Programming in C

Debugging Programs with gdb
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#ifdef DEBON
# define DEBUG(level, fmt, ...) \
if (Debug >= level) \
fprintf (stderr, fmt, __VA_ARGS__)
#else
# define DEBUG(level, fmt, ...)
#endif
When compiling a program containing the previous definition (which you can conveniently
place inside a header file and include in your program), you either define DEBON
or not. If you compile prog.c as follows:
$ gcc prog.c
it compiles in the null definition for DEBUG based on the #else clause shown in the previous
preprocessor statements. On the other hand, if you compile your program like this:
$ gcc –D DEBON prog.c
the DEBUG macro that calls fprintf based on the debug level is compiled in with the rest
of your code.
At runtime, if you have compiled in the debugging code, you can select the debug
level. As noted, this can be done with a command-line option as follows:
$ a.out –d3
Here, the debug level is set to 3. Presumably, you would process this command-line argument
in your program and store the debug level in a variable (probably global) called
Debug.And in this case, only DEBUG macros that specify a level of 3 or greater cause the
fprintf calls to be made.
Note that a.out -d0 sets the debugging level to zero and no debugging output is
generated even though the debugging code is still in there.
To summarize, you have seen here a two-tiered debugging scheme: Debugging code
can be compiled in or out of the code, and when compiled in, different debugging levels
can be set to produce varying amounts of debugging output.
Debugging Programs with gdb
gdb is a powerful interactive debugger that is frequently used to debug programs compiled
with GNU’s gcc compiler. It allows you to run your program, stop at a predetermined
location, display and/or set variables, and continue execution. It allows you to
trace your program’s execution and even execute it one line at a time. gdb also has a
facility for determining where core dumps occur. A core dump occurs due to some abnormal
event, possibly division by zero or attempts to access past the end of an array.This
results in the creation of a file named core that contains a snapshot of the contents of the
process’s memory at the time it terminated. 1
1.Your system might be configured to disable the automatic creation of this core file, often due to
the large size of these files. Sometimes, this has to do with the maximum file creation size, which
can be changed with the ulimit command.