TEE(2) Linux Programmer's Manual TEE(2)

tee - duplicating pipe content

#define _GNU_SOURCE         /* See feature_test_macros(7) */
#include <fcntl.h>
ssize_t tee(int fd_in, int fd_out, size_t len, unsigned int flags);

tee() duplicates up to len bytes of data from the pipe referred to by the file descriptor fd_in to the pipe referred to by the file descriptor fd_out. It does not consume the data that is duplicated from fd_in; therefore, that data can be copied by a subsequent splice(2).
flags is a bit mask that is composed by ORing together zero or more of the following values:
Currently has no effect for tee(); see splice(2).
Do not block on I/O; see splice(2) for further details.
Currently has no effect for tee(), but may be implemented in the future; see splice(2).
Unused for tee(); see vmsplice(2).

Upon successful completion, tee() returns the number of bytes that were duplicated between the input and output. A return value of 0 means that there was no data to transfer, and it would not make sense to block, because there are no writers connected to the write end of the pipe referred to by fd_in.
On error, tee() returns -1 and errno is set to indicate the error.

SPLICE_F_NONBLOCK was specified in flags or one of the file descriptors had been marked as nonblocking (O_NONBLOCK), and the operation would block.
fd_in or fd_out does not refer to a pipe; or fd_in and fd_out refer to the same pipe.
Out of memory.

The tee() system call first appeared in Linux 2.6.17; library support was added to glibc in version 2.5.

This system call is Linux-specific.

Conceptually, tee() copies the data between the two pipes. In reality no real data copying takes place though: under the covers, tee() assigns data to the output by merely grabbing a reference to the input.

The example below implements a basic tee(1) program using the tee() system call. Here is an example of its use:

$  date |./a.out out.log | cat
Tue Oct 28 10:06:00 CET 2014
$  cat out.log
Tue Oct 28 10:06:00 CET 2014

#define _GNU_SOURCE
#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <errno.h>
#include <limits.h>
int main(int argc, char *argv[]) { int fd; int len, slen;
if (argc != 2) { fprintf(stderr, "Usage: %s <file>\n", argv[0]); exit(EXIT_FAILURE); }
fd = open(argv[1], O_WRONLY | O_CREAT | O_TRUNC, 0644); if (fd == -1) { perror("open"); exit(EXIT_FAILURE); }
do { /* * tee stdin to stdout. */ len = tee(STDIN_FILENO, STDOUT_FILENO, INT_MAX, SPLICE_F_NONBLOCK);
if (len < 0) { if (errno == EAGAIN) continue; perror("tee"); exit(EXIT_FAILURE); } else if (len == 0) break;
/* * Consume stdin by splicing it to a file. */ while (len > 0) { slen = splice(STDIN_FILENO, NULL, fd, NULL, len, SPLICE_F_MOVE); if (slen < 0) { perror("splice"); break; } len -= slen; } } while (1);
close(fd); exit(EXIT_SUCCESS); }

splice(2), vmsplice(2), pipe(7)
2019-03-06 Linux