tee — duplicating pipe content
#define _GNU_SOURCE #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 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).
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
() 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); }