The TinyOS printf Library (2.0.2 Release)

This lesson demonstrates how to use the printf library located in tos/lib/printf to debug TinyOS applications by printing messages over the serial port.

Overview

Anyone familiar with TinyOS knows that debugging applications has traditionally been a very arduous, if not stressful process. While simulators like TOSSIM can be used to help verify the logical correctness of a program, unforseen problems inevitably arise once that program is deployed on real hardware. Debugging such a program typically involves flashing the three available LEDs in some intricate sequence or resorting to line by line analysis of a running program through the use of a JTAG.

It is common practice when developing desktop applications to print output to the terminal screen for debugging purposes. While tools such as gdb provide means of stepping though a program line by line, often times developers simply want to quickly print something to the screen to verify that the value of a variable has been set correctly, or determine that some sequence of events is being run in the proper order. It would be absurd to suggest that they only be allowed three bits of information in order to do so.

The TinyOS printf library provides this terminal printing functionality to TinyOS applications through motes connected to a pc via their serial interface. Messages are printed by calling printf commands using a familiar syntax borrowed from the C programming language. In order to use this functionality, developers simply need to include a single component in their top level configuration file (PrintfC), and include a "printf.h" header file in any components that actually call printf().

Currently, the printf library is only supported on msp430 and atmega128x based platforms (e.g. mica2, micaZ, telos, eyesIFX). In the future we hope to add support for other platforms as well.

The TinyOS printf Library

This section provides a basic overview of the TinyOS printf library, including the components that make it up and the interfaces they provide. In the following section we walk you through the process of actually using these components to print messages from a mote to your pc. If you dont care how printf works and only want to know how to use it, feel free to skip ahead to the next section.

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The entire printf library consists of only 4 files located in the tos/lib/printf directory: one module, one configuration, one interface file, and one header file.

• PrintfC.nc -- Configuration file providing printf functionality to TinyOS applications
• PrintfP.nc -- Module implementing the printf functionality
• PrintfFlush.nc -- Interface for flushing printf messages over the serial port to a pc
• printf.h -- Header file specifying the printf message format and size of the flush buffer

The PrintfC configuration is the only component an application needs to wire in order to use the functionality provided by the TinyOS printf library. Below is the component graph of the PrintfC configuration:

Figure 1: The component graph of the PrintfC configuration.

Conceptually, the operation of the TinyOS printf library is very simple. Developers supply strings to printf() commands in a distributed fashion throughout any of the components that make up a complete TinyOS application. These strings are buffered in a central location inside the PrintfP component and flushed out to a PC in the form of TinyOS SerialMessages upon calling the flush() command of the PrintfFlush interface.

By encapsulating the strings produced by calls to printf() inside standard TinyOS SerialMessages, applications that use the serial stack for other purposes can share the use of the serial port. Alternate implementations were considered in which printf would have had exclusive access to the serial port, and explicit flushing would not have been necessary. In the end, we felt it was better to give developers the freedom to decide exactly when messages should be printed, as well as allow them to send multiple types of SerialMessages in a single application.

Currently, only a single buffer is used to store the strings supplied to calls to printf before flushing them. This means that while the buffer is being flushed, any calls to printf will fail. In the future, we plan to implement a double buffered approach so that strings can continue to be buffered at the same time they are being printed.

There are also plans to provide a means of flushing messages out to a PC without requiring developers to make an explicit flush() call. This would allow developers to simply wire in the PrintfC component without having to make any calls to any interfaces it provides. In fact, the PrintfC component would not need to provide any interfaces at all. It would start itself up and then run in a loop, periodically flushing the contents of the printf buffer. Such functionality is useful in applications that do not really care when messages are printed or how long a delay the process of printing introduces to other sections of code. Explicit flushing would still be recommended in applications where the sections of code under examinatation are very timing sensitive (e.g. inside the CC2420 radio stack).

Using the TinyOS printf Library

To help guide the process of using the printf library, a TestPrintf application has been created. At present, this application is not included in the official TinyOS distribution (<= 2.0.2). If you are using TinyOS from a cvs checkout, you will find it located under apps/tutorials/Printf. Otherwise, you can obtain it from cvs by running the following set of commands from a terminal window:

cd $TOSROOT/apps/tutorials
cvs -d:pserver:anonymous@tinyos.cvs.sourceforge.net:/cvsroot/tinyos login
cvs -z3 -d:pserver:anonymous@tinyos.cvs.sourceforge.net:/cvsroot/tinyos co -P -d Printf tinyos-2.x/apps/tutorials/Printf

Just hit enter when prompted for a CVS password. You do not need to enter one.

If you are not using cvs, you will also have to apply the patch found here in order to allow the printf library to compile correctly for atmega128x based platforms (i.e. mica2, micaz):

cp tinyos-2.0-printf.patch $TOSROOT/..
cd $TOSROOT/..
patch -p0 < tinyos-2.0-printf.patch

Note that you may have to use 'sudo' when applying the patch if you run into permission problems.

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The TestPrintf application demonstrates everything necessary to use the printf library. Go ahead and open the TestPrintfAppC configuration to see how the various interfaces provided by the PrintfC component have been wired in. You will want to do something similar in your own applications.

configuration TestPrintfAppC{
}
implementation {
  components MainC, TestPrintfC, LedsC;
  components PrintfC;

  TestPrintfC.Boot -> MainC;
  TestPrintfC.Leds -> LedsC;
  TestPrintfC.PrintfControl -> PrintfC;
  TestPrintfC.PrintfFlush -> PrintfC;
}

First, the PrintfControl interface has been wired in to enable turning on and off the service providing printf functionality. Turning on the Printf service implicity turns on the serial port for sending messages. Second, the PrintfFlush interface has been wired in to allow the application to control when printf messages should be flushed out over the serial line. In this application, all printf() commands are called directly within the TestPrintfC component. In general, printf() commands can be called from any component as long as they have included the "printf.h" header file.

