Get data from shared memory after getting signal from child process

Question

int *sharedmem(key_t *key,char k,int size){
  int shmid;
  int *segptr;
  *key=ftok('.',k);
  if((shmid=shmget(*key,size,IPC_CREAT|IPC_EXCL|0666))==-1){
    printf("Shared memory segment exists - opening as client\n");
    if((shmid = shmget(*key, size, 0)) == -1){
      perror("bad shmget");
      exit(1);
    }
  }
  else
    printf("Creating new shared memory segment\n");
  if((segptr = shmat(shmid, 0, 0)) == NULL){
    perror("bad shmat");
    exit(1);
  }
  return segptr;
}
int main(){
  ...
  mem=sharedmem(&key,'a',2);
  fpid=getpid();
  signal(SIGHUP,sighup);
  if(pid=fork()==0){
    ...
    mem[0]=1;
    kill(fpid,SIGHUP);
  }
  else {//parent
     //read data from mem[0] when receiving the signal.
  }

How can I read the data from mem[0] in parent upon receiving the signal from the child process? I want to notify the parent that the data has been written to the shared memory and the parent can read it.

Answer 1

In your parent, you can just read the shared data within the signal handler. However, depending on what you want to do with this data, you might want to adopt a different strategy. For instance:

• Increment a global static variable in your signal handler (make it a volatile sig_atomic_t ). Then in them main loop of your program test this, and take action appropriately.

• Nicer, use a self pipe . Explanation here . In essence create a non-blocking pipe() and write a character to it from your signal handler. select() on the read side of the pipe in the normal way in your main loop.

• If your parent process is multithreaded, consider abandoning the signal handler and use semaphores or similar.