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:
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.
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_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.
This page is part of release 5.10 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/.