Pipe buffer size is 4k or 64k?

Question

I read in multiple places that the default buffer size for a pipe is 4kB (for instance, here), and my ulimit -a tends to confirm that statement:

Answer 1

The other answers tell you that the pipe size is 64 KB. The reason why PIPE_BUF is 4KB is that PIPE_BUF is the largest size for which writes are guaranteed to be atomic. See http://pubs.opengroup.org/onlinepubs/9699919799/functions/write.html