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 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 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.
The
tee() system call first appeared in Linux 2.6.17; library support was
added in glibc 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 <errno.h>
#include <fcntl.h>
#include <limits.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
int
main(int argc, char *argv[])
{
int fd;
ssize_t 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);
}
for (;;) {
/*
* 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);
}
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");
exit(EXIT_FAILURE);
}
len -= slen;
}
}
close(fd);
exit(EXIT_SUCCESS);
}
splice(2),
vmsplice(2),
pipe(7)