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Before examining the TestPrintfC component, first install the application on a mote and see what kind of output it produces. Note that the instructions here are only valid for installation on a telosb mote on a linux based TinyOS distribution. For installation on other systems or for other mote platforms, please refer to lesson 1 for detailed instructions.

To install the application on the mote, run the following set of commands.

cd $TOSROOT\apps\tests\TestPrintf
make telosb install bsl,/dev/ttyUSBXXX

You will notice during the installation process that a pair of java files are compiled along with the TinyOS application. The first java file, PrintfMsg.java, is generated by mig to encapsulate a TinyOS printf message received over the serial line (for more information on mig and how it generates these files, please refer to the section entitled "MIG: generating packet objects" in lesson 4). The second file, PrintfClient.java is used to read printf messages received from a mote and print them to your screen.

To see the output generated by TestPrintf you need to start the PrintfClient by running the following command:

cd $TOSROOT\apps\tests\TestPrintf
java PrintfClient -comm serial@/dev/ttyUSBXXX:telosb

After resetting the mote, the following output should be printed to your screen:

Hi I am writing to you from my TinyOS application!!
Here is a uint8: 123
Here is a uint16: 12345
Here is a uint32: 1234567890
I am now iterating: 0
I am now iterating: 1
I am now iterating: 2
I am now iterating: 3
I am now iterating: 4
This is a really short string...
I am generating this string to have just less than 250
characters since that is the limit of the size I put on my
maximum buffer when I instantiated the PrintfC component.
Only part of this line should get printed bec

Note that the 'tty' device (i.e. COM port) specified when starting the PrintfClient MUST be the one used for communicating with a mote over the serial line. On telos and mica motes this is the same port that the mote is programmed from. Other motes, such as eyesIFX, have one port dedicated to programming and another for communication. Just make sure you use the correct one.

If for some reason you do not receive the output shown above, please refer to lesson 4 to verify you have done everything necessary to allow serial communication between your pc and the mote. Remember that when using the MIB510 programming board that the switch on the very front of the board must be set to the OFF position in order to send messages from the mote to the pc.

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Go ahead and open up TestPrintfC to see how this output is being generated.

Upon receiving the booted event, the Printf service is started via a call to PrintfControl.start()

event void Boot.booted() {
  call PrintfControl.start();
}

Once the Printf service has been started, a PrintfControl.startDone() event is generated. In the body of this event the first four lines of output are generated by making successive calls to printf and then flushing the buffer they are stored in.

event void PrintfControl.startDone(error_t error) {
  printf("Hi I am writing to you from my TinyOS application!!\n");
  printf("Here is a uint8: %u\n", dummyVar1);
  printf("Here is a uint16: %u\n", dummyVar2);
  printf("Here is a uint32: %ld\n", dummyVar3);
  call PrintfFlush.flush();
}

Once these first four lines have been flushed out, the PrintfFlush.flushDone() event is signaled. The body of this event first prints the next 5 lines in a loop, followed by the last five lines. Finally, once all lines have been printed, the Printf service is stopped via a call to PrintfControl.stop().

event void PrintfFlush.flushDone(error_t error) {
  if(counter < NUM_TIMES_TO_PRINT) {
    printf("I am now iterating: %d\n", counter);
    call PrintfFlush.flush();
  }
  else if(counter == NUM_TIMES_TO_PRINT) {
    printf("This is a really short string...\n");
    printf("I am generating this string to have just less ...
    printf("Only part of this line should get printed bec ...
    call PrintfFlush.flush();
  }
  else call PrintfControl.stop();
  counter++;
}

Notice that the last line of output is cut short before being fully printed. If you actually read the line printed above it you can see why. The buffer used to store TinyOS printf messages before they are flushed is by default limited to 250 bytes. If you try and print more characters than this before flushing, then only the first 250 characters will actually be printed. This buffer size is configurable, however, by specifying the proper CFLAGS option in your Makefile.

CFLAGS += -DPRINTF_BUFFER_SIZE=XXX

Once the the Printf service has been stopped, the PrintfControl.stopDone() event is signaled and Led 2 is turned on to signify that the application has terminated.

event void PrintfControl.stopDone(error_t error) {
  counter = 0;
  call Leds.led2Toggle();
  printf("This should not be printed...");
  call PrintfFlush.flush();
} 

Notice that the call to printf() inside the body of the PrintfControl.stopDone() event never produces any output. This is because the Printf service has been stopped before this command is called.

Conclusion

A few points are worthy of note before jumping in and writing your own applications that use the functionality provided by the printf library.

1. In order to use the printf library, the tos/lib/printf directory must be in your include path. The easiest way to include it is by adding the following line directly within the Makefile of your top level application:

CFLAGS += -I$(TOSDIR)/lib/printf 

2. Remember that changing the printf buffer size is done similarly:

CFLAGS += -DPRINTF_BUFFER_SIZE=XXX

3. You MUST be sure to #include "printf.h" header file in every component in which you would like to call the printf() command. Failure to do so will result in obscure error messages making it difficult to identify the problem.

Hopefully you now have everything you need to get going with the TinyOS printf library. All questions (or comments) about the use of this library should be directed to tinyos-help mailing list.

Enjoy!!

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