Debugging Techniques
Use of the Pre-processor #define Statements
This concept can be improved, building off of the previous example.
Say there are a lot of debugging print statements in the source code.
Wouldn't it be nice to be able to turn them all
on or off quickly, rather that commenting each and every one on and off.
Well this can be done easily...
Source Code Before:
#include <stdio.h>
int main()
{
int array[10];
int i;
for (i = 0 ; i < 20 ; i++)
{
#ifdef DEBUG_ON
printf("In for loop: i = %d\n", i);
#endif
array[i]=1;
}
for (i = 0 ; i < 10 ; i++)
{
printf("%d", array[i]);
}
return 0;
}
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Compilation & Execution Before:
linux1[37]% gcc -ansi -Wall main.c
linux1[38]% a.out
Segmentation fault (core dumped)
linux1[39]%
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"#ifdef DEBUG_ON" and "#endif" have been added around
all of the print statements. Basically this tells the pre-processor, if
"DEBUG_ON" has been defined, do include what is between the
"#ifdef" and "#endif" statements. "DEBUG_ON"
could be anything, but as with any code, use something logical.
In the above example "DEBUG_ON" has not been defined, and the
output shows no change.
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Source Code After:
#include <stdio.h>
#define DEBUG_ON
int main()
{
int array[10];
int i;
for (i = 0 ; i < 20 ; i++)
{
#ifdef DEBUG_ON
printf("In for loop: i = %d\n", i);
#endif
array[i]=1;
}
for (i = 0 ; i < 10 ; i++)
{
printf("%d", array[i]);
}
return 0;
}
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Compilation & Execution After:
linux1[40]% gcc -ansi -Wall main.c
linux1[41]% a.out
In for loop: i = 0
In for loop: i = 1
In for loop: i = 2
In for loop: i = 3
In for loop: i = 4
In for loop: i = 5
In for loop: i = 6
In for loop: i = 7
In for loop: i = 8
In for loop: i = 9
In for loop: i = 10
In for loop: i = 11
In for loop: i = 12
In for loop: i = 13
In for loop: i = 14
In for loop: i = 15
In for loop: i = 16
In for loop: i = 17
In for loop: i = 18
In for loop: i = 19
Segmentation fault (core dumped)
linux1[42]%
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With "#define DEBUG_ON" placed in the
source code, the pre-processor will recognize "DEBUG_ON" as being
defined, and will include all statements nested within the
"#ifdef" and "#endif" pre-processor directives.
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Notes:
- Multiple "#define" statements
- Multiple #define statements can be used to print
multiple levels of debugging information.
"#define DEBUG_LEVEL_1" might only print
when calling a function... "in Foo()"
"#define DEBUG_LEVEL_2" might print what values are passed in...
"calling Foo( x=1, y=2 )" etc...
It would be necessary to have each #ifdef check for the appropriate
level.
Modified by:
Dawn Block,
September 2003
Created by:
Daniel J. Hood,
February 2000
http://www.csee.umbc.edu/~cshc/resources/debug/defines.shtml
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