No, this is not correct. The idea of sendfile() is that you can send a physical address (probably something in the buffer cache) directly to a network adapter

No, this is not correct. The idea of sendfile() is that you can send a physical address (probably something in the buffer cache) directly to a network adapter in a zero copy manner. That's current impossible with a send()/write() to a file descriptor in Linux because you can't construct an SKB from a userspace address without forcefully pinning memory. Pinning memory from userspace is a privileged operation. OTOH, since buffer cache is not part of the memory of a process, you can obtain a physical address of it. There's no magic in the kernel that avoids context switches. If a kernel thread has to switch to another kernel thread, that's still a context switch. And syscalls are ridiculously cheap on modern hardware. sysenter is like < 100 cycles. That said, there's no theoretical advantage to doing this via sendfile verses doing a send() from an mmap()'d file. If Linux was smarter, it could realize instantly that the address given to send is file backed and could construct an SKB from the physical memory without pinning. This is what the reference to a "5 minute hack" was vs. Rob Pike's claim that the interface exists to work around a problem in Linux. He's completely correct.