tee(2)

Synopsis

#define _GNU_SOURCE
#include <fcntl.h>

ssize_t tee(int fd_in, int fd_out, size_t size, unsigned int flags);

Rod's take

tee(1) reads stdin and writes it twice, through userspace both times. tee(2) does it inside the kernel: it duplicates what's sitting in one pipe into another without consuming the source, so a following splice can still drain the original. Both ends must be pipes. Log fanout is tee into a scratch pipe, splice the original to one place, splice the copy to the other, and nobody forks a shell pipeline. Only page pointers get duplicated, never the pages. Small, obscure, and exactly right for the job.

Flags and arguments that matter

Errors you will actually hit

Example

/* stdin and stdout are pipes; also copy everything to logfd */
for (;;) {
    ssize_t n = tee(STDIN_FILENO, STDOUT_FILENO, INT_MAX, SPLICE_F_NONBLOCK);
    if (n < 0 && errno == EAGAIN)
        continue;               /* real code: poll() first */
    if (n < 0)
        err(1, "tee");
    if (n == 0)
        break;
    while (n > 0) {             /* consume what tee duplicated */
        ssize_t s = splice(STDIN_FILENO, NULL, logfd, NULL, n, 0);
        if (s <= 0)
            err(1, "splice");
        n -= s;
    }
}

Trimmed from the full walkthrough in splice and tee: zero-copy data movement, 2.6.17 edition.

BSD equivalent

none