.\" This manpage is Copyright (C) 2006 Jens Axboe .\" and Copyright (C) 2006 Michael Kerrisk .\" .\" SPDX-License-Identifier: Linux-man-pages-copyleft .\" .TH tee 2 2022-12-04 "Linux man-pages 6.03" .SH NAME tee \- duplicating pipe content .SH LIBRARY Standard C library .RI ( libc ", " \-lc ) .SH SYNOPSIS .nf .BR "#define _GNU_SOURCE" " /* See feature_test_macros(7) */" .B #include .PP .BI "ssize_t tee(int " fd_in ", int " fd_out ", size_t " len \ ", unsigned int " flags ); .fi .\" Return type was long before glibc 2.7 .SH DESCRIPTION .\" Example programs http://brick.kernel.dk/snaps .\" .\" .\" add a "tee(in, out1, out2)" system call that duplicates the pages .\" (again, incrementing their reference count, not copying the data) from .\" one pipe to two other pipes. .BR tee () duplicates up to .I len bytes of data from the pipe referred to by the file descriptor .I fd_in to the pipe referred to by the file descriptor .IR fd_out . It does not consume the data that is duplicated from .IR fd_in ; therefore, that data can be copied by a subsequent .BR splice (2). .PP .I flags is a bit mask that is composed by ORing together zero or more of the following values: .TP 1.9i .B SPLICE_F_MOVE Currently has no effect for .BR tee (); see .BR splice (2). .TP .B SPLICE_F_NONBLOCK Do not block on I/O; see .BR splice (2) for further details. .TP .B SPLICE_F_MORE Currently has no effect for .BR tee (), but may be implemented in the future; see .BR splice (2). .TP .B SPLICE_F_GIFT Unused for .BR tee (); see .BR vmsplice (2). .SH RETURN VALUE Upon successful completion, .BR 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 .IR fd_in . .PP On error, .BR tee () returns \-1 and .I errno is set to indicate the error. .SH ERRORS .TP .B EAGAIN .B SPLICE_F_NONBLOCK was specified in .I flags or one of the file descriptors had been marked as nonblocking .RB ( O_NONBLOCK ) , and the operation would block. .TP .B EINVAL .I fd_in or .I fd_out does not refer to a pipe; or .I fd_in and .I fd_out refer to the same pipe. .TP .B ENOMEM Out of memory. .SH VERSIONS The .BR tee () system call first appeared in Linux 2.6.17; library support was added in glibc 2.5. .SH STANDARDS This system call is Linux-specific. .SH NOTES Conceptually, .BR tee () copies the data between the two pipes. In reality no real data copying takes place though: under the covers, .BR tee () assigns data to the output by merely grabbing a reference to the input. .SH EXAMPLES The example below implements a basic .BR tee (1) program using the .BR tee () system call. Here is an example of its use: .PP .in +4n .EX $ \fBdate | ./a.out out.log | cat\fP Tue Oct 28 10:06:00 CET 2014 $ \fBcat out.log\fP Tue Oct 28 10:06:00 CET 2014 .EE .in .SS Program source \& .\" SRC BEGIN (tee.c) .EX #define _GNU_SOURCE #include #include #include #include #include #include int main(int argc, char *argv[]) { int fd; ssize_t len, slen; if (argc != 2) { fprintf(stderr, "Usage: %s \en", 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); } .EE .\" SRC END .SH SEE ALSO .BR splice (2), .BR vmsplice (2), .BR pipe (7)