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tee() - Unix, Linux System Call
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NAME
tee - duplicating pipe content
SYNOPSIS
#define _GNU_SOURCE
#include <fcntl.h>
long tee(int fd_in, int fd_out, size_t len
", unsigned int " flags );
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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):
| Tag | Description |
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SPLICE_F_MOVE |
Currently has no effect for
tee(); see
splice(2).
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SPLICE_F_NONBLOCK |
Do not block on I/O; see
splice(2)
for further details.
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SPLICE_F_MORE |
Currently has no effect for
tee(), but may be implemented in the future; see
splice(2).
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SPLICE_F_GIFT |
Unused for
tee(); see
vmsplice(2).
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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
| Tag | Description |
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EINVAL |
fd_in or
fd_out does not refer to a pipe; or
fd_in and
fd_out refer to the same pipe.
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ENOMEM |
Out of memory.
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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(2)
system call.
#define _GNU_SOURCE
#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <assert.h>
#include <errno.h>
#include <limits.h>
int
main(int argc, char *argv[])
{
int fd;
int len, slen;
assert(argc == 2);
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);
}
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HISTORY
The
tee(2)
system call first appeared in Linux-2.6.17.
CONFORMING TO
This system call is Linux specific.
SEE ALSO
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