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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | STANDARDS | HISTORY | NOTES | EXAMPLES | SEE ALSO |
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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 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.
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 <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)
Linux man-pages (unreleased) (date) 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 2023-12-22 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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