diff --git a/libavutil/avstring.h b/libavutil/avstring.h
index ea7d1dcb43..ffb7aa6bfa 100644
--- a/libavutil/avstring.h
+++ b/libavutil/avstring.h
@@ -203,22 +203,22 @@ char *av_strtok(char *s, const char *delim, char **saveptr);
 /**
  * Locale-independent conversion of ASCII isdigit.
  */
-int av_isdigit(int c);
+av_const int av_isdigit(int c);
 
 /**
  * Locale-independent conversion of ASCII isgraph.
  */
-int av_isgraph(int c);
+av_const int av_isgraph(int c);
 
 /**
  * Locale-independent conversion of ASCII isspace.
  */
-int av_isspace(int c);
+av_const int av_isspace(int c);
 
 /**
  * Locale-independent conversion of ASCII characters to uppercase.
  */
-static inline int av_toupper(int c)
+static inline av_const int av_toupper(int c)
 {
     if (c >= 'a' && c <= 'z')
         c ^= 0x20;
@@ -228,7 +228,7 @@ static inline int av_toupper(int c)
 /**
  * Locale-independent conversion of ASCII characters to lowercase.
  */
-static inline int av_tolower(int c)
+static inline av_const int av_tolower(int c)
 {
     if (c >= 'A' && c <= 'Z')
         c ^= 0x20;
@@ -238,7 +238,7 @@ static inline int av_tolower(int c)
 /**
  * Locale-independent conversion of ASCII isxdigit.
  */
-int av_isxdigit(int c);
+av_const int av_isxdigit(int c);
 
 /**
  * Locale-independent case-insensitive compare.