lavu: add av_strtok()

The function strtok_r() is part of the POSIX.1 specification, but is not
available on some platforms. We provide an internal implementation, so we
do not need to rely on a platform implementation.

libavutil/avstring.h

@@ -141,4 +141,28 @@ char *av_d2str(double d);
  */
 char *av_get_token(const char **buf, const char *term);
 
+/**
+ * Split the string into several tokens which can be accessed by
+ * successive calls to av_strtok().
+ *
+ * A token is defined as a sequence of characters not belonging to the
+ * set specified in delim.
+ *
+ * On the first call to av_strtok(), s should point to the string to
+ * parse, and the value of saveptr is ignored. In subsequent calls, s
+ * should be NULL, and saveptr should be unchanged since the previous
+ * call.
+ *
+ * This function is similar to strtok_r() defined in POSIX.1.
+ *
+ * @param s the string to parse, may be NULL
+ * @param delim 0-terminated list of token delimiters, must be non-NULL
+ * @param saveptr user-provided pointer which points to stored
+ * information necessary for av_strtok() to continue scanning the same
+ * string. saveptr is updated to point to the next character after the
+ * first delimiter found, or to NULL if the string was terminated
+ * @return the found token, or NULL when no token is found
+ */
+char *av_strtok(char *s, const char *delim, char **saveptr);
+
 #endif /* AVUTIL_AVSTRING_H */