define in c Tutorial 2

 c preprocessor (Part 2)

 3.# undef
suppose you have something like this..

view sourceprint?

01	# define x 4

02

03	# define x 5

04

05	int main(){

06

07

int z=x;

08

09	printf("%d",x); // output 5

10

11

}

although the output is 5 you can’t trust these type of declarations.If you want to redefine the same macro then first undefine it using #undef
correct way of doing

01	# define x 8

02

03	# undef x // x is now unknown to compiler

04

05	# define x 9 // x is 9 now

06

07	int main()

08

{

09

10	int z = x;

11

12	printf("%d",z); // output 9

13

14

}

so if you have your own macros and you are in doubt that their names can conflict with the macros defined in header files then first use # undef to undefine these macros.

1	# undef conflictingname // undefining macro conflictingname in a headerfile.

2

3	# define conflictingname

4

5	int main()

6

{

7	// do some stuff here

8

}

1	# undef anyname // not an error

2	int main()

3

{

4	// do some stuff here

5

}

4.# pragma
The directive is a compiler specific directive which compiler vendors may provide .

1	# pragma GCC poison x //variable cannot be used

2

3	int main(){

4

5	/*error x is poisoned .It cannot be used anywhere in GCC compiler system.*/

6

7

int x;

8

9

}

here GCC poison is called pragmas.Remember these are  compiler dependent.
So GCC poison will not work on microsoft compilers (will not give an error).You have to read manual of the compiler.
Predefined macro

1

_LINE_

2

_FILE_

3

_TIME_

4

_DATE_

5

__STDC__

working example

01	# define _LINE_ 98 // error predefined cannot be redefined

02	int main()

03

{

04	// note this is __ not _

05	printf("%d\n",__LINE__); // will output current line number

06	printf("%s\n",__FILE__); // file name

07	printf("%s\n",__DATE__); // date of execution

08	printf("%s\n",__TIME__); // time of execution

09	printf("%d\n",__STDC__); // Whether the compiler is standard conforming (1) or not

10

}