tee(2) — Linux manual page

NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | VERSIONS | CONFORMING TO | NOTES | EXAMPLES | SEE ALSO | COLOPHON

TEE(2)                  Linux Programmer's Manual                 TEE(2)

NAME         top

       tee - duplicating pipe content

SYNOPSIS         top

       #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);

DESCRIPTION         top

       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).

RETURN VALUE         top

       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         top

       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.

VERSIONS         top

       The tee() system call first appeared in Linux 2.6.17; library
       support was added to glibc in version 2.5.

CONFORMING TO         top

       This system call is Linux-specific.

NOTES         top

       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.

EXAMPLES         top

       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 <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);
       }

SEE ALSO         top

       splice(2), vmsplice(2), pipe(7)

COLOPHON         top

       This page is part of release 5.12 of the Linux man-pages project.
       A description of the project, information about reporting bugs,
       and the latest version of this page, can be found at
       https://www.kernel.org/doc/man-pages/.

Linux                          2020-06-09                         TEE(2)

Pages that refer to this page: pipe(2)splice(2)syscalls(2)vmsplice(2)pipe(7)