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For portability to machines where a pointer (such as to a label,
which is how g77 implements
ASSIGN and its relatives,
GOTO and assigned-
is wider (bitwise) than an
uses a different memory location to hold the
ASSIGNed 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.
See 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.