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 <sys/types.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)