Programming Forums - unix signal processing

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unix signal processing

The following is a program I'm trying to get working. My problem isn't the code itself. I can't seem to understand signal handling no matter what I read or try. Will someone please help me understand how to handle signals?

The following program spawns three processes which are passed as arguments. The program is suppose to initially spawn each process and then restart each of the processes (up to 4 times) when the SIGCHLD signal is received.

Please note that the p1xxx variables aren't being used at the moment and I'm aware of this fact. I originally had this program working using the waitpid() and the WNOHANG option (no signal handling) and I was using the p1xxx variables as counters.

#include <iostream>

#include <unistd.h>

#include <sys/wait.h>



using namespace std;



void sig_handler( int signo ); //signal handler

int spawnProcess(char *name); //spawns a particular process by name, returns PID

void syserr(const char *msg); //ignore this



int main(int argc, char * argv[]) {

        if(argc != 4){

                cerr << "Lab5: Program takes 3 processes as arguments" << endl;

                //syserr("Program takes 3 arguments");

                return 1;

        }



        //Initial spawn of our processes

        int pids[3];

        for(int i = 1; i < 4; i++){

                pids[i-1] = spawnProcess(argv[i]);

        }



        //Keep track of how many times each process has been restarted

        int p1Res=1, p2Res=1, p3Res=1;



   struct sigaction act;



        act.sa_handler = sig_handler;

        sigemptyset( &act.sa_mask );

        act.sa_flags = SA_NOCLDWAIT;



        if( sigaction( SIGCHLD , &act, NULL ) == -1 ) {



            perror( "sigaction" );

            exit( 1 );

        }



        cout << "Parent has exited" << endl;

return 0;

}

These are the functions

void sig_handler( int signo ) {

        }



//Spawn the process "name" and return its PID

int spawnProcess(char *name) {

        int pid;

        switch( (pid = vfork()) ){

                case -1:

                        //syserr("vfork");

                        cerr << "Error calling vfork()" << endl;

                        return 1;

                case 0:

                        if(execl(name, NULL) == -1){

                        //syserr("execl");

                        cerr << "Error calling execl" << endl;

                        return 1;

                        }

        };

return pid;

}

Thank you for your time.

Its late so you get no explanation but this. You set up the signal handler properly you just needed to actual check on the child processes in and start back up the right one. I use some global data so the signal handler would know what pids to check each time. You also need to sleep the parent so it can wait for the children. (sorry for mixed tabs and spaces, but I use spaces ;))

#include <iostream>

#include <string>

#include <unistd.h>

#include <sys/wait.h>



using namespace std;



// These need to be global so the signal handler can access them as well

int run_count[3] = {0, 0, 0};

int child_pids[3];

string processes[3];





void sig_handler( int signo ); //signal handler

int spawnProcess(const char *name); //spawns a particular process by name, returns PID



int main(int argc, char * argv[]) {

        if(argc != 4){

                cerr << "Lab5: Program takes 3 processes as arguments" << endl;

                return 1;

        }



    // Setup the signal handler

    struct sigaction act;

    act.sa_handler = sig_handler;

        sigemptyset( &act.sa_mask );

        act.sa_flags = SA_NOCLDWAIT;



    if( sigaction( SIGCHLD , &act, NULL ) == -1 ) {

            perror( "sigaction" );

            exit( 1 );

        }



        // Initial spawn of our processes

        for(int i = 1; i < 4; i++){

            processes[i-1] = string(argv[i]);

                child_pids[i-1] = spawnProcess(argv[i]);

        }





    // Sleep for 20 seconds even if the timer is interupted by a signal

        int sleep_time = 20;

        while (0 != (sleep_time = sleep(sleep_time)))

        {}



        cout << "Parent has exited" << endl;



    return 0;

}



void sig_handler( int signo ) {

    int status;

    for (int i = 0; i < 4; ++i)

    {

        // If we get negative one the child is dead

        if ((-1 == waitpid(child_pids[i], &status, WNOHANG)) &&

            (run_count[i] < 4))

        {

            // Increment the run count

            run_count[i]++;



            cout << "Restarting child '" << processes[i] << "' for the "

                <<  run_count[i] << " time" << endl;

            // Run the process again and record its new pid, then

            child_pids[i] = spawnProcess(processes[i].c_str());

        }

    }

}



//Spawn the process "name" and return its PID

int spawnProcess(const char *name) {

        int pid;

        switch( (pid = vfork()) ){

                case -1:

                        cerr << "Error calling vfork()" << endl;

                        return 1;

                case 0:

                        if(execl(name, NULL) == -1){

                        cerr << "Error calling execl" << endl;

                        return 1;

                        }

        };

return pid;

}

Oh yes, and there might be way to do this without global data. See the "wait" function in this page of the gnu c manual.

Thanks for your reply. That code you posted is making my head hurt so I'll have to stare at it for awhile. The actual code you have in the sig_handler() looks similar to the code I had before this program used signals. I'm going to play with this for awhile longer. I'll post back if I have any more questions.

Once I understand this I'll probably try to change it so it doesn't use globals. I have a feeling that my professor will reject it if I use globals.

Using the GCC code I linked to you should be able to removed the need to store the pids globally but you somehow have to know what command to run when a certain child quits. The signal handler can't be passed any other information so it will be quite a trick for it to figure out what command it should run to "restart" a process.

It seems to be working just fine with the globals. There were some bugs but the logic of the code is sound. Why can't the signal handler be passed anything more than int signo? Is that just the way it's implemented?