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  Dinil Divakaran <>
  1.1, 2002-08-29

  RTLinux Installation and writing realtime programs in Linux

  Table of Contents

  1. Introduction

     1.1 Purpose
     1.2 Who should read this HOWTO
     1.3 Acknowledgement
     1.4 Feedback
     1.5 Distribution Policy

  2. Installing RTLINUX

  3. Why RTLinux

  4. Writing RTLinux Programs

     4.1 Introduction to writing modules
     4.2 Creating RTLinux Threads
     4.3 An example program

  5. Compiling and Executing

  6. Inter-Process Communication

     6.1 Realtime FIFO
     6.2 Application Using FIFO

  7. What next


  1.  Introduction

  1.1.  Purpose

  This document aims at getting the novice user up and running with
  RTLinux in as painless a manner as possible.

  1.2.  Who should read this HOWTO

  This document is meant for all those who wish to know the working of a
  realtime kernel. For those of you already familiar with module
  programming, the document wouldn't appear as a difficult one. And for
  others, you needn't worry; since only the basic concepts of module
  programming are required, which we would indeed discuss, as and when

  1.3.  Acknowledgement

  First of all I would like to thank my advisor, Pramode C. E, for his
  encouragement and help. Also I express my sincere appreciation to
  Victor Yodaiken.  This document would not have been possible without
  all the information gathered from different papers contributed by
  Victor Yodaiken. I am also grateful to Michael Barabanov for his
  thesis on "A Linux-base Real-Time Operating System".

  1.4.  Feedback

  Any doubt or comment about this document, is always welcome. Please
  feel free to email me <>. If there
  is any mistake in this document, please let me know so that I can
  correct it in the next revision. Thanks.

  1.5.  Distribution Policy

  Copyright (C)2002 Dinil Divakaran.

  This document  is free; you can redistribute it and/or modify it under
  the terms of the GNU General Public License as published by the Free
  Software Foundation; either version 2 of the License, or (at your
  option) any later version.

  This document is distributed in the hope that it will be useful, but
  WITHOUT ANY WARRANTY; without even the implied warranty of
  General Public License for more details.

  2.  Installing RTLINUX

  The first step in the compilation of RTLinux kernel, is to download a
  pre-patched kernel 2.2.18 (x86 only) 2.2.18
  <> (x86
  only) or 2.4.0-test1
  (x86, PowerPC, Alpha) into /usr/src/ and untar it. Also put a fresh
  copy of RTLinux kernel (version 3.0) from
  <> in /usr/src/rtlinux/.  (We will use $ to
  represent the prompt).

  1. Now, configure the Linux kernel :

                     $ cd /usr/src/linux
                     $ make config
                     $ make menuconfig
                     $ make xconfig

  2. For building the kernel image, type :

                     $ make dep
                     $ make bzImage
                     $ make modules
                     $ make modules_install
                     $ cp arch/i386/boot/bzImage /boot/rtzImage

  3. Next step is to configure LILO. Type in the following lines in the
     file /etc/lilo.conf


  WARNING: replace /dev/hda1 in the above with your root filesystem. The
  easiest way to find out which filesystem it should be, take a look at
  the existing entry in your /etc/lilo.conf for "root=".

  4. Now, restart your computer and load the RTLinux kernel by typing
     'rtl' at the LILO prompt. Then 'cd' to /usr/src/rtlinux/ and
     configure RTLinux.

                     $ make config
                     $ make menuconfig
                     $ make xconfig

  5. Finally, for compiling RTLinux

                     $ make
                     $ make devices
                     $ make install

  The last step will create the directory:

  /usr/rtlinux-xx (xx denotes the version)

  which contains the default installation directory for RTLinux which is
  needed to create and compile user programs (that is, it contains the
  include files, utilities, and documentation). It will also create a
  symbolic link:


  which points to /usr/rtlinux-xx.  In order to maintain future
  compatibility, please make sure that all of your own RTLinux programs
  use /usr/rtlinux as its default path.

  NOTE : If you change any Linux kernel options, please don't forget to

                  $ cd /usr/src/rtlinux
                  $ make clean
                  $ make
                  $ make install

  3.  Why RTLinux

  The reasons for the design of RTLinux can be understood by examining
  the working of the standard Linux kernel. The Linux kernel separates
  the hardware from the user-level tasks. The kernel uses scheduling
  algorithms and assigns priority to each task for providing good
  average performances or throughput.  Thus the kernel has the ability
  to suspend any user-level task, once that task has outrun the time-
  slice allotted to it by the CPU. This scheduling algorithms along with
  device drivers, uninterruptible system calls, the use of interrupt
  disabling and virtual memory operations are sources of
  unpredictability. That is to say, these sources cause hindrance to the
  realtime performance of a task.

