CODEWITHSUNDEEP
linuxsegmentation-faultpthreads
Michael Miner
Michael Miner

Reputation: 964

pthread Segmentation Faults

I am doing an assignment using pthreads and mutual exclusion. I have to create n print servers and m print clients, who each have 5 print jobs. We are to create the threads, pass the jobs through a queue of size 4 to the print servers which then print the job (ie busy work in this case). Here is the code for passing the jobs and servicing the jobs.

These are the client and server threads

void *PrintClient(void *arg){
int i;
char str[NUMJOBSPERCLIENT][100];

for(i=1;i<NUMJOBSPERCLIENT;i++){
    pthread_mutex_lock(&mutex);
    req.clientID = pthread_self();
    req.fileSize = rand_int(FILEMIN,FILEMAX);
    sprintf(str[i], "File_%d_%d",pthread_self(),i);
    req.fileName = str[i];
    append(req);
    pthread_mutex_unlock(&mutex);
    sleep(rand_int(1,3));
}//for
pthread_exit(NULL);
} // end PrintClient

void *PrintServer(void *arg){

pthread_mutex_lock(&mutex);
pthread_cond_wait(&cond,&mutex);
while(count > 0){
    take();
    count = count -1;
}
pthread_mutex_unlock(&mutex);
pthread_exit(NULL);
} // end PrintServer

And this is code which adds or removes a job from the queue. I know the error is here and it had to do with the threads themselves but I can not find it for the life of me. So far the debugger has been almost no help (I am running on a university linux server which is showing no compile errors).

void append(PrintRequest item){
BoundBuffer[count] = req;
printf("I am client %s\n",req.fileName);
count++;
if(count == BUFSIZE){
    printf("Buffer Size Reached\n");
    pthread_cond_signal(&cond);
}
} // end append

PrintRequest take(){
printf("Printing %s\n", BoundBuffer[count].fileName);
usleep(BoundBuffer[count].fileSize/PRINTSPEED);
printf("Finished Printing %s\n", BoundBuffer[count].fileName);
} // end take

Upvotes: 0

Views: 740

Answers (1)

Frank Wu
Frank Wu

Reputation: 21

I guess the segmentation fault is signaled around printf("Printing %s\n", BoundBuffer[count].fileName);, right?

In your PrintClient, you store file name to local variable str[][] and copy the pointer to this local variable as one parameter of the request req.fileName = str[i];. Thus the address pointed by req.fileName is allocated on the stack of the client thread.

When the requests are processed in the server thread PrintServer, it is possible that the client thread which generated the request is no longer present. The result is that req.fileName points to an address which doesn't exists (the stack memory has already been de-allocated with the exiting of the client thread), then when you de-reference such address in printf("Printing %s\n", BoundBuffer[count].fileName);, segmentation fault is signaled.

Upvotes: 1

Related Questions