libc: avoid weak_alias() warnings from gcc 9.

gcc 9 started to warn about alias created with different "attributes"
(__attribute__(...)) from the source symbol. Unfortunately, in a couple
of cases gcc also started to add a gratuituous "nothrow" attribute to
our C code (which can never throw in any case...) and as a result,
weak_alias throws.

I tried to fix this in a number of ways, but none of them worked
satisfactorily. This patch does something ugly, that works - add
the nothrow attribute manually in the two places which had this
problem. Unfortunately it means we edited yet another Musl source
file (stdio/sscanf.c).

Signed-off-by: Nadav Har'El <[email protected]>
Message-Id: <[email protected]>

Makefile

@@ -1515,7 +1515,7 @@ musl += stdio/setlinebuf.o
 musl += stdio/setvbuf.o
 musl += stdio/snprintf.o
 musl += stdio/sprintf.o
-musl += stdio/sscanf.o
+libc += stdio/sscanf.o
 libc += stdio/stderr.o
 libc += stdio/stdin.o
 libc += stdio/stdout.o

libc/stdio/sscanf.c

@@ -0,0 +1,19 @@
+#include <stdio.h>
+#include <stdarg.h>
+#include "libc.h"
+
+int sscanf(const char *restrict s, const char *restrict fmt, ...)
+{
+	int ret;
+	va_list ap;
+	va_start(ap, fmt);
+	ret = vsscanf(s, fmt, ap);
+	va_end(ap);
+	return ret;
+}
+
+#if __GNUC__ >= 9
+weak_alias(sscanf,__isoc99_sscanf) __attribute__((nothrow));
+#else
+weak_alias(sscanf,__isoc99_sscanf);
+#endif

libc/stdio/vsscanf.c

@@ -15,4 +15,8 @@ int vsscanf(const char *restrict s, const char *restrict fmt, va_list ap)
 	return vfscanf(&f, fmt, ap);
 }
 
+#if __GNUC__ >= 9
+weak_alias(vsscanf,__isoc99_vsscanf) __attribute__((nothrow));
+#else
 weak_alias(vsscanf,__isoc99_vsscanf);
+#endif