unable to create pthreads for chat ap

[Fillis52]

hello,
i have built a basic chat application, which at this point accepts a connection from a user, but once the user is connected the pthreads are not created properly and exits the client program

the error message shown is '1Ex'

here is the client code, i have highlighted where the problem is

void *threadWrite(void *arg)
{
	struct threadWriteArgs* toWrite = (struct threadWriteArgs*) arg;
	int sock = toWrite->sock;
	int n = toWrite->n;
	struct sockaddr_in server = toWrite->server;
	struct hostent *hp = toWrite->hp;
	int serverPort = toWrite->serverPort;
	size_t length;char buffer[256];

	while(1)
		{
		bcopy((char *)hp->h_addr, (char *)&server.sin_addr, hp->h_length);
   		server.sin_port = serverPort;
   		length=sizeof(struct sockaddr_in);
   		printf("Please enter the message: ");
   		bzero(buffer,256);
   		fgets(buffer,255,stdin);
   		n=sendto(sock,buffer, strlen(buffer), 0, &server, length);
  			if (n < 0) error("Sendto");
		}
	pthread_exit((void*) 27);
}

void *threadWait(void *arg)
{
	struct threadWaitArgs* toWait = (struct threadWaitArgs*) arg;
	int sock = toWait->sock;
	int n = toWait->n;
	struct sockaddr_in from=toWait->from;
	size_t length = sizeof(struct sockaddr_in);
	char buffer[256];

	while(1)
		{
		n = recvfrom(sock,buffer,256,0,&from, &length);
   		if (n < 0) error("recvfrom");
  			// write(1,"Got an ack: ",12);
   			write(1,buffer,n);
		}
}

void join(int sock,int n,struct sockaddr_in server, struct sockaddr_in from , struct hostent *hp, int serverPort )
{
	size_t length;
	char buffer[256];
	bcopy((char *)hp->h_addr, (char *)&server.sin_addr, hp->h_length);
   	server.sin_port = serverPort;
   	length = sizeof(struct sockaddr_in);
   	printf("Please enter join plus your nickname: ");
   	bzero(buffer, 256);
   	fgets(buffer, 255, stdin);
   	n = sendto(sock, buffer, strlen(buffer), 0, &server, length);

   	if (n < 0) error("Sendto");
   		n = recvfrom(sock, buffer, 256, 0, &from, &length);
   	if (n < 0) error("recvfrom");
  		 write(1, buffer, n);

//the write message is printed out before the user is kicked off

	pthread_t waitingThread;
	pthread_t writingThread;

	threadWaitArgs toWait;
		toWait.sock = sock;
		toWait.n =n;
		toWait.from = from;

	threadWriteArgs toWrite;
	toWrite.sock = sock;
	toWrite.n = n;
	toWrite.server = server;
	toWrite.from=from;
	toWrite.hp = hp;
	toWrite.serverPort = serverPort;

	int waitingResult = pthread_create(
		&waitingThread,
		NULL,
		&threadWait,
		(void*) &toWait);

	int writingResult = pthread_create(
		&writingThread,
		NULL,
		&threadWrite,
		(void*) &toWrite);

	if(waitingResult !=0)
	{
		printf("Could not create waiting thread\n");
		exit(1);
	}

	if(writingResult !=0)
	{
		printf("Could not create writing thread\n");
		exit(1);
	}
}

the code compiles, so i am not sure why the threads arent being created.

also, i should note that 'join' is the first method to be called

many thanks,
Fiona

[Salem]

SourceForge.net: Indentation - cpwiki
Your code is presently too unreadable to spend too much time on it.

[rags_to_riches]

That's not a standard socket error message that I've ever seen. From where is it originating?

[smasherprog]

Well, looking at your code for 20 seconds, I noticed that you have while loops that will never exit.

Do your threads get created?

or do you get the "Could not create writing thread" ?

[Salem]

> threadWaitArgs toWait;
This is a local variable.
It goes out of scope as soon as the function exits.

