diff --git a/include/string.h b/include/string.h
index 9a38656b2600..3844d41738ec 100644
--- a/include/string.h
+++ b/include/string.h
@@ -58,21 +58,21 @@ __BEGIN_DECLS
 #if __POSIX_VISIBLE >= 200112 || __XSI_VISIBLE
 void	*memccpy(void * __restrict, const void * __restrict, int, size_t);
 #endif
-void	*memchr(const void *, int, size_t);
-int	 memcmp(const void *, const void *, size_t);
+void	*memchr(const void *, int, size_t) __pure;
+int	 memcmp(const void *, const void *, size_t) __pure;
 void	*memcpy(void * __restrict, const void * __restrict, size_t);
 void	*memmove(void *, const void *, size_t);
 void	*memset(void *, int, size_t);
 #if __BSD_VISIBLE
 char	*stpcpy(char *, const char *);
-char	*strcasestr(const char *, const char *);
+char	*strcasestr(const char *, const char *) __pure;
 #endif
 char	*strcat(char * __restrict, const char * __restrict);
-char	*strchr(const char *, int);
-int	 strcmp(const char *, const char *);
+char	*strchr(const char *, int) __pure;
+int	 strcmp(const char *, const char *) __pure;
 int	 strcoll(const char *, const char *);
 char	*strcpy(char * __restrict, const char * __restrict);
-size_t	 strcspn(const char *, const char *);
+size_t	 strcspn(const char *, const char *) __pure;
 #if __POSIX_VISIBLE >= 200112 || __XSI_VISIBLE
 char	*strdup(const char *);
 #endif
@@ -84,24 +84,24 @@ int	 strerror_r(int, char *, size_t);
 size_t	 strlcat(char *, const char *, size_t);
 size_t	 strlcpy(char *, const char *, size_t);
 #endif
-size_t	 strlen(const char *);
+size_t	 strlen(const char *) __pure;
 #if __BSD_VISIBLE
 void	 strmode(int, char *);
 #endif
 char	*strncat(char * __restrict, const char * __restrict, size_t);
-int	 strncmp(const char *, const char *, size_t);
+int	 strncmp(const char *, const char *, size_t) __pure;
 char	*strncpy(char * __restrict, const char * __restrict, size_t);
 #if __BSD_VISIBLE
-char	*strnstr(const char *, const char *, size_t);
+char	*strnstr(const char *, const char *, size_t) __pure;
 #endif
-char	*strpbrk(const char *, const char *);
-char	*strrchr(const char *, int);
+char	*strpbrk(const char *, const char *) __pure;
+char	*strrchr(const char *, int) __pure;
 #if __BSD_VISIBLE
 char	*strsep(char **, const char *);
 char	*strsignal(int);
 #endif
-size_t	 strspn(const char *, const char *);
-char	*strstr(const char *, const char *);
+size_t	 strspn(const char *, const char *) __pure;
+char	*strstr(const char *, const char *) __pure;
 char	*strtok(char * __restrict, const char * __restrict);
 #if __POSIX_VISIBLE >= 199506 || __XSI_VISIBLE >= 500
 char	*strtok_r(char *, const char *, char **);