Parallel programming in gfortran, strange? result in hello program

Question

I compile the next hello program, but I don't get the result that I may expect.

program hello

print*, " Hello parallel world"

!$omp parallel

print*, omp_get_thread_num()

!$omp end parallel

print*, "Back to the sequential world"

end

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To compile I use:

mpifort -fopenmp -o hello hello.f90

---

When I run it, it results:

Hello parallel world

0.00000000

0.00000000

0.00000000

0.00000000

Back to the sequential world

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, but according to a manual (Parallel Programming in OpenMP by Rohit Chandra, Ramesh Menon, Leo Dagum, David Kohr, Dror Maydan, Jeff McDonald) I could get numbers between 0 and 3, not only zeros. There is something wrong?

Answer 1

You are missing use omp_lib and the compiler implicitly assumes that omp_get_num_thread() is a function that returns a floating-point value, because its name begins with o. It is not though - it returns an integer. Adding implicit none right after program hello will turn off implicit declarations and trigger a compile-time error without use omp_lib.

program hello
   use omp_lib
   implicit none
   print*, "Hello parallel world"
!$omp parallel
   print*, omp_get_thread_num()
!$omp end parallel
   print*, "Back to the sequential world"
end