  You might already be familiar with the non-realtime performance, say,
  when you are listening to the music played using 'mpg123' or any other
  player.  After executing this process for a pre-determined time-slice,
  the standard Linux kernel could preempt the task and give the CPU to
  another one (e.g. one that boots up the X server or Netscape).
  Consequently, the continuity of the music is lost. Thus, in trying to
  ensure fair distribution of CPU time among all processes, the kernel
  can prevent other events from occurring.

  A realtime kernel should be able to guarantee the timing requirements
  of the processes under it.  The RTLinux kernel accomplishes realtime
  performances by removing such sources of unpredictability as discussed
  above. We can consider the RTLinux kernel as sitting between the
  standard Linux kernel and the hardware. The Linux kernel sees the
  realtime layer as the actual hardware. Now, the user can both
  introduce and set priorities to each and every task. The user can
  achieve correct timing for the processes by deciding on the scheduling
  algorithms, priorities, frequency of execution etc. The RTLinux kernel
  assigns lowest priority to the standard Linux kernel. Thus the user-
  task will be executed in realtime.

  The actual realtime performance is obtained by intercepting all
  hardware interrupts. Only for those interrupts that are related to the
  RTLinux, the appropriate interrupt service routine is run. All other
  interrupts are held and passed to the Linux kernel as software
  interrupts when the RTLinux kernel is idle and then the standard Linux
  kernel runs. The RTLinux executive is itself nonpreemptible.

  Realtime tasks are privileged (that is, they have direct access to
  hardware), and they do not use virtual memory. Realtime tasks are
  written as special Linux modules that can be dynamically loaded into
  memory. The initialization code for a realtime tasks initializes the
  realtime task structure and informs RTLinux kernel of its deadline,
  period, and release-time constraints.

  RTLinux co-exists along with the Linux kernel since it leaves the
  Linux kernel untouched. Via a set of relatively simple modifications,
  it manages to convert the existing Linux kernel into a hard realtime
  environment without hindering future Linux development.

  4.  Writing RTLinux Programs

  4.1.  Introduction to writing modules

  So what are modules? A Linux module is nothing but an object file,
  usually created with the -c flag argument to gcc. The module itself is
  created by compiling an ordinary C language file without the main()
  function. Instead there will be a pair of init_module/cleanup_module

  �  The init_module() which is called when the module is inserted into
     the kernel.  It should return 0 on success and a negative value on

  �  The cleanup_module() which is called just before the module is

  Typically, init_module() either registers a handler for something with
  the kernel, or it replaces one of the kernel function with its own
  code (usually code to do something and then call the original
  function).  The cleanup_module() function is supposed to undo whatever
  init_module() did, so the module can be unloaded safely.

  For example, if you have written a C file called module.c (with
  init_module() and cleanup_module() replacing the main() function), the
  code can be converted into a module by typing :

          $ gcc -c {SOME-FLAGS} my_module.c

  This command creates a module file named module.o, which can now be
  inserted into the kernel by using the 'insmod' command :

          $ insmod module.o

  Similarly, for removing the module, you can use the 'rmmod' command :

          $ rmmod module

  4.2.  Creating RTLinux Threads

  A realtime application is usually composed of several ``threads'' of
  execution.  Threads are light-weight processes which share a common
  address space.  In RTLinux, all threads share the Linux kernel address
  space.  The advantage of using threads is that switching between
  threads is quite inexpensive when compared with context switch. We can
  have complete control over the execution of a thread by using
  different functions as will be shown in the examples following.

  4.3.  An example program

  The best way to understand the working of a thread is to trace a
  realtime program. For example, the program shown below will execute
  once every second, and during each iteration it will print 'Hello

  The Program code (file - hello.c) :

  #include <rtl.h>
  #include <time.h>
  #include <pthread.h>

  pthread_t thread;

  void * thread_code(void)
          pthread_make_periodic_np(pthread_self(), gethrtime(), 1000000000);

          while (1)
                  pthread_wait_np ();
                  rtl_printf("Hello World\n");

          return 0;

  int init_module(void)
     return pthread_create(&thread, NULL, thread_code, NULL);

  void cleanup_module(void)

  So, let us start with the init_module(). The init_module() invokes
  pthread_create(). This is for creating a new thread that executes
  concurrently with the calling thread. This function must only be
  called from the Linux kernel thread (i.e., using init_module()).

          int  pthread_create(pthread_t  * thread,
                              pthread_attr_t * attr,
                              void * (*thread_code)(void *),
                              void * arg);

  The new thread created is of type pthread_t, defined in the header
  pthread.h. This thread executes the function thread_code(), passing it
  arg as its argument. The attr argument specifies thread attributes to
  be applied to the new thread.  If attr is NULL, default attributes are

  So here, thread_code() is invoked with no argument. thread_code has
  three components - initialization, run-time and termination.

