Getting silly with C, part (void*)2

C[++] never ceases to amaze me. Just yesterday I saw this, totally valid, code snippet:

``` void g(); void f() { return g(); } ```

hehe. similar to

How to Get Fired Using Switch Statements & Statement Expressions:

https://blog.robertelder.org/switch-statements-statement-exp...

Without information about how identifiers are declared, you do not know how to parse this:

  (A)(B);

Or this (like the puts(puts) in the article):

  A(B):

Back in 1999 I made a small C module called "sfx" (side effects) which parses and identifies C expressions that could plausibly contain side effects. This is one of the bits provided in a small collection called Kazlib.

This can be used to make macros safer; it lets you write a #define macro that inserts an argument multiple times into the expansion. Such a macro could be unsafe if the argument has side effects. With this module, you can write the macro in such a way that it will catch the situation (albeit at run time!). It's like a valgrind for side effects in macros, so to speak.

https://git.savannah.gnu.org/cgit/kazlib.git/tree/sfx.c

In the sfx.c module, there is a rudimentary C expression parser which has to work in the absence of declaration info. In other words it has to make sense of an input like (A)(B).

I made it so that when the parser encounters an ambiguity, it will try parsing it both ways, using backtracking via exception handling (provided by except.c). When it hits a syntax error, it can backtrack to an earlier point and parse alternatively.

Consider (A)(A+B). When we are looking at the left part (A), that could plausibly be a cast or declaration. In recursive descent mode, we are going left to right and looking at left derivations. If we parse it as a declaration, we will hit a syntax error on the +, because there is no such operator in the declarator grammar. So we backtrack and parse it as a cast expression, and then we are good.

Hard to believe that was 26 years ago now. I think I was just on the verge of getting into Lisp.

I see the sfx.c code assumes it would never deal with negative character values, so it cheerfully uses the <ctype.h> functions without a cast to unsigned char. It's a reasonable assumption there since the inputs under the intended use case would be expressions in the user's program, stringified by the preprocessor. Funny bytes would only occur in a multi-byte string literal (e.g. UTF-8). When I review code today, this kind of potential issue immediately stands out.

The same exception module is (still?) used in the Ethereal/Wireshark packet capture and analysis tool. It's used to abort "dissecting" packets that are corrupt or truncated.

Can anyone explain how the last snippet works?

C syntax is already way too rich and complex.

We need a C- ore µC:

No implicit cast except for literals and void* (explicit compile time/runtime casts), one loop statement (loop{}), no switch/enum/generic/_thread/typeof/etc, no integer promotion, only sized primitive types (u64 s32 f32 etc...), no anonymous code block, real compiler hard/compile time constant declaration, many operators have to go (--,++, a?b:c, etc)... and everything I am forgetting right now (the dangerous struct pack attribute...). But we need inline keywords for memory barriers, atomics for modern hardware architecture programming.

Note that this is GNU C, not standard C. GNU has extended the normal C language with features such as forward parameter declarations and numeric ranges in switch cases. Lots of people don't know about these things.