Certain Simple C Concepts

C Preprocessor Directives

    C Preprocessor is a separate program invoked by the compiler as first part of the compilation. It includes the directives for source file inclusion (ie #include), macro definition (#define) and conditional inclusion (ie #if).

    Preprocessor directives starts with a "#" sign, and that statement will be executed prior to the actual compilation of the code.

  (1) Source code inclusion

    #include Directive
      Preprocessor will replace the #include directive by entire content of the specified file.
     If the file name is enclosed in angle brackets, compiler will search the file in standard directories where it is configured to search.

 #include < stdio.h> 

     If the file name is enclosed in single quotes, compiler will search the file first in current directory and then if not found search in standard directories.

 #include "Timer.h" 

     Inorder to search in directories other than standard directories, use -I flag in the Makefile.

 CFLAGS += -I$(TOSDIR)/lib/printf

  (2) Macro definition 

    "#define" Directive

       #define Directive Defines the processor to replace all subsequent occurrence of a macro with a specific token.

 #define ADXL345_DEVID 0x00 

     The above example causes the preprocessor to replace the subsequent occurrence of the identifier "ADXL345_DEVID" with constant hex value 0x00.

     "#undef"Directive
        Defines macros are not affected by the block structure of C. A macro once defined will last until a "#undef" directive for that particular token. After undefining you can again redefine that token.

 #define ADXL345_DEVID 0x00 
 #undef ADXL345_DEVID 
 #define ADXL345_DEVID 0x01 

  (3) Conditional Inclusions

         Allows to include or discard certain code based on some conditions.

       #ifdef

 #ifdef NUM
 int amount [NUM];
 #endif

        Here the middle line will be compiled only if the NUM variable is already defined with #define.

        #ifndef

 #ifndef NUM
 #define NUM 46
 int amount [NUM];
 #endif

        Here the NUM variable will be defined only if it is not previously defined.

       #if, #else, #elif

 #if NUM>100
 #undef NUM
 #define NUM 100
 
 #elif NUM<50
 #undef NUM
 #define NUM 50

 #else
 #undef NUM
 #define NUM 200
 #endif 

       Here depending on certain condition the code is compiled.

Race condition

   Usually occurs in threaded programs. When two or more threads try to access same resource and change its value at same time, race condition occurs. Inorder to avoid this condition put a lock around the variable before using.

Printing in C

     To print the remainder

Use % for printing remainder of a division operation.

printf("%d",564%10);

rvalue Vs lvalue

   rvalue: rvalue of a variable is the value stored in that variable.
   lvalue: lvalue of a variable is the value of its address (where it is stored).

j=6;
k=j;

   Here in first line 'j' is at the left side of the equation and it denotes the address to which value 6 is to be copied. In the second line 'j' is at the right side of the equation and it denotes the value stored in 'j' to be copied to the address denoted by 'k'.

Function Pointer

   Declaring function pointer

      "ptFunction" is the function pointer declared and initialized to Null value.

int (*pt2Function)(float, char, char) = NULL; 

   Defining function pointer

      "ptFunction" is initialized to "DoIt" function whose definition is given below

ptFunction = DoIt;        // short form
ptFunction = &DoIt;   // correct assignment using address operator

void DoIt(float a, char b, char c) { }

   Comparing function pointer 

      This checks whether the  "ptFunction" is defined or not.

if(ptFunction >0) { }

   Calling function using function pointer 

       Both the methods will work.

int result1 = ptFunction(12, 'a', 'b'); 
int result2 = (*ptFunction) (12, 'a', 'b');

   Pass function pointer as argument 

       Both the function call and the function definition are given below.

PassPtr(&DoIt);  //Call the function

void PassPtr(int (*pt2Func)(float, char, char)) { } // Definition for the called function

Enumerations (enum)

   It is a set of named integer constants. Is defined like structures.

   Enum definition

enum coin {penny, dollar, paisa=15, dime, quater}

   Each of the enum symbol stands for an integer value starting from 0.

   Printing the values

printf("%d %d %d %d %d\n",penny,dollar,paisa,dime,quater);

   Here penny=0, dollar=1, paisa=15, dime=16 and quater=17.

   Declare variable

enum coin money;

   "money" is an object of type coin. 

   Initialize the variable

money = paisa;

   "money" is initialized to value "paisa". Printing money will print value 15.