Re: C scopes, another C-like language? was Compilers :)

On 09/01/2023 04:48, Hans-Peter Diettrich wrote:
> On 1/8/23 8:21 PM, David Brown wrote:
>
>> In other words, it can combine all the variables declared in nested
>> scope and act as though they were all defined at the start of the
>> function.
>
> AFAIR nested scopes were introduced just to allow for space saving
> memory overlays. Regardless of whether a compiler really takes that
> optimization *option*.


I don't know the history as to /why/ block scope variables were
introduced in C - that was all before my time. But they certainly allow
you to write better structure in your code (in my opinion, anyway - I am
aware that some people think "declare all variables at the start of the
function before any code" gives better code). IMHO, local variables
should have as small a scope as practically possible. So even if I have
to use C90 (which I dislike - C99 was a big improvement), if I have a
variable that is only needed inside a loop or a conditional, I'll
declare it there.


Compilers have always been free to use lifetime analysis to merge or
overlap variables, regardless of how and where they are declared inside
a function. There have never been any requirements from the language
forcing compilers to have separate "stack slots" for variables that are
declared at the start of a function or within the same block. Of
course, compilers of old were not as sophisticated as modern compilers,
and debuggers of old were also more limited, so it was quite common to
have fixed and dedicated stack slots for each variable declared at the
start of the function. But it was never /required/ for correct
implementation of the language.


> Of course problems can arise from malware assuming memory contents as
> left over from a previous block, as it's not required that the compiler
> initializes all local variables on block entry.
>


You are required to initialise or assign to all local variables that you
read. Failing to do so is undefined behaviour (or in some
circumstances, unspecified behaviour). And it doesn't matter whether
you initialise your local variable at the start of a block, or assign to
it later when you use it - the compiler is under no obligation to make
any initialisation at the start of the block, regardless of what you
write. It only has to make sure you get the same results in the end as
you would have had if it had followed your source code directly.


If malware can execute its code inside your functions (from code
injections, buffer overflows, etc.), then all sorts of things can go
wrong. Initialising local variables at the start of a block can reduce
the risks and consequences a bit, but it will not save you.

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