tee(2)                        System Calls Manual                       tee(2)

NAME
       tee - duplicating pipe content

LIBRARY
       Standard C library (libc, -lc)

SYNOPSIS
       #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
       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
       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
       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.

STANDARDS
       Linux.

HISTORY
       Linux 2.6.17, glibc 2.5.

NOTES
       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
       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);
       }

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

Linux man-pages 6.7               2023-10-31                            tee(2)