting to be awakened, which hints that they are not child processes rather other processes running on the system, and would require a fifo to communicate.
Nevertheless, the following is what I could conclude from the limited information.
You need to create a function which would be invoked 10 times(loop) by the coordinator on the first process(waiting to be awakened)
The function would recursively invoke the second process and so on till the last sleeping process.
You'll have to use SIGUSR1, and define action for it in a custom signal handler,
eg.
signal(SIGUSR1,custom_handler)
You will need to keep marker as a global variable.
Because C is a procedural language and kernel's scheduling is not in your hands once a process terminates you cannot recall it or ensure same PID for a process on forking.
So if you are thinking of creating processes inside functions which will be paused and on getting a signal shall resume, fine.....!, But it would be a one-off.
That's all I can say by the limited information your question presents.
Following is the above idea in C.
Initialise count = 5 (no. of processes)in the caller.
mypid points to the first process's PID.
void party_time(int count, pid_t* mypid, int mysig)
{
if(count == 0)
return;
else
{
printf("Signal sent :: to PID : %d\n",*mypid);
kill(*mypid,SIGUSR1);
party_time(count - 1 ,++mypid,SIGUSR1);
}
}