为什么MPI_REDUCE在编译时返回语法错误？

如评论中所述，您的一行内容太长。至少如果您将警告提高到最大（使用开发代码时就应该如此），则使用gfortran可以获得更多信息

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)

我建议您移动“新”（即30年可用）的免费格式，该格式比基于传统打孔卡的格式更灵活。实际上，总的来说，这将是学习现代安全实践的绝好机会，这比您几年前使用的方法要多（我从您的口中猜出我们是一个非常相似的年份）。这是您代码的某种现代化版本

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$