> pthread_create(...&toWait);
This has a pointer to that local variable.
By the time it gets to look at it, it's probably gone.

If you're still at the stage where normal sequential programming and debugging is still quite hard, or you have to think about it a lot, then thread programming really isn't for you.

[Fillis52]

this is the first exercise in a C programming course i am on, and they have so far taught us nothing but the basics so i am on a steep learning curve.

as for the while loops, they are meant to never end as the threads need to continually be waiting for a message from other chat clients or waiting to send a message.

I understand threadWaitArgs toWait and threadWriteArgs toWrite are local variables, but as they are created in the same function in which they are called, i thought they would still exist. Unless the point exists at the time of creating the thread, then it is lost when trying to be passed into the thread function.

The messages to be displayed when a thread is not able to be created are not shown, so i assume they do get created.

[Salem]

Well yes, they exist at the point the threads are created.
But they will NOT exist for the lifetime of the threads.

How many times is join() likely to be called?
If it's just once, then making the thread data variables static would work.
If not, then the best way out is to malloc them, and pass the pointer to the created thread.

Also, since you're new at this, are threads an absolute part of the question?
Because there are other ways of doing this which are not so mind-numbingly complex as threads.

[Fillis52]

join() is called once in the main method, and not at all by any other methods. so I will make the variables static and see the effect.

also, it was hinted at to use threads, which is why that was the path i chose

thanks for everyone's help by the way, i really appriciate it

[Fillis52]

ok, when changed to

static struct threadWaitArgs{
	int sock;
	int n;
	struct sockaddr_in from;
}threadWaitArgs;

static struct threadWriteArgs{
	int sock;
	int n;
	struct sockaddr_in server;
	struct sockaddr_in from;
	struct hostent *hp;
	int serverPort;
}threadWriteArgs;

get compiler errors as follows:

cc1: warnings being treated as errors
client_udp.c: In function 'join':
client_udp.c:147: warning: statement with no effect
client_udp.c:147: error: expected ';' before 'toWait'
client_udp.c:148: error: 'toWait' undeclared (first use in this function)
client_udp.c:148: error: (Each undeclared identifier is reported only once
client_udp.c:148: error: for each function it appears in.)
client_udp.c:152: warning: statement with no effect
client_udp.c:152: error: expected ';' before 'toWrite'
client_udp.c:153: error: 'toWrite' undeclared (first use in this function)

but when change to

static threadWaitArgs toWait;
	toWait.sock = sock;
	toWait.n =n;
	toWait.from = from;

static threadWriteArgs toWrite;
	toWrite.sock = sock;
	toWrite.n = n;
	toWrite.server = server;
	toWrite.from=from;
	toWrite.hp = hp;
	toWrite.serverPort = serverPort;

get error from debugger as follows:
13[New Thread 0xb7ffbb90 (LWP 11643)]
[New Thread 0xb75fab90 (LWP 11644)]
[Thread 0xb75fab90 (LWP 11644) exited]
[Thread 0xb7ffbb90 (LWP 11643) exited]

[rags_to_riches]

Latest post was also posted here

[smasherprog]

You should make sure you have code to wait on those threads to complete -- I didnt see that. This would cause your program to exit
This is merely psudo code to explain why your program might be exiting

int main(){
    thread a, b;
    createp_thread(a);
    createp_thread(b);
return 0;

Do you see how your program will exit? creating the threads is not enough, you need to wait for the threads to complete, then exit the program. Otherwise, in the case above, your program will create the threads, and then exit.

I forget the function to wait for the thread to complete, and since you have your threads runnning for ever, they never will. So, try this

volitile bool arunning, b running;
int main(){
    thread a, b;
  arunning=brunning=false;
    if(createp_thread(a) == 0) arunning =true;
    if(createp_thread(b)== 0) brunning =true;
while(arunning && brunning ) {}// this will prevent your program from exiting if the threads are created properly. 
return 0;

Then your threads need to set the booleans to false to make the program exit. Now, this is by far the worst method, but you have to start somewhere :P