Prohibit use of break/continue in a statement

Prohibit use of break/continue in a statement

[Hirrolot]

I have a macro that expands to a for-loop. It is used as follows:

    MACRO(...) {
        // User code
    }

The for-loop is used to open a new scope with a new variable; thus,
`MACRO(...) { ... }` would be a proper C statement. The loop itself is
executed only once.

The problem is that if a user uses `break` or `continue` like this:

    while (i < 10) {
        MACRO(...) {
           break;
            // User code...
       }
    }

Then that `break` will apply to the for-loop generated by `MACRO`, not
to the outer while-loop. The same holds for `continue`. This is
unexpected behaviour, and I would like to prohibit the use of the
`break`/`continue` statements in a user statement placed after
`MACRO`. Ideally, this should somehow trigger a compilation
error/warning.

It should be clarified that `MACRO` is not of a loop itself: it is not
a for-each macro or something like this; using that for-loop is just
an implementation detail. I would be also happy with getting rid of
that for-loop but I have no idea how. If I just generate a variable
like this:

    #define MACRO(...) int x; /* Some other stuff */

Then `MACRO(...) { ... }` will no longer be a single C statement.
Moreover, two occurrences of `MACRO` calls in a single scope will
result in a compilation error, which is also quite unfortunate.

Is it possible to trigger a compilation warning/error for
`break`/`continue` with GCC?

Re: Prohibit use of break/continue in a statement

[David Brown]

On 31/01/2022 16:54, Hirrolot via Gcc-help wrote:
> I have a macro that expands to a for-loop. It is used as follows:
> 
>     MACRO(...) {
>         // User code
>     }
> 
> The for-loop is used to open a new scope with a new variable; thus,
> `MACRO(...) { ... }` would be a proper C statement. The loop itself is
> executed only once.
> 
> The problem is that if a user uses `break` or `continue` like this:
> 
>     while (i < 10) {
>         MACRO(...) {
>            break;
>             // User code...
>        }
>     }
> 
> Then that `break` will apply to the for-loop generated by `MACRO`, not
> to the outer while-loop. The same holds for `continue`. This is
> unexpected behaviour, and I would like to prohibit the use of the
> `break`/`continue` statements in a user statement placed after
> `MACRO`. Ideally, this should somehow trigger a compilation
> error/warning.
> 
> It should be clarified that `MACRO` is not of a loop itself: it is not
> a for-each macro or something like this; using that for-loop is just
> an implementation detail. I would be also happy with getting rid of
> that for-loop but I have no idea how. If I just generate a variable
> like this:
> 
>     #define MACRO(...) int x; /* Some other stuff */
> 
> Then `MACRO(...) { ... }` will no longer be a single C statement.
> Moreover, two occurrences of `MACRO` calls in a single scope will
> result in a compilation error, which is also quite unfortunate.
> 
> Is it possible to trigger a compilation warning/error for
> `break`/`continue` with GCC?
> 

The only thing I can think of is:

#define break DON'T USE BREAK HERE
#define continue DON'T USE CONTINUE HERE

That should cause a compilation error if "break" or "continue" are used.
 Of course, you need to use the #define and a matching #undef in the
places MACRO is used.  And it's undefined behaviour to use keywords as
macro names, so other bad things might happen.

A far better idea would be to explain what you are trying to achieve
with your MACRO, and show your current definition.  Then people could
perhaps give you useful advice!

David

Fwd: Prohibit use of break/continue in a statement

[Hirrolot]

---------- Forwarded message ---------
From: Hirrolot <hirrolot@gmail.com>
Date: Mon, Jan 31, 2022 at 10:59 PM
Subject: Re: Prohibit use of break/continue in a statement
To: David Brown <david.brown@hesbynett.no>


Ah, actually, the implementation is rather involved. I tried to
explain it in my blog post [1]. The section "Pattern matching" should
give you a clear sense of what is happening.

Yes, `#define` directives for `break` and `continue` should work, but
defining and undefining them every time would be a true hassle.

[1] https://hirrolot.github.io/posts/compiling-algebraic-data-types-in-pure-c99.html

Re: Prohibit use of break/continue in a statement

[Al K]

Why not use statement expressions [1] ?

[1] https://gcc.gnu.org/onlinedocs/gcc/Statement-Exprs.html

On Mon, Jan 31, 2022 at 11:02 AM Hirrolot via Gcc-help <gcc-help@gcc.gnu.org>
wrote:

> ---------- Forwarded message ---------
> From: Hirrolot <hirrolot@gmail.com>
> Date: Mon, Jan 31, 2022 at 10:59 PM
> Subject: Re: Prohibit use of break/continue in a statement
> To: David Brown <david.brown@hesbynett.no>
>
>
> Ah, actually, the implementation is rather involved. I tried to
> explain it in my blog post [1]. The section "Pattern matching" should
> give you a clear sense of what is happening.
>
> Yes, `#define` directives for `break` and `continue` should work, but
> defining and undefining them every time would be a true hassle.
>
> [1]
> https://hirrolot.github.io/posts/compiling-algebraic-data-types-in-pure-c99.html
>

Re: Prohibit use of break/continue in a statement

[Hirrolot]

How statement expressions can help? The documentation says
`break`/`continue` are allowed in statement expressions, if I
understand it correctly.

Re: Prohibit use of break/continue in a statement

[Al K]

You can use statement expression to get rid of the for-loop within MACRO(),
since it'll provide a new
scope for variables you define within the statement expr, i.e. instead of
`for (...) { /* code */ }` use
`({ /* code */ })`.

Cheers,
Al

On Mon, Jan 31, 2022 at 12:44 PM Hirrolot <hirrolot@gmail.com> wrote:

> How statement expressions can help? The documentation says
> `break`/`continue` are allowed in statement expressions, if I
> understand it correctly.
>