Debug Library in NXC

September  2011


One of the biggest challenges developing programs on the NXT is fixing them when they go wrong. The NXT has a small screen with only 8 lines of text displayed. It’s difficult to know what is causing a program to fail, and to have a record of what a program did.

I created this logging library in NXC to solve this problem. It presents a very simple API that you can use in your NXC programs to log data into a text log file as your program runs. Each entry is timestamped with the current NXT tick counter, so you can see when your program reached a certain point. The library supports logging text strings, values and byte arrays. Once your program has finished running you can download the log file to your PC using Bricxx or NeXTTools on the Mac.

How to use the logging library

To use the logging library you include it at the start of your NXC program as follows:


bool LOGGING=true;

#define LOGFILE "mylogfile.txt"

#define LOGFILE_SIZE 30000

#include "logging.nxc"


The boolean variable LOGGING enables or disables the logging functions. If set to true then data is logged into the log file. If false then nothing happens. Why add this? Well logging data to a file on the NXT takes time, and makes your program run slower. You can use the LOGGING variable to debug a program and then once you are satisfied that the program is correct simply set it to false and your program will no longer generate a logfile.

The name of the log file to create is defined in the LOGFILE macro. I suggest you name the logfile to be the same name as the program that creates it so you can keep track of which log files belong to which programs on your NXT.

The NXT firmware requires that every file must have a fixed size when it is created. In the example above the LOGFILE_SIZE macro is set to 30000 bytes. The logfile named mylogfile.txt will grow in size until it reaches 30000 bytes of content, at which point no new data is saved into the logfile.

Download the logfile library: logging.nxc.

Debug Logging Library API

The logging library contains the following functions:

  1. openLogFile(filename, filesize): this function creates a logfile with the given filename and filesize. It should be the first function called in your main() task.

  2. closeLogFile(): this function closes the logfile previously opened by openLogFile(). Only one logfile can be open at any one time in a program.

  3. writeLogString(s): write the string s to the logfile.

  4. writeLogByte(b): write a single byte value to the logfile.

  5. writeLogBytes(b): write a byte array to the logfile. A byte array can be any size. The byte array is written in hexadecimal format with each byte separated by a space.

  6. writeLogValue(s, v): write a (name,value pair) to the logfile. Used to record the value of a named variable. For example: writeLogValue(“foo is now”, foo);

  7. writeLogValue2(s, v1, v2): write a (name,value1,value2) triple to the logfile. Used to record the value of two variables, for example writeLogValue2(“Coordinates”, x, y)

  8. writeLogValue3(s, v1, vw, v3): write a (name,value1,value2,value3) quad to the logfile. Used to record the value of three variables, for example writeLogValue3(“Index”, i, j, k)

Sample Program

To see the logging library in operation I’ve provided an example program named logme.nxc below. It simple calls each of the logging functions to demonstrate how they work.


bool LOGGING=true;

#define LOGFILE "logme.txt"

#define LOGFILE_SIZE 3000

#include "logging.nxc"


task main() {

int i, j, k;

byte b[16];


writeLogString("I have started main");

i = 5;

j = 17;

writeLogValue2("i, j", i, j);

writeLogString("initialising array…");

for(i=0; i < 16; i++) {

b[i] = i * 5;






Sample logfile output

So what does the logfile look like from the program above? First of all we launch NeXTTools on the Mac (or Bricxx on your PC) to view the contents of the NXT filesystem.

As you can see running the logme program create a logfile named logme.txt. Now let’s see what’s in the logfile:

Logfile opened at 100175 ticks

100184 : I have started main

100186 : i, j = 5,  17

100187 : initialising array…

100191 : Done

100192 : 0 5 a f 14 19 1e 23 28 2d 32 37 3c 41 46 4b 

Logfile closed at 100205 ticks


Last updated 6 September, 2011

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