[enriquto]

Ignoring the return of printf is a "bug". For the hello-world example, you can simply pass the printf value to main: "return printf(...) > 0;"

[dataflow]

It's not necessarily an error to print less than you intended though. The consumer might have simply decided that they didn't need the rest of the input. Whether or not it's an error depends on why the write failed to occur. Usually out-of-space is an error, whereas pipe-is-closed/has-reached-EOF is not.

[jwilk]

That's insufficient, because printf() is buffered.