You should certainly be able to have nested cilk_for loops! However, whether it's needed depends on the range of the loops involved. I can't say without knowing more.

Possible ways to debug this:

• Does the program work properly if you set the number of workers to 1? This can be done by either setting the CILK_NWORKERS environment variable or using the Cilk runtime API call __cilkrts_set_param("nworkers", "1");
• You could have a race condition. Have you run Cilkscreen?

- Barry

Generally, you're correct. Cilkscreen should be run on programs that work with 1 worker. Butif you're racing on a pointer, then Cilkscreen might help you diagnose the problem.

And you haven't told me whether the problem reproduces with 1 worker, which may make it much easier to debug.

What does the callstack look like at the segfault?

- Barry

Racing on a pointer is no different than racing on any other shared variable. Consider two threads executing the following code, where "my_struct" is a pointer to a structure:

for (my_struct = list_root; NULL != my_struct; my_struct = my_struct->next) { if (my_struct->data == what_im_looking_for) { my_struct->found_it = true; break; } }

If thread 1 and thread 2 execute the loop at the same time, there are any number of ways that bad things could occur, including a segfault. For example, consider that thread 1 executes the if statement which derefences the pointer just after thread 2 has reached the end of the list.Thread 1 willdereference NULL, which results in a segfault.

Obviously this is an exagerated case, but finding races in your code is really difficult. That's why there are tools like Cilkscreen or Inspector.

- Barry

Do your parallel loops manipulate data structures that are shared?
If so, you may need to add critical sections or reduction operations.

SegFaults are generally the result of a pointer gone wrong (or not what it is thought to be).
An example can be your serial version of the (nested) loop having an exception path for the first iteration where it initializes "something" for use on subsequent iterations. And then the runtime circumstance results in the thread not containing the first iteration indication (e.g. index==0) to pass around the initialization section prior to initialization. This would result in the thread using a non-initialized structure. You have a similar case if the first time indicator is a flag (or NULL pointer) where multiple threads see the indicator as fist time state prior to setting it to initialized (resulting in multiple and concurrent initializations of the shared structure).

Jim Dempsey

>>Please give me a short example that I can use and manipulate

Consult the Cilk Plus examples (for simple programs that work).

Assume you own a valet parking garage where your employee(s) park the cars...
and you are responsible for any damage.

Having one employee is similar to single threaded programming.
Having multiple employees is similar to multi-threaded programming.

What is different between the two situations?
What are potential problems?

You have similar issues with parallel programming.

trying to park two cars in an empty slot...
Having two employees tally the total number of cars by looking at the old tally, then adding/subtracting one while other employee doing same thing, then both of them posting posting the new (incorrect) tally.

You have to look at similar issues in your code.

Jim Dempsey