Fortran floating point equality - fortran

Question

I have a Fortran program that tests equality in two floating point numbers. It can be condensed to what is shown below. When this program is run with "0.1" given as a command line argument, I expect it to print "what I expected" but instead it prints "strange". I understand that this is probably due to a floating point rounding issue, but am hoping someone might be able to explain exactly how I should change inputvariable to make this code print "what I expected" with a command line argument of 0.1
program equalitytest
character(len=3) :: arg1
real*8 :: inputvariable
CALL GET_COMMAND_ARGUMENT(1,arg1)
READ(arg1,*) inputvariable
IF (inputvariable.EQ.0.1) THEN
PRINT*, "what I expected"
ELSE
PRINT*, "strange"
ENDIF
end program equalitytest
Run as follows:
./equalitytest 0.1
strange

Answer 1

As a general point, there should be very few reasons why one should need to compare equality with real numbers. If I ever find myself writing such code, I tend to pause and have a think about what I am trying to achieve. What real-world condition is actually a reflection of this?
The exception to the above relate to zeros, either when writing robust code which checks for and handles possible divisions by zero, or for cases searching for a convergent solution to an equation - in the latter case, this should be considered using a delta anyway.
If there really is a need for this check, why not outsource it to a standard library within the project, e.g.
module mylib
use iso_fortran_env
implicit none
private
public :: isReal4EqualReal4
public :: isReal4EqualReal8
public :: isReal8EqualReal4
public :: isReal8EqualReal8
real(real32), parameter :: delta4 = 0.001
real(real64), parameter :: delta8 = 0.0000000001
contains
logical function isReal4EqualReal4(lhs, rhs) result(equal)
real(real32), intent(in) :: lhs
real(real32), intent(in) :: rhs
equal = (abs(lhs - rhs) .le. delta4)
end function isReal4EqualReal4
logical function isReal4EqualReal8(lhs, rhs) result(equal)
real(real32), intent(in) :: lhs
real(real64), intent(in) :: rhs
equal = (abs(lhs - real(rhs,4)) .le. delta4)
end function isReal4EqualReal8
logical function isReal8EqualReal4(lhs, rhs) result(equal)
real(real64), intent(in) :: lhs
real(real32), intent(in) :: rhs
equal = isReal4EqualReal8(rhs, lhs)
end function isReal8EqualReal4
logical function isReal8EqualReal8(lhs, rhs) result(equal)
real(real64), intent(in) :: lhs
real(real64), intent(in) :: rhs
equal = (dabs(lhs - rhs) .le. delta8)
end function isReal8EqualReal8
end module mylib
EDIT: Forgot to add that one of the benefits of the above is the compiler will warn me if I'm attempting to compare two real numbers of different types while using the wrong interface
EDIT: Updated to use portable real number definitions.