These examples illustrate a number of issues that are intended as helpers when getting started using OpenMP. The Fortran compiler must be configured to process OpenMP directives, i.e. those source lines starting with '!$OMP'.
Two threads are used. There is a danger that thread 1 uses x before thread 0 defines it to have the value 5. This has not happened here, but it could.
Four threads are used. The question is: Which thread processes a do loop index with the upper limit of value n = 101?
Thread 0 asks for input. Each of the alternate threads wait for a (random) fraction of a second before pausing on thread 0 to complete the input. When input is completed all threads print a message. This uses a VOLATILE routine argument to support the pausing step.
A sum of weighted vector dot or inner products is computed. The vectors are random and the generator is entered in a critical section so that the results are repeatable. The outer sum of the weighted dot products uses an ATOMIC designation so that a race condition will not occur.
A parallel do loop records the loop index associated with each thread number + 1. These values here match, but that is not assured.
A subprogram needs work space. This is allocated in the main program and passed to the threads. Each thread uses its index to construct a pointer that uses a specified part of the global work space for its local work space.
A loop for computing a dot product has a race condition. The accumulation step is invalid. This is a purposeful error and it is fixed in the example dotpAtomic.
By using an ATOMIC designation before the accumulation step, the race condition of example dotp is eliminated.
Race condition: Thread: 1, x = 5 After barrier: Thread: 1, x = 5 After barrier: Thread: 0, x = 5
Thread 3 processes index 101
Enter an integer -- 1 Thread 10 waited 0.345 secs before (Volatile) FLAG was tested Thread 9 waited 0.833 secs before (Volatile) FLAG was tested Thread 14 waited 0.701 secs before (Volatile) FLAG was tested Thread 15 waited 0.735 secs before (Volatile) FLAG was tested Thread 11 waited 0.871 secs before (Volatile) FLAG was tested Thread 4 waited 0.963 secs before (Volatile) FLAG was tested Thread 6 waited 0.335 secs before (Volatile) FLAG was tested Thread 2 waited 0.353 secs before (Volatile) FLAG was tested Thread 13 waited 0.888 secs before (Volatile) FLAG was tested Thread 1 waited 0.025 secs before (Volatile) FLAG was tested Thread 8 waited 0.796 secs before (Volatile) FLAG was tested Thread 12 waited 0.090 secs before (Volatile) FLAG was tested Thread 7 waited 0.915 secs before (Volatile) FLAG was tested Thread 0 did no waiting after input. Thread 5 waited 0.838 secs before (Volatile) FLAG was tested Thread 3 waited 0.667 secs before (Volatile) FLAG was tested
Weighted dot product = 0.1296754E+04
Natural Numbers for Each Thread =
1 1.00
2 2.00
3 3.00
4 4.00
5 5.00
6 6.00
7 7.00
8 8.00
9 9.00
10 10.00
11 11.00
12 12.00
13 13.00
14 14.00
15 15.00
16 16.00
If no "Error" messages, work assignment succeeded.
Dot product - n*6 = -0.2060E+05 Thread number = 0 Race condition, stop!
No race condition with ATOMIC construct!