C Token example program:

int main()
{
 int x, y, sum;
 x = 5, y = 10;
 sum = x + y;
 Printf (“Sum = %d \n”, sum);
}

From above program C Tokens can be categorized as follows,
main – identifier, which is usually used by linker as the entry point of the regular program
{,}, (,) – delimiter
int – keyword
x, y, sum – Variables
+, = – Operators
main, {, }, (, ), int, x, y, sum – tokens

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Global variables are helpful when many functions in the program use the same data. Storage for global variables is in a fixed region of memory set aside for this purpose by the compiler.

In the following program, the variable count has been declared outside of all functions.

Example:

#include "stdio.h"
int count;  /* count is global  */
void func1(void);
int main(void)
{
  count = 100;
  func1();
  return 0;
}
void func1(void)
{
  printf("count is %
d", count); /* will print 100 */
}

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Local variables exist only while the block of code in which they are declared is executing. That is, a local variable is created upon entry into its block and destroyed upon exit. Furthermore, a variable declared within one code block has no bearing on or relationship to another variable with the same name declared within a different code block.

Example:

void func1(void) 
{ 
  int y; 
  y = 10;
}
void func2(void)
{ 
  int y;
  y = -199;
}

The integer variable y is declared twice, once in func1( ) and once in func2( ). The y in func1( ) has no relationship to the y in func2( ). This is because each y is known only to the code within the block in which it is declared.

The C language contains the keyword auto, which can be used to declare local variables. However, since all variables are, by default, assumed to be auto, this keyword is virtually never used.

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Here are few examples,

Example 1:

main()
 {
  char c;
 
  printf("%d,%d\n", sizeof c, sizeof(int)); /* returns 1, 4*/
 }

Example 2:

(main)
 {
	 struct 
	 {
	   int a;
	   int b;
	 }s;
	
	printf ("% d \ n", sizeof (s)); /* returns 8*/
	}

Example 3:

main()
 {
  short array [] = {1, 2, 3, 4, 5};

  short la = sizeof (array) / * returns 10 * /
 }
 

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Pointers defined using specific data type cannot hold the address of the some other type of variable i.e., it is incorrect in C++ to assign the address of an integer variable to a pointer of type float.

Example:

float *f; //pointer of type float
int i;  //integer variable
f = &i; //compilation error

The above problem can be solved by general purpose pointer called void pointer.

Void pointer can be declared as follows:

 
void *v // defines a pointer of type void

The pointer defined in this manner do not have any type associated with them and can hold the address of any type of variable.

Example:

void *v; 
int *i;
int ivar;
char chvar;
float fvar;
v = &ivar; // valid
v = &chvar; //valid
v = &fvar; // valid
i = &ivar; //valid
i = &chvar; //invalid
i = &fvar; //invalid

The post what is Void Pointer? first appeared on Student Projects.

In many applications memory is allocated for holding data objects. After using these objects, tha aplication will de-allocate this memory so that the memory can be re-used. In some cases the alications may use a pointer to an object whose memory is already de-allocated. This may lead to application crash or an unpredictable behavior.

scenarios which leads to dangling pointer

1. Application makes use of a object after it has been released, and there by access to an invalid memory location.
2. A function returns a pointer to one of its local variables, and since this local variable is defined only fro the function, the pointer becomes invalid once the function ends.

The most common result of this bug is the crash of the application or its running thread.

Examle 1:

#include "stdlib.h"
 
void func()
{
    char *dp = malloc(A_CONST);
    /* ... */
    free(dp);         /* dp now becomes a dangling pointer */
    /* ... */
}

Example 2:

{
   char *dp = NULL;
   /* ... */
   {
       char c;
       dp = &c;
   } /* c falls out of scope */
     /* dp is now a dangling pointer */
} 

Example 3:

#include "stdio.h"

int *call();
void main(){

int *ptr;
ptr=call();

fflush(stdin);
printf("%d",*ptr);
}

int * call(){

int x=25;
++x;

return &x;
}

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#define Sunday 0
#define Monday 1
#define Tuesday 2
#define Wednesday 3
#define Thursday 4
#define Friday 5
#define Saturday 6

The above lines can be replaced by:

enum AllDays{Sunday, Monday, Tuesday, Wednesday, Thursday, Friday, Saturday};

Example program:

#include "stdio.h" 
int main()
{
	enum AllDays{Sunday, Monday, Tuesday, Wednesday, Thursday, Friday, Saturday};

	AllDays aDay;
	int i = 0;
	printf("Please enter the day of the week (0 to 6)\n");
	scanf("%d",&i);
	aDay = AllDays(i);

	if(aDay == Sunday || aDay == Saturday)
	printf("It is weekend :) \n");
	else
	printf("It is not weekend :( \n");

	return 0;
}

The post How to use Enum in C? first appeared on Student Projects.

To use bool in C, we can use enum as below.

enum bool {
    false, true
};

bool value;
value = bool(0); // False
value = bool(1); // True

if(value == false)
	printf("Value is false");
else
	printf("Value is true");

Here we can use variable ‘value’ as boolean variable.

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2. In case of structure, each member have their own memory space but In union, one block is used by all the member of the union.

Detailed Example:

struct stu
{
	char c;
	int l;
	float p;
};

union emp
{
	char c;
	int l;
	float p;
};

In the above example size of the structure stu is 7 and size of union emp is 4.

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