  In the initialization phase, is the call to

          int pthread_make_periodic_np(pthread_t thread,
                                       hrtime_t start_time,
                                       hrtime_t period);

  pthread_make_periodic_np marks the thread as ready for execution.  The
  thread will start its execution at start_time and will run at
  intervals specified by period given in nanoseconds.
  gethrtime returns the time in nanoseconds since the system bootup.

         hrtime_t gethrtime(void);

  This time is never reset or adjusted. gethrtime always gives
  monotonically increasing values. hrtime_t is a 64-bit signed integer.

  By calling the function pthread_make_periodic_np(), the thread tells
  the scheduler to periodically execute this thread at a frequency of 1
  Hz. This marks the end of the initialization section for the thread.

  The while() loop begins with a call to the function pthread_wait_np(),
  which suspends execution of the currently running realtime thread
  until the start of the next period. The thread was previously marked
  for execution using pthread_make_periodic_np. Once the thread is
  called again, it executes the rest of the contents inside the while
  loop, until it encounters another call to pthread_wait_np().

  Because we haven't included any way to exit the loop, this thread will
  continue to execute forever at a rate of 1Hz. The only way to stop the
  program is by removing it from the kernel with the rmmod command. This
  invokes the cleanup_module(), which calls pthread_delete_np() to
  cancel the thread and deallocate its resources.

  5.  Compiling and Executing

  In order to execute the program, hello.c, (after booting rtlinux,
  ofcourse) you must do the following :

  1. Compile the source code and create a module using the GCC compiler.
     To simplify things, however, it is better to create a Makefile.
     Then you only need to type 'make' to compile the code.

     Makefile can be created by typing in the following lines in a file
     named 'Makefile'.

          all: hello.o
              rm -f *.o
          hello.o: hello.c
              $(CC) ${INCLUDE} ${CFLAGS} -c hello.c

  2. Locate and copy the file into the same directory as your
     hello.c and Makefile.  The file is an include file which
     contains all the flags needed to compile the code. You can copy it
     from the RTLinux source tree and place it alongside of the hello.c

  3. For compiling the code, use the command 'make'.

             $ make

  4. The resulting object binary must be inserted into the kernel, where
     it will be executed by RTLinux. Use the command 'rtlinux' (you need
     to be the 'root' to do so).

             $ rtlinux start hello

  You should now be able to see your hello.o program printing its
  message every second. Depending on the configuration of your machine,
  you should either be able to see it directly in your console, or by

          $ dmesg

  To stop the program, you need to remove it from the kernel. To do so,

          $ rtlinux stop hello

  Alternate ways for inserting and removing the module is to use insmod
  and rmmod respectively.

  Here, we have made our example program too simple. Contrary to what we
  have seen, there may be multiple threads in a program. Priority can be
  set at thread creation or modified later. Also, we can select the
  scheduling algorithm that should be used. In fact, we can write our
  own scheduling algorithms!

  In our example, we can set priority of the thread as 1, and select
  FIFO scheduling by inserting the following lines in the beginning of
  the function, thread_code() :

          struct sched_param p;
          p . sched_priority = 1;
          pthread_setschedparam (pthread_self(), SCHED_FIFO, &p);

  6.  Inter-Process Communication

  The example program that we have seen above is what is known as a
  realtime process.  Not every part of a application program need be
  written in realtime. It is found that only that part of a program
  which requires precise time restrictions should be written as a
  realtime process. Others can be written and executed in user space.
  User spaces processes are often easier to write, execute and debug
  than realtime threads. But then, there should be a way to communicate
  between user space Linux processes and realtime thread.

  There are several ways for inter-process communication. We will
  discuss the most important and common way of communication - the
  realtime FIFO.

