size_t scan_utf8(const char *src,size_t len,uint32_t *dest);
scan_utf8 decodes an unsigned integer in UTF-8 encoding from a memory area holding binary data. It writes the decode value in dest and returns the number of bytes it read from src.
scan_utf8 never reads more than len bytes from src. If the sequence is longer than that, or the memory area contains an invalid sequence, scan_utf8 returns 0 and does not touch dest.
The length of the longest UTF-8 sequence is 5. If the buffer is longer than that, and scan_utf8 fails, then the data was not a valid UTF-8 encoded sequence.
fmt_utf8 and scan_utf8 implement the encoding from UTF-8, but are meant to be able to store integers, not just Unicode code points. Values above 0x10ffff are not valid UTF-8. If you are using this function to parse UTF-8, you need to reject them (see RFC 3629).