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NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | VERSIONS | CONFORMING TO | NOTES | EXAMPLE | SEE ALSO | COLOPHON |
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:
SPLICE_F_MOVE Currently has no effect for tee(); see splice(2).
SPLICE_F_NONBLOCK Do not block on I/O; see splice(2) for further
details.
SPLICE_F_MORE Currently has no effect for tee(), but may be
implemented in the future; see splice(2).
SPLICE_F_GIFT 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.
EAGAIN SPLICE_F_NONBLOCK was specified in flags, and the operation
would block.
EINVAL fd_in or fd_out does not refer to a pipe; or fd_in and fd_out
refer to the same pipe.
ENOMEM 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
Program source
#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)
This page is part of release 4.12 of the Linux man-pages project. A
description of the project, information about reporting bugs, and the
latest version of this page, can be found at
https://www.kernel.org/doc/man-pages/.
Linux 2017-07-13 TEE(2)
Pages that refer to this page: pipe(2), splice(2), syscalls(2), vmsplice(2), pipe(7)