[o11c]

In my experience, the biggest offender is programs trying to do syscalls directly (possibly for async-signal-safety), but not being aware of `writev`. Especially programs that do colored output can be really stupid here. Sometimes there are stupid programs that use multiple processes to do colored output even (IIRC CMake is a big offender here, but CMake is infamous for refusing to fix bugs)!

The pipe buffer is big enough that sane programs aren't likely to run into problems. The math:

PIPE_BUF is 512 per POSIX but in practice 4096 on Linux (probably others too?). If we assume a horrible-and-unlikely 12 formatting characters per real character (and assume a real character is non-BMP and thus 4 bytes, but still single-column), Linux has enough for 64 characters. With more reasonable assumptions (mostly ascii, no more than 4 formatting changes per line) we get more like 6 lines of output being atomic on Linux, and even POSIX being likely to get at least one whole line.