  6.1.  Realtime FIFO

  Realtime FIFOs are unidirectional queues (First In First Out). So at
  one end a process writes data into the FIFO, and from the other end of
  the FIFO, information is read by another process. Usually, one of
  these processes is the realtime thread and the other is a user space

  The Realtime FIFOs are actually character devices (/dev/rtf*) with a
  major number of 150. Realtime threads uses integers to refer to each
  FIFO (for example - 2 for /dev/rtf2). There is a limit to the number
  of FIFOs. There are functions such as rtf_create(), rtf_destroy(),
  rtf_get(), rtf_put() etc for handling the FIFOs.

  On the other hand, the Linux user process sees the realtime FIFOs as
  normal character devices. Therefore the functions such as open(),
  close(), read() and write() can be used on these devices.

  6.2.  Application Using FIFO

  First, let us consider a simple C program (filename pcaudio.c) to play
  music (of just two tones) through the PC speaker. For the time being,
  let us assume that for playing the note, we need only write to the
  character device /dev/rtf3.  (Later, we will see a realtime time
  process that reads from this FIFO (/dev/rtf3) and sends to the PC

  #include <sys/types.h>
  #include <sys/stat.h>
  #include <fcntl.h>
  #include <unistd.h>

  #define DELAY 30000

  void make_tone1(int fd)
          static char buf = 0;
          write (fd, &buf, 1);

  void make_tone2(int fd)
          static char buf = 0xff;
          write (fd, &buf, 1);

          int i, fd = open ("/dev/rtf3", O_WRONLY);
          while (1)
                  for (i=0;i<DELAY;i++);
                  for (i=0;i<DELAY;i++);

  Now, if the above shown program (pcaudio.c) is compiled and run, it
  should create regular sound patters corresponding to a square wave.
  But before that we need a module for reading from '/dev/rtf3' and
  sending the corresponding data to the PC speaker. This realtime
  program can be found at the rtlinux source tree
  (/usr/src/rtlinux/examples/sound/) . Insert the module sound.o using
  the command 'insmod'.

  Since we have inserted a module for reading from the device, we can
  now execute our program (compile using 'gcc' and then execute the
  corresponding 'a.out'.  So the process produces somewhat regular
  tones, when there is no other (time consuming) process in the system.
  But, when the X server is started in another console, there comes more
  prolonged silence in the tone. Finally, when a 'find' command (for a
  file in /usr directory) is executed, the sound pattern is completely
  distorted. The reason behind this is that, we are writing the data
  onto the FIFO in non-realtime.

  We will, now, see how to run this process in realtime, so that the
  sound is produced without any kind of disturbance. First, we will
  convert the above program into a realtime program.  (Filename

  #include <rtl.h>
  #include <pthread.h>
  #include <rtl_fifo.h>
  #include <time.h>

  #define FIFO_NO 3
  #define DELAY 30000
  pthread_t thread;

  void * sound_thread(int fd)
          int i;
          static char buf = 0;
          while (1)
                  for(i=0; i<DELAY; i++);
                  buf = 0xff;
                  rtf_put(FIFO_NO, &buf, 1);

                  buf = 0x0;
                  rtf_put(FIFO_NO, &buf, 1);
          return 0;

  int init_module(void)
          return pthread_create(&thread, NULL, sound_thread, NULL);

  void cleanup_module(void)

  If not already done, 'plug in' the module sound.o into the kernel.
  Compile the above program by writing a Makefile for it (as said
  earlier), thus producing the module 'rtaudio.o'.  Before inserting
  this module, one more thing. Note that the above program runs into
  infinite loop. Since, we have not included code for the thread to
  sleep or stop, the thread will not cease its execution. In short, your
  PC speaker will go on producing the tone, and you will have to restart
  your computer for doing anything else.

  So, let us change the code a little bit (only in the function
  sound_thread()) by asking the thread itself to make the delay between

  void * sound_thread(int fd)
          static char buf = 0;
          pthread_make_periodic_np (pthread_self(), gethrtime(), 500000000);

          while (1)
                  buf = (int)buf^0xff;
                  rtf_put(FIFO_NO, &buf, 1);
          return 0;


  This time we can stop the process by just removing the module by using
  the 'rmmod' command.

  Here, we have seen how realtime FIFOs can be used for inter-process
  communication. Also the real need for RTLinux can be understood from
  the above example.

  7.  What next

  This document has gone through the basics of programming in RTLinux.
  Once you have understood the basic concept it is not difficult to make
  steps by your own. So you can go through all other examples available
  along with the rtlinux source. Then you should be able to write
  modules and test them. For more information regarding module
  programming, you can refer to 'Linux Kernel Module Programming Guide'
  by Ori Pomerantz.

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