debian man page for tee

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
       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 series of modifier flags, which share the name space with splice(2) and vmsplice(2):

       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
       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 in the output by merely grabbing a reference to the input.

EXAMPLE
       The following example implements a basic tee(1) program using the tee() system call.

       #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>
", 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 3.44 of the Linux man-pages project.  A description of the project, and information about reporting bugs,  can
       be found at http://www.kernel.org/doc/man-pages/.

Linux								    2012-05-04								    TEE(2)