Stdout buffer in gfortran

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Timing several versions give interesting results:

program test
  implicit none
  integer :: i,k
  logical :: multi
  multi = command_argument_count() > 0
  do k=1,10000000
    if (multi) then
!      WRITE(*,'(*(A1))') (achar(i),i=49,90)
!      WRITE(*,'(42A1)') (achar(i),i=49,90)
      WRITE(*,*) (achar(i),i=49,90)
    else
      do i=49,90
        WRITE(*,"(A)",advance="no")achar(i)
      enddo
      print*,""
    endif
  enddo
end program test

>time ./a.out > /dev/null
68.902u 25.599s 1:34.50 99.9%
using @kargl's hint:
>time ./a.out 1 > /dev/null
 7.908u 0.584s 0:08.49  99.8%   using @kargl's hint, list-directed (*) format
12.923u 0.547s 0:13.47  99.9%  as above, (42A1) format
14.673u 0.543s 0:15.21 100.0% as above, (*(A1)) format

@FortranFan’s version, with 10,000,000 executions of the inner loop, completes in

time ./stdio_m > /dev/null
2.236u 0.016s 0:02.25 99.5%

So, it seems, there is a huge overhead added by executing WRITE statement (43 times more in OP version vs. @kargl’s), less dependent on the actual number of characters output. NB., the Fortran-C version is much faster even with 430 million calls to putchar/puts

NOTE: there is a strange problem with Fortran-C version. Compiled with gfortran v. 10 outputs lines ending with extra ‘D’ character:

123456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZD

ifort gives expected output ending each line on ‘Z’.
This can be fixed by modifying the @FortranFan’s program unit to

program stdio
  use, intrinsic :: iso_c_binding, only : c_char, c_int
  use stdio_m, only : putchar, puts
  integer(c_int) :: i,k, r,nl
  nl = iachar(new_line('a'))
  do k=1,10000000
    do i = 49, 90
      r = putchar( i )
    end do
    r = putchar(nl)
  end do
end program stdio

Is puts(char_c_"") OK?