pthread_create without separate function

Question

I have an array of pthread_ts which are started within a for-loop via pthread_create.

I have a ton of variables that are declared beforehand and are important for the inner workings of the thread. I would like to have an anonymous inner function as start-routine of the pthread_create like so:

pthread_create(threads[i], NULL, 

     { inner function here }

, NULL);

I know that C++ doesn't have this in particular so I thought maybe lambdas could be of help or maybe someone has another idea so that I do not have to create a seperate method and hand over all those variables that come before pthread_create.

Why not use std::thread instead? The standard threads support lambdas, IIRC. Since you mention lambdas, you probably have std::thread too. — Incomputable
– Incomputable, Commented Jun 9, 2018 at 12:28
I suggest you do some more research about lambda expressions, a non-capturing lambda can be used as a function pointer. And read about std::thread as well. — Some programmer dude
– Some programmer dude, Commented Jun 9, 2018 at 12:28
Unfortunately I cannot use std::thread. Is it possible with pthreads? — leonheess
– leonheess, Commented Jun 9, 2018 at 12:30
Can you elaborate why std::thread would not be usable/possible while lambdas are? — JVApen
– JVApen, Commented Jun 9, 2018 at 12:33
If you have access to lambdas, you should have access to std::thread as well. They both came with the C++11 standard. Can you please elaborate on why you "cannot use std::thread"? — Some programmer dude
– Some programmer dude, Commented Jun 9, 2018 at 12:34

Florian Weimer · Accepted Answer · 2018-06-09 13:53:55Z

5

If a lambda expression does not capture anything, the lambda object can be converted to a C function pointer, so something like this will work:

pthread_t thr;
pthread_create (&thr, NULL,
                [] (void *closure) -> void * {
                  return nullptr;
                }, NULL);

The explicit return type of void * is needed because the inferred one usually will not be correct. Since you cannot use captures, you need to use the closure parameter to pass along a pointer to an object (and use static_cast to cast it to the correct type in the lambda), and for life-time reasons, it is probably necessary to allocate that on the heap.

(Also note that pthreads[i] as the first element to pthread_create does not look right, it's a pointer used to return part of the result, the ID of the new thread.)

answered Jun 9, 2018 at 13:53

Florian Weimer

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