a2i(): Implement default arguments with macro magic

[alejandro-colomar]

This needs some ISO C23 feature (__VA_OPT__), but we're already using that in stprintf_a(), which we have since 4.15.0, so it should be fine.

alx@devuan:~/src/shadow/shadow/master$ git blame -- lib/string/sprintf/snprintf.h | grep __VA_OPT__
ba3456c9fd lib/string/sprintf/snprintf.h (Alejandro Colomar       2025-05-21 13:33:37 +0200 22) 	snprintf_(s, countof(s), fmt __VA_OPT__(,) __VA_ARGS__)               \
alx@devuan:~/src/shadow/shadow/master$ git show ba3456c9fd | grep __VA_OPT__
-	snprintf_(s, NITEMS(s), fmt __VA_OPT__(,) __VA_ARGS__)                \
+	snprintf_(s, countof(s), fmt __VA_OPT__(,) __VA_ARGS__)               \
alx@devuan:~/src/shadow/shadow/master$ git blame ba3456c9fd^ -- lib/string/sprintf/snprintf.h | grep __VA_OPT__
22272347b6 lib/string/sprintf/snprintf.h (Alejandro Colomar 2024-06-27 11:29:56 +0200 21) 	snprintf_(s, NITEMS(s), fmt __VA_OPT__(,) __VA_ARGS__)                \
alx@devuan:~/src/shadow/shadow/master$ git show 22272347b6 | grep -e __VA_OPT__ -e ^diff | grep -B1 -e ^- -e ^+
diff --git a/lib/string/sprintf.h b/lib/string/sprintf/snprintf.h
-	snprintf_(s, NITEMS(s), fmt __VA_OPT__(,) __VA_ARGS__)
+	snprintf_(s, NITEMS(s), fmt __VA_OPT__(,) __VA_ARGS__)                \
alx@devuan:~/src/shadow/shadow/master$ git blame 22272347b6^ -- lib/string/sprintf.h | grep __VA_OPT__
8c6634d9bc lib/string/sprintf.h (Alejandro Colomar 2023-08-26 14:54:44 +0200 23) 	snprintf_(s, NITEMS(s), fmt __VA_OPT__(,) __VA_ARGS__)
alx@devuan:~/src/shadow/shadow/master$ git show 8c6634d9bc | grep -e __VA_OPT__ -e ^diff | grep -B1 -e ^- -e ^+
diff --git a/lib/string/sprintf.h b/lib/string/sprintf.h
-	len_ = snprintf(s, sz_, fmt __VA_OPT__(,) __VA_ARGS__);               \
+	snprintf_(s, NITEMS(s), fmt __VA_OPT__(,) __VA_ARGS__)
alx@devuan:~/src/shadow/shadow/master$ git blame 8c6634d9bc^ -- lib/string/sprintf.h | grep __VA_OPT__
ce4c4d4ad5 lib/string/sprintf.h (Alejandro Colomar 2023-08-26 12:11:59 +0200 27) 	len_ = snprintf(s, sz_, fmt __VA_OPT__(,) __VA_ARGS__);               \
alx@devuan:~/src/shadow/shadow/master$ git show ce4c4d4ad5 | grep -e __VA_OPT__ -e ^diff | grep -B1 -e ^- -e ^+
diff --git a/lib/string/sprintf.h b/lib/string/sprintf.h
+	len_ = snprintf(s, sz_, fmt __VA_OPT__(,) __VA_ARGS__);               \
alx@devuan:~/src/shadow/shadow/master$ git ref ce4c4d4ad5
ce4c4d4ad592 (2023-12-15; "lib/string/sprintf.h: Add SNPRINTF() macro")
alx@devuan:~/src/shadow/shadow/master$ git describe --contains ce4c4d4ad5
4.15.0-rc1~73

I've tested this in liba2i, and it works like a charm.

---

Revisions:

v1b

• Rebase

$ git rd 
1:  71ee7d526 = 1:  a4ce7786a lib/macro.h: DEFAULT(): Add macro
2:  439ccd4ae ! 2:  1d838ce38 lib/atoi/a2i.h: a2i(): Specify default parameter values
    @@ lib/atoi/a2i.h
     -  T            max_ = max;                                      \
     +  T            min_ = DEFAULT(type_min(T), min);                \
     +  T            max_ = DEFAULT(type_max(T), max);                \
    -                                                                       \
    +                                                                 \
        int  status;                                                  \
    -                                                                       \
    +                                                                 \
     @@
      #define a2ul(...)   a2i(unsigned long, __VA_ARGS__)
      #define a2ull(...)  a2i(unsigned long long, __VA_ARGS__)
3:  5e3800fcf ! 3:  6316a8396 lib/, src/: Use default argument values in a2i() calls
    @@ lib/limits.c: static int setrlimit_value (unsigned int resource,
     +          if (a2i(rlim_t, &l, value, NULL, 10, 0,) == -1
                    && errno != ENOTSUP)
                {
    -                   return 0;  // FIXME: We could instead throw an error, though.
    +                   return 0;  // FIXME: we could instead throw an error, though.
     
      ## lib/shadow/shadow/sgetspent.c ##
     @@ lib/shadow/shadow/sgetspent.c: sgetspent(const char *s)

[alejandro-colomar]

For the curious, here's a small example of how this preprocessor magic works:

#include <a2i.h>

#define VA_IFNOT_(...)       __VA_ARGS__
#define VA_IFNOT_1(...)
#define VA_IFNOT(cond, ...)  VA_IFNOT_ ## cond (__VA_ARGS__)

#define DEFAULT_(def, ...)                                            \
(                                                                     \
	/* default:  */ VA_IFNOT(__VA_OPT__(1), def)                  \
	/* override: */ __VA_OPT__(__VA_ARGS__)                       \
)

#define DEFAULT(def, ovr)  DEFAULT_(def, ovr)

#define foo(a, b)  bar(DEFAULT(42, a), DEFAULT(43, b))


int bar(int, int);


int
main(void)
{
	long  l;

	l = foo(7, 111);
	l = foo(77,);
	l = foo(,11);
	l = foo(,);
	l = foo();  // Too few arguments
	l = foo(,,);  // Too many arguments
}

alx@devuan:~/tmp$ gcc -Wall -Wextra -E def.c | grepc main
def.c:30:17: error: macro ‘foo’ requires 2 arguments, but only 1 given
   30 |         l = foo();  // Too few arguments
      |                 ^
def.c:15:9: note: macro ‘foo’ defined here
   15 | #define foo(a, b)  bar(DEFAULT(42, a), DEFAULT(43, b))
      |         ^~~
def.c:31:19: error: macro ‘foo’ passed 3 arguments, but takes just 2
   31 |         l = foo(,,);  // Too many arguments
      |                   ^
def.c:15:9: note: macro ‘foo’ defined here
   15 | #define foo(a, b)  bar(DEFAULT(42, a), DEFAULT(43, b))
      |         ^~~
(standard input):int
main(void)
{
 long l;

 l = bar(( 7 ), ( 111 ));
 l = bar(( 77 ), ( 43 ));
 l = bar(( 42 ), ( 11 ));
 l = bar(( 42 ), ( 43 ));
 l = foo;
 l = foo;
}

[alejandro-colomar]

@kees