vasnprintf: Fix handling of # flag in %b, %B directives

[Bruno Haible via Gnulib discussion list]

When building a testdir of vasnprintf-posix, configured on glibc with
the command

  gl_cv_func_printf_precision=no gl_cv_func_printf_enomem=no ./configure

I get this test failure:

  test-vasnprintf-posix.c:4583: assertion 'strcmp (result, 
"0b00000011000000111001 33") == 0' failed
  Aborted (core dumped)

This patch fixes it.


2025-04-10  Bruno Haible  <br...@clisp.org>

        vasnprintf: Fix handling of # flag in %b, %B directives.
        * lib/vasnprintf.c (VASNPRINTF): In the prec_ourselves code, treat a 0b
        prefix like a 0x prefix.

diff --git a/lib/vasnprintf.c b/lib/vasnprintf.c
index e7db39c74c..a07c1377dd 100644
--- a/lib/vasnprintf.c
+++ b/lib/vasnprintf.c
@@ -6876,10 +6876,13 @@ VASNPRINTF (DCHAR_T *resultbuf, size_t *lengthp,
                                 || *prec_ptr == ' '))
                           prefix_count = 1;
                         /* Put the additional zeroes after the 0x prefix if
-                           (flags & FLAG_ALT) || (dp->conversion == 'p').  */
+                           (flags & FLAG_ALT) || (dp->conversion == 'p'), or
+                           after the 0b prefix if (flags & FLAG_ALT).  */
                         else if (count >= 2
                                  && prec_ptr[0] == '0'
-                                 && (prec_ptr[1] == 'x' || prec_ptr[1] == 'X'))
+                                 && (prec_ptr[1] == 'x' || prec_ptr[1] == 'X'
+                                     || prec_ptr[1] == 'b'
+                                     || prec_ptr[1] == 'B'))
                           prefix_count = 2;
 
                         move = count - prefix_count;