[cfe-commits] [Request for approval] Warning for recursive macro expansion attempt

[Abramo Bagnara]

Il 01/01/2012 18:56, Douglas Gregor ha scritto:
> On Dec 22, 2011, at 10:32 AM, Abramo Bagnara wrote:
> 
>> > 
>> > The attached patch add a warning (disabled by default) to show the
>> > (mostly unwanted) attempt to recursively expand a macro.
>> > 
>> > Ok to commit?
> I'm having a hard time understanding the motivation behind this warning, because I've never encountered a place where it would have helped find a problem. On the other hand, I've seen a number of macros that wrap functions (with the same name) that benefit from the suppression of recursive macro instantiations, and those would come up as false positives with this warning.
> 

As you write there might be legitimate needs for that detail of C
preprocessor behavior (and this is the reason why I've left it disabled
by default), but there are coding standars that prohibits that, e.g.
NASA JPL Institutional Coding Standard for the C Programming Language at
page 16 of

http://lars-lab.jpl.nasa.gov/JPL_Coding_Standard_C.pdf

is written:

"Rule 20 (preprocessor use)
Use of the C preprocessor shall be limited to file inclusion and simple
macros.

...

Specifically, the use of token pasting (cf. MISRA-C:2004 Rules 19.12 and
19.13), variable argument lists (ellipses) (cf. MISRA-C:2004 Rule 16.1),
and recursive macro calls are excluded by this rule. All macros are
required to expand into complete syntactic units (cf. MISRA-C:2004 Rule
19.4)."

Implementing this check using clang library is very hard (if not
impossible) and this is the reason why I've proposed to add this warning.