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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | STANDARDS | HISTORY | NOTES | EXAMPLES | SEE ALSO | COLOPHON |
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tee(2) System Calls Manual tee(2)
tee - duplicating pipe content
Standard C library (libc, -lc)
#define _GNU_SOURCE /* See feature_test_macros(7) */
#include <fcntl.h>
ssize_t tee(int fd_in, int fd_out, size_t size, unsigned int flags);
tee() duplicates up to size 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 or one of the file
descriptors had been marked as nonblocking (O_NONBLOCK),
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.
Linux.
Linux 2.6.17, glibc 2.5.
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 <errno.h>
#include <fcntl.h>
#include <limits.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <unistd.h>
int
main(int argc, char *argv[])
{
int fd;
ssize_t size, ssize;
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);
}
for (;;) {
/*
* tee stdin to stdout.
*/
size = tee(STDIN_FILENO, STDOUT_FILENO,
INT_MAX, SPLICE_F_NONBLOCK);
if (size < 0) {
if (errno == EAGAIN)
continue;
perror("tee");
exit(EXIT_FAILURE);
}
if (size == 0)
break;
/*
* Consume stdin by splicing it to a file.
*/
while (size > 0) {
ssize = splice(STDIN_FILENO, NULL, fd, NULL,
size, SPLICE_F_MOVE);
if (ssize < 0) {
perror("splice");
exit(EXIT_FAILURE);
}
size -= ssize;
}
}
close(fd);
exit(EXIT_SUCCESS);
}
splice(2), vmsplice(2), pipe(7)
This page is part of the man-pages (Linux kernel and C library
user-space interface documentation) project. Information about
the project can be found at
⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report
for this manual page, see
⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
This page was obtained from the tarball man-pages-6.15.tar.gz
fetched from
⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on
2025-08-11. If you discover any rendering problems in this HTML
version of the page, or you believe there is a better or more up-
to-date source for the page, or you have corrections or
improvements to the information in this COLOPHON (which is not
part of the original manual page), send a mail to
[email protected]
Linux man-pages 6.15 2025-05-17 tee(2)
Pages that refer to this page: io_uring_enter2(2), io_uring_enter(2), pipe(2), splice(2), syscalls(2), vmsplice(2), io_uring_prep_tee(3), pipe(7)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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