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Using and Porting GNU Fortran

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15.5.1 Backslash in Constants

In the opinion of many experienced Fortran users, `-fno-backslash' should be the default, not `-fbackslash', as currently set by g77.

First of all, you can always specify `-fno-backslash' to turn off this processing.

Despite not being within the spirit (though apparently within the letter) of the ANSI FORTRAN 77 standard, g77 defaults to `-fbackslash' because that is what most UNIX f77 commands default to, and apparently lots of code depends on this feature.

This is a particularly troubling issue. The use of a C construct in the midst of Fortran code is bad enough, worse when it makes existing Fortran programs stop working (as happens when programs written for non-UNIX systems are ported to UNIX systems with compilers that provide the `-fbackslash' feature as the default--sometimes with no option to turn it off).

The author of GNU Fortran wished, for reasons of linguistic purity, to make `-fno-backslash' the default for GNU Fortran and thus require users of UNIX f77 and f2c to specify `-fbackslash' to get the UNIX behavior.

However, the realization that g77 is intended as a replacement for UNIX f77, caused the author to choose to make g77 as compatible with f77 as feasible, which meant making `-fbackslash' the default.

The primary focus on compatibility is at the source-code level, and the question became "What will users expect a replacement for f77 to do, by default?" Although at least one UNIX f77 does not provide `-fbackslash' as a default, it appears that the majority of them do, which suggests that the majority of code that is compiled by UNIX f77 compilers expects `-fbackslash' to be the default.

It is probably the case that more code exists that would not work with `-fbackslash' in force than code that requires it be in force.

However, most of that code is not being compiled with f77, and when it is, new build procedures (shell scripts, makefiles, and so on) must be set up anyway so that they work under UNIX. That makes a much more natural and safe opportunity for non-UNIX users to adapt their build procedures for g77's default of `-fbackslash' than would exist for the majority of UNIX f77 users who would have to modify existing, working build procedures to explicitly specify `-fbackslash' if that was not the default.

One suggestion has been to configure the default for `-fbackslash' (and perhaps other options as well) based on the configuration of g77.

This is technically quite straightforward, but will be avoided even in cases where not configuring defaults to be dependent on a particular configuration greatly inconveniences some users of legacy code.

Many users appreciate the GNU compilers because they provide an environment that is uniform across machines. These users would be inconvenienced if the compiler treated things like the format of the source code differently on certain machines.

Occasionally users write programs intended only for a particular machine type. On these occasions, the users would benefit if the GNU Fortran compiler were to support by default the same dialect as the other compilers on that machine. But such applications are rare. And users writing a program to run on more than one type of machine cannot possibly benefit from this kind of compatibility. (This is consistent with the design goals for gcc. To change them for g77, you must first change them for gcc. Do not ask the maintainers of g77 to do this for you, or to disassociate g77 from the widely understood, if not widely agreed-upon, goals for GNU compilers in general.)

This is why GNU Fortran does and will treat backslashes in the same fashion on all types of machines (by default). See section 8.1 Direction of Language Development, for more information on this overall philosophy guiding the development of the GNU Fortran language.

Of course, users strongly concerned about portability should indicate explicitly in their build procedures which options are expected by their source code, or write source code that has as few such expectations as possible.

For example, avoid writing code that depends on backslash (`\') being interpreted either way in particular, such as by starting a program unit with:


Then, use concatenation of `BACKSL' anyplace a backslash is desired. In this way, users can write programs which have the same meaning in many Fortran dialects.

(However, this technique does not work for Hollerith constants--which is just as well, since the only generally portable uses for Hollerith constants are in places where character constants can and should be used instead, for readability.)

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