Hello, all

While comp.lang.c is being choked by spammers, let me ask it
in .c++ as well. Is it possible with the C preprocessor to
generate code patterns of the following form:

if( <expr_1> )
{ ret = 1;
goto PREPROC;
RET_1:
<stmt_1>;
return;
}
if( <expr_2> )
{ ret = 2;
goto PREPROC;
RET_2:
<stmt_2>;
return;
}
... // many lines of intermediate code
switch( ret )
{ case 1: goto RET_1; break;
case 2: goto RET_2; break;
...
}

where the programmer (more or less) only specifies the test
expressions and corresponding statements, e.g.:

#TEST( <expr_1> )
<stmt_1>; // may bec compund, many lines
#TEST( <expr_2> )
<stmt_2>;
#INTERMEDIATE
#TEST_END

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I wrote:

> if( <expr_1> )
> { ret = 1;
> goto PREPROC;
> RET_1:
> <stmt_1>;
> }
> if( <expr_2> )
> { ret = 2;
> goto PREPROC;
> RET_2:
> <stmt_2>;
>
> }
> ...
> switch( ret )
> { case 1: goto RET_1; break;
> case 2: goto RET_2; break;
> ...
> }

In the code above, label PREPROC is missing. It should be
placed before the `switch' statement.

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On 2023-10-08 2:22 PM, Anton Shepelev wrote:
> Hello, all
>
> While comp.lang.c is being choked by spammers, let me ask it
> in .c++ as well. Is it possible with the C preprocessor to
> generate code patterns of the following form:
>
> if( <expr_1> )
> { ret = 1;
> goto PREPROC;
> RET_1:
> <stmt_1>;
> return;
> }
> if( <expr_2> )
> { ret = 2;
> goto PREPROC;
> RET_2:
> <stmt_2>;
> return;
> }
> ... // many lines of intermediate code
PREPROC:
> switch( ret )
> { case 1: goto RET_1; break;
> case 2: goto RET_2; break;
> ...
> }
>
> where the programmer (more or less) only specifies the test
> expressions and corresponding statements, e.g.:
>
> #TEST( <expr_1> )
> <stmt_1>; // may bec compund, many lines
> #TEST( <expr_2> )
> <stmt_2>;
> #INTERMEDIATE
> #TEST_END
>

What's the purpose of the jumps to the `switch` and back? It does nothing.

- Alf

Alf P. Steinbach:

> What's the purpose of the jumps to the `switch` and back?
> It does nothing.

Shall I quote my entire code from the previous article, or
may I count on everybody's smart newsreaders that can step
up the thread at a single keystroke?

To answer your question, I was thinking of implementing a
state-machine, where the entire cycle or tick (entry, exit,
transition) is located in a single function and executed in
a single invocation of it. The transition-handling code is
specific for each transition leading out of the current
state. The exit-handling code is common to the state and
must be executed /before/ the transition-handling code, but
/only/ if a transition occurs, therefore:

if( trans_cond_1 )
{ ret = 1; /* a transition must be performed */
goto EXIT; /* but we must exit current state first */
TRANS_1:
/* transition-handler here */
return;
}
...
return; /* no transition */
switch( ret )
{ case 1: goto TRANS_1; break;
...
}

See what I mean?

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