Re: Gcc Digest, Vol 29, Issue 7

[Yair Lenga <yair.lenga@gmail.com>]

Hi,

Wanted to get some feedback on an idea that I have - trying to address the
age long issue with type check on VA list function - like 'scanf' and
friends. In my specific case, I'm trying to build code that will parse a
list of values from SELECT statement into list of C variables. The type of
the values is known (by inspecting the result set meta-data). My ideal
solution will be to implement something like:

int result_set_read(struct result_set *p_result_set, ...);

Which can be called with

int int_var ; float float_var ; char c[20] ;
result_set_read(rs1, &int_var, &float_var, c) ;

The tricky part is to verify argument type - make sure . One possible path
I thought was - why not leverage the ability to describe scanf like
functions (
result_set_read(rs1, const char *format, ...) __attribute((format (scanf,
2, 3)) ;

And then the above call will be
result_set-read(rs1, "%d %f %s", &int_var, &float_var, c) ;

With the added benefit that GCC will flag as error, if there is mismatch
between the variable and the type. My function parses the scanf format to
decide on conversions (just the basic formatting '%f', '%d', '%*s', ...).
So far big improvement, and the only missing item is the ability to enforce
check on string sizes - to support better checks against buffer overflow
(side note: wish there was ability to force inclusion of the max string
size, similar to the sscanf_s).

My question: does anyone know how much effort it will be to add a new GCC
built-in (or extension), that will automatically generate a descriptive
format string, consistent with scanf formatting, avoiding the need to
manually enter the formatting string. This can be thought of as "poor man
introspection". Simple macro can then be used to generate it

#define RESULT_SET_READ(rs, ...) result_set_read(rs,
__builtin_format(__VA_ARGS__),  __VA_ARGS__)

Practically, making the function "safe" (with respect to buffer overflow,
type conversions) for most use cases.

Any feedback, pointers, ... to how to implement will be appreciated

Yair