trusty (2) tee.2.gz

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>\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 3.54 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/.