Probing further at printf-surprise.sh, it appears that the problem is:

* musl is unaffected by the printf bug being tested

* therefore, it does successfully output 20 MiB of blank space

* `head -c 10 fifo` will read the first 10 characters of the fifo

* the rest of the 20 MiB cannot be written, so SIGPIPE is received

* 'exit=141' (- 128 = 13 = SIGPIPE)


I have attached a patch that fixes both csplit-io-err and
printf-surprise on my test box.

-- 
A. Wilcox (awilfox)
Project Lead, Adélie Linux
http://adelielinux.org