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



NAME
       tee - duplicating pipe content

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  descrip-
       tor  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 imple-
			  mented 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,  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
       The  tee()  system call first appeared in Linux 2.6.17; library support
       was added to glibc in version 2.5.

CONFORMING TO
       This system call is Linux-specific.

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.

EXAMPLE
       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
       splice(2), vmsplice(2)

COLOPHON
       This  page  is  part of release 4.10 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				  2014-12-31				TEE(2)