Conditional pre-processing

The C preprocessor provide these conditional pre-processing directives:

Conditional directive Meaning ========================================================== #ifdef Name Pre-process ... when ... "Name" has been #defined #endif #ifndef Name Pre-process ... when ... "Name" has not been #defined #endif #if Condition Pre-process ... when ... "Condition" is true #endif

I will not discuss the
#if Condition
clause because it is rarely used.

The #ifdef conditional pre-processing directive

Syntax of the #ifdef conditional directive:

#ifdef NAME-1 text segment 1 [#elif NAME-2 text segment 2] ... [#else text segment 3] #endif

Meaning:

If NAME-1 was #defined, then the C pre-processor will process text segnment1
Otherwise, if NAME-2 was #defined, then the C pre-processor will process text segnment2
And so on (if you have more #elif directives)
Otherwise, the the C pre-processor will process text segnment3

Example of using the #ifdef conditional directive

I personally like to use ifdef around print( ) statements that I use for debugging:

#define DEBUG // This defines DEBUG = empty string int main( ) { int x, y; x = 3; y = 4; #ifdef DEBUG printf("x = %d\n", x); // Print variable x for debugging printf("y = %d\n", y); // Print variable x for debugging #endif }

When I want the C program to print out debug information, I would #define DEBUG at the top of my program

Example of using the #ifdef conditional directive

The conditional directive will generate the following C source:

int main( ) { int x, y; x = 3; y = 4; printf("x = %d\n", x); // Print variable x for debugging printf("y = %d\n", y); // Print variable x for debugging }

and the C program will print out the variables so I can check and find bugs

DEMO: demo/C/1-C-pre/ifdef1.c

Example of using the #ifdef conditional directive

When I do not want to see the debug information, I would rename the macro-name:

#define DEBUGx // Now DEBUG is not defined ! int main( ) { int x, y; x = 3; y = 4; #ifdef DEBUG printf("x = %d\n", x); // Print variable x for debugging printf("y = %d\n", y); // Print variable x for debugging #endif }

Comment: there is an even better way to manage printing of debug messages!

Example of using the #ifdef conditional directive

The conditional directive will generate the following C source:
int main( ) { int x, y; x = 3; y = 4; }
and the C program will not have print( ) statements for debugging

The define symbol C compiler option

The C compiler has a define symbol compile option that allow you to define a macro name before the C compiler translates a C program
C compiler syntax for the define symbol option:
gcc -Dmacro[=pattern] ......

The C program does not contain the #define DEBUG directive:

int main( ) { int x = 3, y = 4; #ifdef DEBUG printf("x = %d\n", x); // Print variable x for debugging printf("y = %d\n", y); // Print variable x for debugging #endif }

To include the print( ) statement, compile with: gcc -DDEBUG prog.c

DEMO: demo/C/1-C-pre/ifdef2.c

The #ifndef conditional pre-processing directive

Syntax of the #ifndef conditional directive:

#ifndef NAME-1 text segment 1 [#elif NAME-2 text segment 2] ... [#else text segment 3] #endif

Meaning:

If NAME-1 was not #defined, then the C pre-processor will process text segnment1
Otherwise, if NAME-2 was not #defined, then the C pre-processor will process text segnment2
And so on (if you have more #elif directives)
Otherwise, the the C pre-processor will process text segnment3

Common use of the #ifndef conditional directive

The #ifndef conditional directive is commonly used to prevent multiple inclusion of the same header file
Commonly used technique in C programming
The #ifndef conditional directive can break a recursive include chain of include files
I will show you how in the next set of slides