[joosters]

You'd have to check the various POSIX standards to be sure - and have to then verify that the OS/libc actually follows them, but you can pretty much rely on every libc's I/O wrapper functions to handle interrupted system calls or incomplete writes. I've never seen any code check the return value of a printf() to verify that all the characters were printed.

As you say, the GNU versions definitely handle EINTR - the linux man page for puts() just says it returns a non-negative value on success, it's not even specified whether it returns the number of bytes written or not.