libroot: Add stpncpy.

Imported from musl.

It is in POSIX.1-2017.

headers/posix/string.h

@@ -62,6 +62,7 @@ extern char		*strcasestr(const char *string, const char *searchString);
 extern char		*strdup(const char *string);
 extern char		*strndup(const char* string, size_t size);
 extern char		*stpcpy(char *dest, const char *source);
+extern char		*stpncpy(char *dest, const char *source, size_t size);
 
 extern size_t	strlcat(char *dest, const char *source, size_t length);
 extern size_t	strlcpy(char *dest, const char *source, size_t length);

src/system/libroot/posix/musl/string/Jamfile

@@ -10,6 +10,7 @@ for architectureObject in [ MultiArchSubDirSetup ] {
 
 		MergeObject <$(architecture)>posix_musl_string.o :
 			memrchr.c
+			stpncpy.c
 			strchrnul.c
 			strcspn.c
 			swab.c

src/system/libroot/posix/musl/string/stpncpy.c

@@ -0,0 +1,33 @@
+#include <features.h>
+#include <string.h>
+#include <stdint.h>
+#include <limits.h>
+
+#define ALIGN (sizeof(size_t)-1)
+#define ONES ((size_t)-1/UCHAR_MAX)
+#define HIGHS (ONES * (UCHAR_MAX/2+1))
+#define HASZERO(x) ((x)-ONES & ~(x) & HIGHS)
+
+char *__stpncpy(char *restrict d, const char *restrict s, size_t n)
+{
+#if defined(__GNUC__) && __GNUC__ > 3
+	typedef size_t __attribute__((__may_alias__)) word;
+	word *wd;
+	const word *ws;
+	if (((uintptr_t)s & ALIGN) == ((uintptr_t)d & ALIGN)) {
+		for (; ((uintptr_t)s & ALIGN) && n && (*d=*s); n--, s++, d++);
+		if (!n || !*s) goto tail;
+		wd=(void *)d; ws=(const void *)s;
+		for (; n>=sizeof(size_t) && !HASZERO(*ws);
+		       n-=sizeof(size_t), ws++, wd++) *wd = *ws;
+		d=(void *)wd; s=(const void *)ws;
+	}
+#endif
+	for (; n && (*d=*s); n--, s++, d++);
+tail:
+	memset(d, 0, n);
+	return d;
+}
+
+weak_alias(__stpncpy, stpncpy);
+