print vs write Fortran statements

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The Fortran 2003 standard constitutes a strong foundation of “modern Fortran”. Modern Fortran (Fortran ≥ 2003) is so different in capabilities and coding style from Fortran 77 as to be a distinct, highly backward compatible language. Of course, almost all of Fortran 95 was incorporated into Fortran 2003, except for a few obscure little used and confusing features deprecated and already unsupported by some popular compilers.

Writing to console effectively

The use of write(*,*) grew out of non-standard use of Fortran 66’s write statement that was introduced for device-independent sequential I/O. Although write(*,*) became part of Fortran 77 for printing to console standard output, the Fortran 77 print command is more concise and more importantly visually distinct. That is, where the full versatility of the write command is not needed, print should be used to help make those cases where write is needed more distinct.

Assembly language comparison

print *,'hi' and write(*,*) 'hi' are IDENTICAL in assembly, within each of multiple compilers (gfortran, flang, ifort) as it should be. In general, disassemble Fortran executables with:

gfortran myprog.f90

objdump --disassemble a.out > myprog.s

Fortran 2003

Fortran 2003 finally settled the five-decade old ambiguity over console I/O with the intrinsic iso_fortran_env module, which is often invoked at the top of a Fortran module like:

module mymod

use, intrinsic:: iso_fortran_env, only: stdout=>output_unit, stdin=>input_unit, stderr=>error_unit

The => operators are here for renaming (they have other meanings for other Fortran statements). It’s not necessary to rename, but it’s convenient for the popularly used names for these console facilities.

Recommendation

  • For routine console printing, use:
    print *,'my text'
    
  • For advanced console printing, whether to output errors, use non-advancing text, or toggle between log files and printing to console, use write(stdout,*) or the like.

Examples

Print to stdout console if output filename not specified

program file_or_console
use, intrinsic:: iso_fortran_env, only: stdout=>output_unit

implicit none

character(:), allocatable :: filename
character(256) :: buf
integer :: i, u

call get_command_argument(1,buf,status=i)
if (i==0) then
  filename = trim(buf)  ! Fortran 2003 auto-allocate
  print *, 'writing to ',filename
  open(newunit=u, file=filename, form='formatted')
else
  u = stdout
endif

i = 3 ! test data

write(u,*) i, i**2, i**3

if (u /= stdout) close(u)   ! closing stdout can disable text console output, and writes to file `fort.6` in gfortran
 
print *,'goodbye'

! end program implies closing all file units, but here we close in case you'd use in subprogram (procedure), where the file reference would persist.
end program

Non-advancing stdout/stdin (for interactive Fortran prompts)

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