NAME
tee - duplicating pipe content
SYNOPSIS
#define _GNU_SOURCE
#include <fcntl.h>
ssize_t tee(int fd_in, int fd_out, size_t len, unsigned int flags);
DESCRIPTION
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 series of modifier flags, which share the name space with
splice(2) and vmsplice(2):
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).
RETURN VALUE
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.
ERRORS
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.
VERSIONS
The tee() system call first appeared in Linux 2.6.17.
CONFORMING TO
This system call is Linux-specific.
NOTES
Conceptually, tee() copies the data between the two pipes. In reality
no real data copying takes place though: under the covers, tee()
assigns data in the output by merely grabbing a reference to the input.
EXAMPLE
The following example implements a basic tee(1) program using the tee()
system call.
#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);
}
SEE ALSO
splice(2), vmsplice(2), feature_test_macros(7)
COLOPHON
This page is part of release 3.24 of the Linux man-pages project. A
description of the project, and information about reporting bugs, can
be found at http://www.kernel.org/doc/man-pages/.