diff --git a/libavutil/avstring.h b/libavutil/avstring.h
index bc24ea14d1..f25dd31d03 100644
--- a/libavutil/avstring.h
+++ b/libavutil/avstring.h
@@ -73,4 +73,18 @@ size_t av_strlcpy(char *dst, const char *src, size_t size);
  */
 size_t av_strlcat(char *dst, const char *src, size_t size);
 
+/**
+ * Append output to a string, according to a format. Never write out of
+ * the destination buffer, and and always put a terminating 0 within
+ * the buffer.
+ * @param dst destination buffer (string to which the output is
+ *  appended)
+ * @param size total size of the destination buffer
+ * @param fmt printf-compatible format string, specifying how the
+ *  following parameters are used
+ * @return the length of the string that would have been generated
+ *  if enough space had been available
+ */
+size_t av_strlcatf(char *dst, size_t size, const char *fmt, ...);
+
 #endif /* AVUTIL_STRING_H */
diff --git a/libavutil/string.c b/libavutil/string.c
index 2f4de8ef50..8ee2a6b12f 100644
--- a/libavutil/string.c
+++ b/libavutil/string.c
@@ -19,6 +19,8 @@
  * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
  */
 
+#include <stdarg.h>
+#include <stdio.h>
 #include <string.h>
 #include <ctype.h>
 #include "avstring.h"
@@ -62,3 +64,15 @@ size_t av_strlcat(char *dst, const char *src, size_t size)
         return len + strlen(src);
     return len + av_strlcpy(dst + len, src, size - len);
 }
+
+size_t av_strlcatf(char *dst, size_t size, const char *fmt, ...)
+{
+    int len = strlen(dst);
+    va_list vl;
+
+    va_start(vl, fmt);
+    len += vsnprintf(dst + len, size > len ? size - len : 0, fmt, vl);
+    va_end(vl);
+
+    return len;
+}