On 03/14/2014 04:22 PM, Celelibi wrote:
2014-03-14 18:30 UTC+01:00, Daniel Hilst Selli <danielhilst@xxxxxxxxx>:
On 03/14/2014 12:46 PM, Celelibi wrote:
2014-03-14 16:08 UTC+01:00, Daniel Hilst Selli <danielhilst@xxxxxxxxx>:
Hi, I have a question about passing multiple arguments to pthreads, the
big deal is where the paremeters are kept.. I see two possible
solutions.. keep it on static variables that are never deallocated.. or
on heap.. so here is my first question
Passing local (stack) variables as arguments to thread is trouble, since
the scope of this variables can go away before my thread returns..right?
So forget about local variables
So here is the two options I see, static vs heap...
I'm using this model on one of my applications, is the same senario, a
function that receives 3 ints as arguments and is called as a thread.. I
create a little wrapper... here is the code http://pastebin.com/Air7u0YD
How gurus does this? I free the args on threadfd wrapper since, on my
real application can't join the thread, to be honest, is and deatached
thread.. Is there something wrong with this strategy, it seems ugly to
me....
Cheers,
Hello,
If you don't mind making the start time of the threads a bit slower,
you can make every thread copy its data into its local stack.
You can either allocate one set of arguments on the stack of the main
and then, with a semaphore wait for the thread to copy its data before
erasing it with the data for the second thread and so on.
Or you can allocate enough memory for the arguments of all the
threads, start all the threads, and still with a semaphore wait that
all the threads copied their own data to their stack.
Making parameters local to threads seems an elegant solution for me, how
would I do it? Should I use this?
http://pubs.opengroup.org/onlinepubs/007904975/functions/pthread_getspecific.html
I didn't know about pthread_getspecific. But it seems that they only
store void*. Not very useful to replace function arguments.
I'd just suggest starting your thread routine with something like:
struct thread_arg *a = arg;
int a1 = a->a1;
int a2 = a->a2;
int a3 = a->a3;
sem_post(a->sem);
I'm not using semaphores here, I just create a wrapper over the real
function I want to call with as 3 ints as arguments, something like this
main()
{
int *args = malloc(sizeof(int) * 3);
args[0] = x; args[1] = y; args[2] = z;
pthread_create(th, detached, wrap, args)
}
void *wrap(void *args)
{
real_func(args[0], args[1], args[2]);
free(args);
}
The args are only freed when real_func returns, so I don't see problems
and need to use semaphores, did you? Is just this function that I
execute as a thread, all others on my layer follow a normal flow, I
mean, no parallel stuff...
You can also make something in-between by allocating enough memory for
a fixed number of arguments. But it's becoming complicated to handle
for probably no gain.
This seems what I'm doing right now.. For simple cases seems acceptable
but for complex case, it seems to be trouble to handle...
But actually, I don't really see why you wouldn't join the threads.
You must not terminate the function main while the threads are
running. If you do, all the threads will be terminated.
I have this cenario, I'm wrinting a layer that will sit between an
industrial stack and end user (a programmer)... the stack will call my
callback for any events that ocurr, my callback should forward the call
to user's callback based on event, in other words, my layer will handle
some events, others are passed to user.. The problem is that the stack
call my callback from its context and this blocks stack execution until
my callback returns, this is the reason I'm creating a new thread in
first place.. I can't trust user to return fast, I can't wait for it..
this is why I'm not joining the thread...
Are you sure you need threads? And not just a way to postpone a long
function call until you have time to actually call it?
I mean: introducing threads when you don't really need to perform
several CPU-intensive actions at the very same time is not always
worth it.
Although the idea might seem sexy in the begining, it always lead to
synchronization problems. And bugs with threaded programs are just
harder to spot and to fix.
I agree, I don't like threads when they aren't needed too, but as a
layer I have no control on execution flow, and, the stack is already
full of threads..
Still I'm wondering how to postpone this execution, I can execute a
signal handler as an alarm or something, but it seems as ugly as threads...
Thanks for your answer :-)
Cheers,
Celelibi
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