Cool trick, thanks for sharing. I don't get why there isn't a suitable snscanf function that takes the buffer length as an argument and returns the number of bytes parsed?
fmemopen takes the buffer length, and there is no need to have the buffer \0 terminated, so instead of strlen you can also just give the buffer size.
The number of bytes parsed can be fetched with the scanf %n format specifier.
the proper remedy is to simply wrap the string to parse with fmemopen(3), which makes the temporary FILE object explicit and persistent for the whole parse, and needs just one strlen call.
https://news.ycombinator.com/item?id=26343149