Using underscores to define kind/precision

Question

I've been using using an underscore to define an integer as a specific kind in fortran.

Here is a snippet of code to demonstrate what 1_8 means, for example:

  program main
  implicit none

  integer(2) :: tiny
  integer(4) :: short
  integer(8) :: long

  tiny = 1
  short = 1
  long = 1

  print*, huge(tiny)
  print*, huge(1_2)

  print*, huge(short)
  print*, huge(1_4)

  print*, huge(long)
  print*, huge(1_8)

  end program main

Which returns (with PGI or gfortran):

32767
32767
2147483647
2147483647
9223372036854775807
9223372036854775807

I'm using the huge intrinsic function to return largest number of the given kind. So 1_8 is clearly the same kind as integer(8). This works for real numbers also, although I haven't shown it here.

However, I've been unable to find any documentation of this functionality and I don't remember where I learned it. My question is:

Is using _KIND standard fortran? Does anybody have a source for this?

Edit: It's been pointed out that the kind values I've used (2,4,8) are not portable - different compilers/machines may give different values for huge(1_4), for example.

IanH IanH · Accepted Answer · 2015-05-21T00:23:09

It is standard Fortran, as of Fortran 90, though the set of valid kind values for each type and the meaning of a kind value is processor dependent.

The Fortran standard is the definitive source for what is "standard" Fortran. ISO/IEC 1539-1:2010 is the current edition, which you can buy, or pre-publication drafts of that document are available from various places. https://gcc.gnu.org/wiki/GFortranStandards has a collection of useful links.

I would expect that many compiler manuals (e.g - see section 2.2 of the current PGI Fortran reference manual) and all reasonable textbooks on modern Fortran would also describe this feature.

Using underscores to define kind/precision

3 Answers