diff --git a/include/string.h b/include/string.h
--- a/include/string.h
+++ b/include/string.h
@@ -54,7 +54,7 @@
 #endif
 
 __BEGIN_DECLS
-#if __XSI_VISIBLE >= 600
+#if __XSI_VISIBLE >= 600 || __ISO_C_VISIBLE >= 2023
 void	*memccpy(void * __restrict, const void * __restrict, int, size_t);
 #endif
 void	*memchr(const void *, int, size_t) __pure;
@@ -91,7 +91,7 @@
 int	 strcoll(const char *, const char *);
 char	*(strcpy)(char * __restrict, const char * __restrict);
 size_t	 strcspn(const char *, const char *) __pure;
-#if __POSIX_VISIBLE >= 200112 || __XSI_VISIBLE
+#if __POSIX_VISIBLE >= 200112 || __XSI_VISIBLE || __ISO_C_VISIBLE >= 2023
 char	*strdup(const char *) __malloc_like;
 #endif
 char	*strerror(int);
@@ -115,8 +115,10 @@
 char	*(strncat)(char * __restrict, const char * __restrict, size_t);
 int	 strncmp(const char *, const char *, size_t) __pure;
 char	*(strncpy)(char * __restrict, const char * __restrict, size_t);
-#if __POSIX_VISIBLE >= 200809
+#if __POSIX_VISIBLE >= 200809 || __ISO_C_VISIBLE >= 2023
 char	*strndup(const char *, size_t) __malloc_like;
+#endif
+#if __POSIX_VISIBLE >= 200809
 size_t	 strnlen(const char *, size_t) __pure;
 #endif
 #if __BSD_VISIBLE
diff --git a/include/time.h b/include/time.h
--- a/include/time.h
+++ b/include/time.h
@@ -155,6 +155,8 @@
 #if __POSIX_VISIBLE >= 199506
 char *asctime_r(const struct tm *, char *);
 char *ctime_r(const time_t *, char *);
+#endif
+#if __POSIX_VISIBLE >= 199506 || __ISO_C_VISIBLE >= 2023
 struct tm *gmtime_r(const time_t *, struct tm *);
 struct tm *localtime_r(const time_t *, struct tm *);
 #endif