Make the %V{is} extension handle a NULL pointer like %s does: output "(null)"

Add %M{essage} extension which prints an errno value as the
corresponding string if possible or numerically otherwise.

It is not currently possible to do the syslog(3) like %m extension
because errno would need to get capatured on entry to the first
function in the printf family, so %M requires you to supply errno
as an argument.

Add %Q{uote} extension which will print a string in double quotes with
appropriate back-slash escapes (only) if necessary.

1 files changed, 10 insertions, 0 deletions

diff --git a/lib/libc/stdio/xprintf.c b/lib/libc/stdio/xprintf.c
index 3b27782..c7814fa 100644
--- a/lib/libc/stdio/xprintf.c
+++ b/lib/libc/stdio/xprintf.c
@@ -655,6 +655,16 @@ register_printf_render_std(const unsigned char *specs)
 			    __printf_render_hexdump,
 			    __printf_arginfo_hexdump);
 			break;
+		case 'M':
+			register_printf_render(*specs,
+			    __printf_render_errno,
+			    __printf_arginfo_errno);
+			break;
+		case 'Q':
+			register_printf_render(*specs,
+			    __printf_render_quote,
+			    __printf_arginfo_quote);
+			break;
 		case 'T':
 			register_printf_render(*specs,
 			    __printf_render_time,