fortran - Why does MPI_REDUCE returns a syntax error at compile time?

Question

This question already has an answer here: question from:https://stackoverflow.com/questions/65879756/why-does-mpi-reduce-returns-a-syntax-error-at-compile-time

Answer 1

As stated in the comments one of your lines is too long. With gfortran at least if you turn warnings up to the max (as you should if developing code) you get a more informative message

ian@eris:~/work/stack$ mpif90 --version
GNU Fortran (Ubuntu 7.4.0-1ubuntu1~18.04.1) 7.4.0
Copyright (C) 2017 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

ian@eris:~/work/stack$ mpif90 -O -fcheck=all -std=f2008 -Wall -Wextra long.f 
long.f:8:72:

       f(a) = 4.d0/(1.d0 + a*a)
                                                                        1
Warning: Obsolescent feature: Statement function at (1)
long.f:27:72:

         call MPI_REDUCE(mypi, pi, 1, MPI_DOUBLE_PRECISION, MPI_SUM, 0, MPI_COMM_WORLD, ierr)
                                                                        1
Warning: Line truncated at (1) [-Wline-truncation]
long.f:27:72:

         call MPI_REDUCE(mypi, pi, 1, MPI_DOUBLE_PRECISION, MPI_SUM, 0, MPI_COMM_WORLD, ierr)
                                                                        1
Error: Syntax error in argument list at (1)

I suggest you move the the "new" (i.e. available for 30 years) free format which is more flexible than the archaic punch card based form. In fact in general it would be a great opportunity to learn modern, safer practices than what you used all those years ago (I am guessing from what you say we are of a very similar vintage). Here's a somewhat modernised version of your code

ian@eris:~/work/stack$ cat long.f90
program pi_reduce

  Use, Intrinsic :: iso_fortran_env, Only :  wp => real64
  ! Even better Use mpi_f08
  Use            :: mpi, Only : mpi_init, mpi_comm_rank, mpi_comm_size, mpi_reduce, &
       MPI_INTEGER, MPI_DOUBLE_PRECISION, MPI_COMM_WORLD, MPI_SUM

  Implicit None
  
  Real( wp ), Parameter :: PI25DT = 3.141592535897932384662643_wp

  Real( wp ) :: mypi, pi, h, sum, x
  integer    :: n, myid, numprocs, i, ierr

  call MPI_INIT(ierr)
  call MPI_COMM_RANK(MPI_COMM_WORLD, myid, ierr)
  call MPI_COMM_SIZE(MPI_COMM_WORLD, numprocs, ierr)
  do
     if (myid == 0) then
        print *, 'Enter the number of intervals: (0 quits) '
        read(*,*) n
     endif
     call MPI_BCAST(n, 1, MPI_INTEGER, 0, MPI_COMM_WORLD, ierr)
     if (n .le. 0) exit
     h = 1.0_wp/n
     sum = 0.0_wp
     do i = myid + 1, n, numprocs
        x = h * ( Real( i , wp ) - 0.5_wp)
        sum = sum + f(x)
     enddo
     mypi = h * sum
     call MPI_REDUCE(mypi, pi, 1, MPI_DOUBLE_PRECISION, MPI_SUM, 0, MPI_COMM_WORLD, ierr)
     if (myid .eq. 0) then
        print *, 'pi is ', pi, ' Error is', abs(pi-PI25DT)
     endif
  enddo
  call MPI_FINALIZE(ierr)

Contains

  Pure Function f( a ) 

    Use, Intrinsic :: iso_fortran_env, Only :  wp => real64

    Implicit None

    Real( wp ) :: f

    Real( wp ), Intent( In ) :: a

    f = 4.0_wp / ( 1.0_wp + a * a )
    
  End Function f
  
end program pi_reduce
ian@eris:~/work/stack$ mpif90 --version
GNU Fortran (Ubuntu 7.4.0-1ubuntu1~18.04.1) 7.4.0
Copyright (C) 2017 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

ian@eris:~/work/stack$ mpif90 -O -fcheck=all -std=f2008 -Wall -Wextra long.f90
ian@eris:~/work/stack$ mpirun -np 8 ./a.out
 Enter the number of intervals: (0 quits) 
345435
 pi is    3.1415926535904788       Error is   1.1769254637528093E-007
 Enter the number of intervals: (0 quits) 
0
ian@eris:~/work/stack$