Debugging

Writing C++ routines is no easy task. Many times a program ends abrutly with an error. A common error is a “segmentation fault” error. When a program displays this message it usually means that the program was trying to access a memory location outside its address space. Many other errors may occurr but finding these errors is usually a time consuming task and several debugging techniques are needed to find them.

A common debugging technique is to print messages on your program so that you may debug or trace the execution of the program. Excentury provides three macro functions to aid in future debugging tasks. These functions expand to a command if the DEBUG macro is defined before the inclusion of excentury.h. There are three values that the DEBUG macro can take. These three values are the several levels of debugging which can facilitate the debugging task at hand and in the future.

To explore what each of these levels do we will consider the file example.cpp which looks as follows

#include <excentury/excentury.h>

int main() {
    debug("This message is only seen with DEBUG set to 2\n");
    trace("This message is only seen with DEBUG set to 3\n");
    printf("Hello world\n");
    exitif(true, NULL, "This is a test to check that DEBUG is on.\n");
    printf("Debug was turned off...\n");
}

Notice that a compilation without defining the DEBUG macro will result in the following

$ g++ example.cpp -o example.run
$ ./example.run
Hello world
Debug was turned off...

This is because debug, trace and exitif expanded to nothing, i.e., example.cpp expanded to

#include <excentury/excentury.h>

int main() {
    printf("Hello world\n");
    printf("Debug was turned off...\n");
}

Debugging Level 1: exitif

This is the most basic level and it will allow you to use the exitif function.

exitif(condition, function_call, ...)

This is a macro which expands to the following:

if (condition) {
    printf(...);
    function_call;
    exit(1);
}

To can either define DEBUG before inclusion of excentury/excentury.h or define it during the call to g++

$ g++ -DDEBUG=1 example.cpp -o example.run1
$ ./example.run1
Hello world
ERROR CAUGHT BY example.cpp line 7 executing:

    int main()

The error occurred because:  true

This is a test to check that DEBUG is on.

In this case, notice that since exitif was defined and the condition to exit was satisfied (any statement that evalutes to true) the program printed a statement explaining the error detected and halted the execution of the program.

Debugging Level 2: debug

This level provides the function debug. These messages should only be used while debuging. If the messages could help in the future then we should replace debug with the level 3 function trace. The debug function behaves as printf but will only be expanded in levels 2 and 3.

$ g++ -DDEBUG=2 example.cpp -o example.run2
$ ./example.run2
This message is only seen with DEBUG set to 2
Hello world
ERROR CAUGHT BY example.cpp line 7 executing:

    int main()

The error occurred because:  true

This is a test to check that DEBUG is on.

Notice the message from level 1 was displayed as well as the message provided to the debug function. Again, these messages are meant to be temporary in order to find out what is going on. We could have used the printf function but soon we will not be able to differentiate between the actual statements that we want to display and those that were temporary debugging messages.

Debugging Level 3: trace

Provides the function trace which will display messages when level 3 is active. These messages are meant to be permanent messages and should be designed to help the user have a better idea of what is going on with the program.

$ g++ -DDEBUG=3 example.cpp -o example.run3
$ ./example.run3
This message is only seen with DEBUG set to 2
This message is only seen with DEBUG set to 3
Hello world
ERROR CAUGHT BY example.cpp line 7 executing:

    int main()

The error occurred because:  true

This is a test to check that DEBUG is on.

The debugging level 3 is really meant to be used as a last resort tool. If done correctly, the debugging level 1 should catch the errors. If this is not enough then we may display messages by using level 2. If all fails then we may want to activate the trace messages to see if there are any useful messages.