diff --git a/include/string_checked.h b/include/string_checked.h
index b9bf596ccb1..3683163ec82 100644
--- a/include/string_checked.h
+++ b/include/string_checked.h
@@ -165,6 +165,8 @@ char *strtok(char * restrict s1 : itype(restrict _Nt_array_ptr<char>),
 char *strerror(int errnum) : itype(_Nt_array_ptr<char>);
 size_t strlen(const char *s : itype(_Nt_array_ptr<const char>));
 
+char *strdup(const char *s : itype(_Nt_array_ptr<const char>)) : itype(_Nt_array_ptr<char>);
+
 #include "_builtin_string_checked.h"
 
 #pragma CHECKED_SCOPE pop