Preprocessing

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C/C++ Training
Programming C/C++ on Eclipe
Trình bày: Ths HungNM
The Preprocessor
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Introduction preprocessor.
Preprocessing
Logical Preprocessor Directives
Standard Preprocessing Macros
Error Generation
Using the assert() Macro
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Introduction preprocessor
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The preprocessor is powerful tool.
It also can be a source of hard-to-find bugs.
It can be easily be misused to create program that are
almost impossible to understand.
The preprocessor is controlled by processing directive that
begin a # character.
C Program
Modified C program
Object code
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Preprocessing
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Including Header Files
 A header file is any external file whose contents
 It use of the #include preprocessor directive.
 Syntax :
• #include <standard_library_file_name>
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External Variables and Functions
 A program that’s made up of several source files
 want to use a global variable that’s defined in another file.
 Declaring the variable as external to the current file using the
extern keyword.
 Syntax :
• extern datatype namevariable;
• Example : extern int number;
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Preprocessing
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Macro definition.
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The #define directive defines a macro.
The #undefine directive remove a macro definition.
Syntax :
• #define identifer replacement-list
Example :
• #define PI 3.14159265
• #define Black White
Advance macro definition.
• It make program easier to read.
• It make program easier to modify
• It help avoid inconsistencies and topographical error.
– Example : Numerical constant like : 3.14149, but some time 3.1415
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Preprocessing
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Macro definition.
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Advance macro definition
• Making minor changes to syntax of C program.
– Example : #define BEGIN {
– #define END }
• Renaming types.
– Exampe : #define BOOL int
• Controlling conditional compilation.
– Example : #define DEBUG
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Preprocessing
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Macros That Look Like Functions
Allows parameters to be specified, which may
themselves be replaced by argument values,
 Syntax :
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• #define identifer(x1,x2,..,xn) replacement list
Example :
• #define max(x, y) x>y ? x : y
• Call : result = myval>99 ? myval : 99;
• It mean : result = max(myval, 99);
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Preprocessing
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Preprocessor Directives on Multiple Lines
using the statement continuation character, \.
 Example :
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• #define min(x, y) \
((x)<(y) ? (x) : (y))
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Strings As Macro Arguments
Example: #define MYSTR "This string"
 Call : printf("%s", MYSTR);
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Preprocessing
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Joining Two Results of a Macro Expansion
you may wish to generate two results in a macro and
join them together
 Using two characters ## serves to separate the
parameters.
 Syntax :
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• #define join(a, b) a##b
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Example :
• strlen(join(var, 123));
• It will be result : strlen(var123);
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Logical Preprocessor Directives
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Conditional Compilation
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Syntax :
#if defined identifier
#endif
Or
#if !defined identifier
#endif
Or
#if !defined block1
#define block1
/* Block of code you do not */
/* want to be repeated.
*/
#endif
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Logical Preprocessor Directives
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Directives Testing for Specific Values
Using #if directive to test the value of a constant
expression.
 Syntax :
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#if constant_expression
#endif
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Example :
#if CPU == Pentium4
printf("\nPerformance should be good." );
#endif
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Logical Preprocessor Directives
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Multiple-Choice Selections
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Using #if, #elseif, #else derective for excecute statements.
Syntax :
#if constant_expression
#elseif constant_expression
#elseif constant_expression
#else
#endif
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Example :
#if CPU == Pentium4
printf("\nPerformance should be good." );
#else
printf("\nPerformance may not be so good." );
#endif
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Standard Preprocessing Macros
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Using __DATE__ macro provides a string representation of the
date.
Using __TIME__, provides a string containing the value of the
time.
Using __FILE__ , A string literal containing the name of the file
being compiled.
__LINE__ A decimal constant containing the current source line
number.
__STDC_VERSION__ : This macro expands to the C Standard's
version number.
Example :
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printf("\nProgram last compiled at %s on %s", __TIME__,
__DATE__ );
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Error Generation
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A preprocessor error directive causes the
preprocessor to generate an error message and
causes the compilation to fail.
Syntax :
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# error token-sequence
Example :
#define BUFFER_SIZE 255
#if BUFFER_SIZE < 256
#error "BUFFER_SIZE is too small." #endif
generates the error message:
BUFFER_SIZE is too small.
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Using the assert() Macro
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The assert() macro is defined in the standard library
header file <assert.h>.
Using #ndefine derectives for Switch on or off assertions.
#undef NDEBUG
 Syntax:
#undef NDEBUG
#include <assert.h>
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Example:
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#include <assert.h>
#include <stdio.h>
int main(void)
{
int y = 5;
for(int x = 0 ; x < 20 ; x++)
{
printf("\nx = %d y = %d", x, y);
assert(x<y);
}
return 0;
}
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End
• Thank You
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