g77 treats an exclamation point (!) in column 6 of
a fixed-form source file
as a continuation character rather than
as the beginning of a comment
(as it does in any other column)
when the -fvxt option is specified.
The following program, when run, prints a message indicating whether it is interpreted according to GNU Fortran (and Fortran 90) rules or VXT Fortran rules:
C234567 (This line begins in column 1.)
I = 0
!1
IF (I.EQ.0) PRINT *, ' I am a VXT Fortran program'
IF (I.EQ.1) PRINT *, ' I am a Fortran 90 program'
IF (I.LT.0 .OR. I.GT.1) PRINT *, ' I am a HAL 9000 computer'
END
(In the GNU Fortran and Fortran 90 languages, exclamation point is
a valid character and, unlike space (<SPC>) or zero (0),
marks a line as a continuation line when it appears in column 6.)