create multiple threads dynamically

[www]

hello

I have a client-server program in which I need to accept connections of multiple clients and create a thread to serve each one of them.
I don't know from before the number of the threads that have to be created so for every connection I have to allocate space.
Here's what I do

pthread_t *tids;
thread_counter=0;

if (( tids = new pthread_t (sizeof ( pthread_t )) ) == NULL ) 
  exit(1);
if(thread_counter!=0){    
    tids+1=new pthread_t (sizeof ( pthread_t ));
    tids=tids+1;

    if(tids==NULL)
        cout<<"error with realloc"<<endl;*/
}
        
if(err=pthread_create(tids,NULL,parse,(void*)argument))        
{
    perror("pthread create\n");
    exit(1);
}

thread_counter++;

The above is in a while loop and allocates space for the first thread and then creates it.
For every new connection it allocates space for a new thread(thread_counter>0) and creates it.
The problem seems to appear with the pthread_create for the second thread etc.
(The program is compiled with g++ and that's why new is used,but vectors are not allowed so the rest of the program is in C)

[memcpy]

If you're trying to dynamically re-allocate the array of thread, it's not correct. You need array brackets with new to show that it is an array, and you never free memory.

Also, spawning a new thread for every client is very wasteful when select or poll could get the job done with only one thread.

[www]

if(thread_counter!=0)
{	
	tids =(pthread_t*)realloc(tids,sizeof ( pthread_t )*(thread_counter+1)) ;
	if(tids==NULL)
	cout<<"error with realloc"<<endl;
}

you mean something like that will not work?
I did't quite understand what you mean with brackets.
Could you give me an example?
thank you

[CornedBee]

Why not just use a std::vector?

[Rodaxoleaux]

Why not just use a std::vector?

but vectors are not allowed so the rest of the program is in C

That's why. Also

tids+1=newpthread_t (sizeof( pthread_t ));

Don't you mean

tids+sizeof(pthread_t)=newpthread_t (sizeof( pthread_t ));

[laserlight]

The program is compiled with g++ and that's why new is used,but vectors are not allowed so the rest of the program is in C

Why are vectors not allowed? Why compile with g++ and use new when the rest of the program is in C?

[Elkvis]

you could also try a doubly linked list. no need to re-allocate whole arrays of things. just one thing at a time, and insert/remove/append/prepend operations are computationally very cheap.

[Salem]

FWIW, if you're making this much of a meal of allocating memory to store a few thread handles, you're nowhere near ready for the complexities that threaded code will present you.

Do as memcpy suggested in post #2 and just use a single thread with select. It's a bit of extra effort to get it to work properly, but you'll keep your sanity trying to debug it.

[Elkvis]

the trouble with using a single thread only is that any long running request will block all other clients until it is finished. my preferred solution to this is to use select/poll to find connections that are waiting for some sort of processing, and spin each request off into its own thread.

[Salem]

How so?

Use select() to watch all the open file descriptors.
Use a timeout with select() so it doesn't block if there is no traffic at all.
For each active descriptor, perform a single read() to get available data, and pass that onto the corresponding handling function.

[Elkvis]

For each active descriptor, perform a single read() to get available data, and pass that onto the corresponding handling function.

but if that handler takes a long time to return, you don't get back to the next call to select until it's done, causing all other connections to be potentially unresponsive. spawning the handler in a thread allows it to complete asynchronously, and (nearly) immediately returns to the main loop that calls select.