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PROGRAM ERRORS & DEBUGGING.
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Programming errors inevitably occur due to typing mistakes, the use of
incorrect logic, failure to initialize variables, the use of bad
algorithms, etc. It is fortunate therefore that debugging features are
built into the Turbo C integrated development environment (IDE).
There are three basic types of program bugs:
1. Syntax errors.
These are found by the compiler at compile time and include such
mistakes as the ommision of an end-of-statement semi-colon or the
use of parentheses instead of braces to mark the body of a function.
2. Semantic errors.
These are found at run-time and include dividing by zero.
3. Logical errors.
These can be difficult to find. Perhaps a variable has not been
correctly initialized and hence assumes an unexpected value, or the
algorithm for the solution of the problem is incorrectly chosen.
1. Syntax errors.
When using the integrated development environment (IDE) version of Turbo C
(TC.EXE), the compiler provides a complete list of all the syntax errors and
any warnings and displays them in the Message window at the bottom of the
screen, whilst access to the source code for editing is still available.
The Message window is not displayed immediately, but during compilation a
Compiling window shows the details of the compilation, including the number
of warnings and errors. Eventually the message 'Press any key' invites the
action to display the Message window. The function key F5 allows zoom
action so that the message window occupies the whole screen if required.
It should be noted, however, that sometimes the compiler can be confused.
A true syntax error in one place may cause the compiler to mistakenly think
that it has found other errors. If parentheses are used instead of braces to
enclose the statements of a function, the compiler may mistakenly believe
that several variables have not been correctly declared. Moreover the
compiler does not indicate the incorrect use of parentheses. In cases like
this it is sensible to correct as many errors as understood and recompile to
check the error situation again. Intelligent interpretation of the compiler
messages should soon lead to a program that will compile.
To correct an error, place the highlight bar in the Message window on the
first error message and press 'ENTER'. The cursor shifts to the Edit window
and is placed in the spot that generated the error message. The status line
of the editor now shows the error message, so that the appropriate
correction can be made. Pressing F6 will now return to the Message window
so that the down-arrow key can be used to select the next error for
correction as above. Alternatively, ALT-F8 can be used to move to the next
error.
ALT-F7 allows movement backwards through the error messages if this is
necessary. If curing one error will eliminate many other errors, then the
F6 method is preferred, as several now redundant errors can be missed.
Otherwise the ALT-F8 method is to be preferred.
If the command-line version of Turbo C (TCC.EXE) is used instead of the IDE
version (TC.EXE), the DOS screen display gives line numbers and error
messages, so that an alternative editor can be used to correct the program.
The procedure for compiling and linking from a command line is given on
pages 37-39 of the User's Guide. For the simple demonstration program
EXAMPLE.C the command line is
tcc -IB:\include -LB:\lib example.c
where the -I option allows include files from B:\include and
the -L option allows library files from B:\lib
if the B drive contains the appropriate files.
other options are given in the User's Guide.
The following files must be available on the existing path:
TCC.EXE TLINK.EXE as well as the file(s) for compilation.
2. Semantic errors.
When a compiled program is run, semantic errors which then occur will be
indicated on the screen. A divide by zero is shown as 'Divide error'.
3. Logical errors.
These are the most difficult to find, and would normally require a manual
calculation through the algorithm or the insertion of additional lines in
the program (temporarily) to show values as the program is executed.
Fortunately the Turbo C integrated debugger is included in the IDE, with
three main menus (Run, Debug, and Break/Watch) with suitable hot keys for
the debugger commands. Table 5.3 on pages 97-8 of the User's Guide gives
details of these debugger commands. Perhaps the most significant of these
commands are:
Ctrl-F7 Break/Watch/Add Watch which permits the value of a variable
to be displayed in the Watch window.
F7 Run/Trace Into which allows the program to be executed
one line at a time, so that the values
in the Watch window can be observed.
Appropriate use of these two commands simulate the traditional method of
debugging a program by inserting additional temporary commands in the
program, in order to display the values of significant variables as the
program is run.