how to fool gcc?

[Tim Kientzle]

Suggestions:

1) Convert RETURNS(s) into a function rather than a macro.

2) Use a ternary (someone else suggested this)

3) Remove the printf-like declaration (Yes, this is distasteful.)

4) Add an intermediate variable:
       do {
            const char *_t = (s) == NULL ? "NULL" : (s);
            openpam_log(... , _t);
            return (s);
       } while (0)

That's all I can think of right now, though I'm certain there
are numerous other fixes.

Good luck, (let us know what works!)

Tim


Dag-Erling Smørgrav wrote:
> I'm having trouble with some uncommitted OpenPAM patches that I'd like
> to get into the tree.  The problem actually doesn't occur with a
> normal build, but it prevents me from building a debugging version of
> libpam.
> 
> Part of the patch declares openpam_log(3) as printf-like so gcc can
> check format strings etc.  However, openpam_log(3) is also used in
> debugging macros such as this:
> 
> #define RETURNS(s) do { \
>         if ((s) == NULL) \
>                 openpam_log(PAM_LOG_DEBUG, "returning NULL"); \
>         else \
>                 openpam_log(PAM_LOG_DEBUG, "returning '%s'", (s)); \
>         return (s); \
> } while (0)
> 
> The problem is that when it encounters RETURNS(NULL), gcc complains
> that I'm passing a NULL argument to printf(3), even though it should
> be obvious that I'm not:
> 
> cc -O -pipe -march=pentium2 -I/usr/src/lib/libpam/libpam -I/home/des/projects/openpam/include -DLIB_MAJ=2 -g -DDEBUG -Wsystem-headers -Werror -Wall -Wno-format-y2k -W -Wstrict-prototypes -Wmissing-prototypes -Wpointer-arith -Wreturn-type -Wcast-qual -Wwrite-strings -Wswitch -Wshadow -Wcast-align -Wbad-function-cast -Wchar-subscripts -Winline -Wnested-externs -Wredundant-decls -c /home/des/projects/openpam/lib/openpam_get_option.c
> /home/des/projects/openpam/lib/openpam_get_option.c: In function `openpam_get_option':
> /home/des/projects/openpam/lib/openpam_get_option.c:62: warning: reading through null pointer (arg 4)
> /home/des/projects/openpam/lib/openpam_get_option.c:73: warning: reading through null pointer (arg 4)
> *** Error code 1
> 
> Stop in /usr/src/lib/libpam/libpam.
> 
> I've tried various twists to fool gcc, such as casting (s) to (const
> char *) and adding 0 to it hoping that the addition would defeat its
> NULL pointer check.  Nothing I've tried works, though, and I would
> really hate to have to lower the WANRS level just for this.
> 
> Any suggestions?
> 
> DES