tee — duplicating pipe content
#define _GNU_SOURCE #include <fcntl.h>
long
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 series
of modifier flags, which share the name space with splice(2) and vmsplice(2):
SPLICE_F_MOVECurrently has no effect for tee(); see splice(2).
SPLICE_F_NONBLOCKDo not block on I/O; see splice(2) for further details.
SPLICE_F_MORECurrently has no effect for tee(), but may be implemented in the
future; see splice(2).
SPLICE_F_GIFTUnused 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.
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.
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.
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);
}
splice(2), vmsplice(2), feature_test_macros(7)
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