vasnwprintf: Fix test failures on musl libc.

* m4/vasnprintf.m4 (gl_PREREQ_VASNWPRINTF): Invoke gl_MUSL_LIBC.
* lib/vasnprintf.c (VASNPRINTF): On musl libc, when WIDE_CHAR_VERSION,
- force pad_ourselves to be 1,
- don't use %n.
Fix zero-padding when the result starts with a prefix "0x" or "0b".
* modules/vasnwprintf (Files): Add musl.m4.
* doc/posix-functions/swprintf.texi: Mention two musl libc bugs.

1 files changed, 10 insertions, 0 deletions

diff --git a/doc/posix-functions/swprintf.texi b/doc/posix-functions/swprintf.texi
index 906ee521aa..a87bc06c86 100644
--- a/doc/posix-functions/swprintf.texi
+++ b/doc/posix-functions/swprintf.texi
@@ -26,11 +26,21 @@ This function does not support the @samp{n} directive on some platforms:
 glibc when used with @code{_FORTIFY_SOURCE >= 2} (set by default on Ubuntu),
 macOS 11.1, MSVC 14.
 @item
+This function sometimes returns a wrong value through the @samp{n} directive
+on some platforms:
+@c https://www.openwall.com/lists/musl/2023/03/19/1
+musl libc 1.2.3.
+@item
 On Windows and 32-bit AIX platforms, @code{wchar_t} is a 16-bit type and therefore cannot
 accommodate all Unicode characters.
 @item
 On Windows, this function does not take a buffer size as second argument.
 @item
+This function ignores the minimum field width in the @samp{lc} directive
+on some platforms:
+@c https://www.openwall.com/lists/musl/2023/03/20/1
+musl libc 1.2.3.
+@item
 This function does not support the @samp{b} directive, required by ISO C23,
 on some platforms:
 glibc 2.34, musl libc, macOS 12.5, FreeBSD 13.1, NetBSD 9.0, OpenBSD 7.2,