Re: Discussion about why GNU/Linux system upgrades cause old programs to break

Richard Stallman <[email protected]> writes:

> [[[ To any NSA and FBI agents reading my email: please consider    ]]]
> [[[ whether defending the US Constitution against all enemies,     ]]]
> [[[ foreign or domestic, requires you to follow Snowden's example. ]]]
>
>   > A C99 program is a program that complies with rules of the C99 language.
>   ...
>   > Any program passed to a C99 compiler must be a C99 program.
>
> Accepting that premise, it follows that the old programs some people
> try to use again were not written to be C99 programs.  You're saying
> that recent versions of GCC are is not meant to compile those old C
> programs.

If a correct program of that vintage is given to GCC, it should still
work, if the user passes -std=gnu89 or such.

There could be many relevant details, though.

For instance, old programs had a tendency to make the kind of faulty
assumptions I mentioned before, because compilers once did less work, so
it was easier to predict their output.

A modern compiler will gladly replace, say, a loop copying int values
from one pointer to another of length N with a call to memcpy, or
possibly with a sequence of SIMD instructions that copy many bytes at a
time from one location to another (but, the latter may end up emitting
code that requires four-byte aligned pointers whereas the "naive" loop
wouldn't have, because the language requires that 'int' pointers are
properly aligned when accessed).

> It seems the users are making a mistake when they try to do that.
>
> What should these users do?  "Fix the program to be C99" is often not
> feasible, for various reasons that users have stated.  That suggestion
> is not helpful.
>
> What can we suggest to them that is feasible for them?

GCC still provides both -fpermissive (to recover old behaviour in C99
and onwards) and -std=gnu89/-std=c89 (which enable -fpermissive by
default, and explicitly enables the C dialect GCC accepted by default up
to version 5.1).

Such things were documented as part of the GCC 14 release information,
on https://gcc.gnu.org/gcc-14/porting_to.html (that page should load
fine even in 'eww').
-- 
Arsen Arsenović

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