C - Preprocessor

Preprocessor

• One of the step performed before compilation
• Is a text substitution tool and it instructs the compiler to do required pre-processing before the actual compilation
• Instructions given to preprocessor are called preprocessor directives and they begin with “#” symbol
• Few advantages of using preprocessor directives would be,
• Easy Development
• Readability
• Portability

• Preprocessor – Compilation Stages

• Before we proceed with preprocessor directive let's try to understand the stages involved in compilation. 
• Some major steps involved in compilation are
  
  • Preprocessing (Textual replacement) 
  • Compilation (Syntax and Semantic rules checking) 
  • Assembly (Generate object file(s)).
  • Linking (Resolve linkages) 
  The below image provides the flow of these stages.     

• Preprocessor – Directives

#include             #elif
#define                    #error
#undef                     #warning
#ifdef                       #line
#ifndef                     #pragma
#else                        #
#endif                      ##
#if 
#else

• Preprocessor – Header Files

• A header file is a file containing C declarations and macro definitions to be shared between several source files. 
• Has to be included using C preprocessing directive ‘#include' 
• Header files serve two purposes:
  
  • Declare the interfaces to parts of the operating system by supplying the definitions and declarations you need to invoke system calls and libraries.
  • Your own header files contain declarations for interfaces between the source files of your program.

• Preprocessor – Header Files vs Source Files

  
● Declarations                                 ● Function and variable definitions
● Sharable/reusable                        ● Non sharable/reusable
        ● #defines                                         ● #defines
        ● Datatypes                                       ● Datatypes
● Used by more than 1 file

• Preprocessor – Header Files - Syntax

Syntax

#include <file.h>

• System header files 

• It searches for a file named file in a standard list of system directories

#include “file.h”

• Local (your) header files

• It searches for a file named file first in the directory containing the current file, then in the quote directories and then the same directories used for <file>
  

Header Files – Search Path

• On a normal Unix system GCC by default will look for headers requested with #include <file> in:

/usr/local/include

libdir/gcc/target/version/include
/usr/target/include
/usr/include

• You can add to this list with the -I <dir> command-line option
  

“gcc -print-prog-name=cc1 -v” would search the path info

Header Files – Once-Only

• If a header file happens to be included twice, the compiler will process its contents twice causing an error
  
  • E.g. when the compiler sees the same structure definition twice, This can be avoided like:

Example:

#ifndef NAME
#define NAME
/* The entire file is protected */
#endif

Macro – Object-Like

• An object-like macro is a simple identifier which will be replaced by a code fragment 
• It is called object-like because it looks like a data object in code that uses it. 
• They are most commonly used to give symbolic names to numeric constants
  

Syntax

#define SYMBOLIC_NAME   CONSTANTS
Example
-------
#define BUFFER_SIZE     1024

Macro – Arguments

• Function-like macros can take arguments, just like true functions
• To define a macro that uses arguments, you insert parameters between the pair of parentheses in the macro definition that make the macro function like
  

Example

#include <stdio.h>
#define SET_BIT(num, pos)  (num | (1 << pos))
int main()
{
     SET_BIT(0, 2);
     return 0;
}

Macro – Multiple Lines

• You may continue the definition onto multiple lines, if necessary, using backslash-newline.
• When the macro is expanded, however, it will all come out on one line
  

Example

#include <stdio.h>
#define SWAP(a, b)   \
{                                \
int temp = a;             \
a = b;                         \
b = temp;                   \
}

int main()
{
int num1 = 10, num1= 20;
          SWAP(num1, num2);
          printf(“%d %d\n”, num1, num2);
}

Macro – Standard Predefined

• Several object-like macros are predefined; you use them without supplying their definitions.
• Standard are specified by the relevant language standards, so they are available with all compilers that implement those standards
  

Example

#include <stdio.h>
int main()
{
         printf(“Program: \“%s\” ”, __FILE__);
         printf(“was compiled on %s at %s. ”, __DATE__, __TIME__);
         printf(“This print is from Function: \”%s\” at line %d\n”, __func__, __LINE__);
return 0;
}

Preprocessor – Conditional Compilation

• A conditional is a directive that instructs the preprocessor to select whether or not to include a chunk of code in the final token stream passed to the compiler
• Preprocessor conditionals can test arithmetic expressions, or whether a name is defined as a macro, or both simultaneously using the special defined operator
• A conditional in the C preprocessor resembles in some ways an if statement in C with the only difference being it happens in compile time 
• Its purpose is to allow different code to be included in the program depending on the situation at the time of compilation. 
• There are three general reasons to use a conditional.
  • A program may need to use different code depending on the machine or operating system it is to run on 
  • You may want to be able to compile the same source file into two different programs, like one for debug and other as final 
  • A conditional whose condition is always false is one way to exclude code from the program but keep it as a sort of comment for future reference

Conditional Compilation - ifdef
Syntax

#ifdef MACRO
            /* Controlled Text */
#endif

Conditional Compilation - defined
Syntax

#if defined (ERROR) && (WARNING)
    /* Controlled Text */
#endif

Conditional Compilation - if
Syntax

#if expression
    /* Controlled Text */
#endif

Conditional Compilation - else
Syntax

#if expression
         /* Controlled Text if true */
#else
         /* Controlled Text if false */
#endif

Conditional Compilation - elif
Syntax

#if DEBUG_LEVEL == 1
          /* Controlled Text*/
#elif
DEBUG_LEVEL == 2
          /* Controlled Text */
#else
          /* Controlled Text */
#endif

Preprocessor – Diagnostic

• The directive ‘#error’ causes the preprocessor to report a fatal error. The tokens forming the rest of the line following ‘#error’ are used as the error message
• The directive ‘#warning’ is like ‘#error’, but causes the preprocessor to issue a warning and continue preprocessing. The tokens following ‘#warning’ are used as the warning message

...