g77.info: Assigned Statement Labels

Assigned Statement Labels (ASSIGN and GOTO)

   For portability to machines where a pointer (such as to a label,
which is how `g77' implements `ASSIGN' and its relatives, the
assigned-`GOTO' and assigned-`FORMAT'-I/O statements) is wider
(bitwise) than an `INTEGER(KIND=1)', `g77' uses a different memory
location to hold the `ASSIGN'ed value of a variable than it does the
numerical value in that variable, unless the variable is wide enough
(can hold enough bits).

   In particular, while `g77' implements

I = 10

as, in C notation, `i = 10;', it implements

     ASSIGN 10 TO I

as, in GNU's extended C notation (for the label syntax),
`__g77_ASSIGN_I = &&L10;' (where `L10' is just a massaging of the
Fortran label `10' to make the syntax C-like; `g77' doesn't actually
generate the name `L10' or any other name like that, since debuggers
cannot access labels anyway).

   While this currently means that an `ASSIGN' statement does not
overwrite the numeric contents of its target variable, _do not_ write
any code depending on this feature.  `g77' has already changed this
implementation across versions and might do so in the future.  This
information is provided only to make debugging Fortran programs
compiled with the current version of `g77' somewhat easier.  If there's
no debugger-visible variable named `__g77_ASSIGN_I' in a program unit
that does `ASSIGN 10 TO I', that probably means `g77' has decided it
can store the pointer to the label directly into `I' itself.

   *Note Ugly Assigned Labels::, for information on a command-line
option to force `g77' to use the same storage for both normal and
assigned-label uses of a variable.