What Is #Progma Directive?

Progma is implementation specific directive that is each progma directive has different implementation rule and use. There are the many type of progma directive and varies from one compiler to another compiler. If compiler doesn't recognize particular progma. Than it ignore the progma statement without showing any error or warning message and execute.

#progma Startup #progma exit #progma warn #progma inline #progma argused #progma hdrfile #progma hdrstup #progma saverage

#progma inline only tells the compiler that source code of program contain inline assembly language code.In C we can write assembly language program with help of asm keyword.

#progma warn  In C there are many warning message which can be ON or OFF with help of #progma warn

Syntax:-

#progma warn +XXX #progma warn -XXX #progam warn .XXX

Where
             +   mean ON
             -   mean OFF
             .   mean on/off toggle

XXX indicate particular warning code in three alphabet

                                     

#progam Startup[priority]   #progam exit[priority]

        

Where Priority Is Optional Integer Number.-

Progma startup always execute the function before the main function.#progam exit always execute the Function after the main function for user priority varies from 64 to 255.

 For C library priority varies from 0 to 63 Default priority is 100

void india(); void usa(); #progma startup india 105 #progma startup start usa #progma exit Usa #progma exit india 105 void main() {      printf("\n i am in main");      getch(); } void india() {     printf("\n i am in india");     getch(); } void usa() {            printf("\n i am in usa");            getch(); }

Output:-

i am in usa i am in india i am in main i am in india i am in usa

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Far Pointer in C Programming

The Pointer which can point or access whole the residence memory of RAM i.e. which can access all 16 segments is known as far Pointer.

Far Pointer:-

(If you don’t know what is segment the click here)
Size of far Pointer is 4 byte or 32 bit.

Examples:

(1) What will be output of following c program?

#include<stdio.h> int main() {    int x=10;    int far *ptr;    ptr=&x;    printf("%d",sizeof ptr);    return 0; }

Output:- 4

(2)What will be output of following c program?

#include<stdio.h> int main() {      int far *near*ptr;     printf("%d %d",sizeof(ptr) ,sizeof(*ptr));     return 0; }

 

Output:- 4 2

Explanation: ptr is far pointer while *ptr is near pointer.

(3)What will be output of following c program?

#include<stdio.h> int main() {     int far *p,far *q;    printf("%d %d",sizeof(p) ,sizeof(q));    return 0; }

Output:- 4 4

First 16 bit stores: Segment number
Next 16 bit stores: Offset address

What is segment number and offset address?

Example:-

#include<stdio.h> int main() {      int x=100;      int far *ptr;      ptr=&x;      printf("%Fp",ptr);     return 0; }

Output: -8FD8:FFF4

Here 8FD8 is segment address and FFF4 is offset address in hexadecimal number format. %Fp is used for print offset and segment address of pointer in printf function in hexadecimal number format.In the header file dos.h there are three macro functions to get the offset address and segment address from far pointer and vice versa.

1. FP_OFF(): To get offset address from far address.

2. FP_SEG(): To get segment address from far address.

3. MK_FP(): To make far address from segment and offset address.

Examples:

(1)What will be output of following c program?

#include <dos.h> #include<stdio.h> int main() {     int i=25;    int far*ptr=&i;    printf("%X %X",FP_SEG(ptr),FP_OFF(ptr));    return 0; }

Output: Any segment and offset address in hexadecimal number format respectively.

(2)What will be output of following c program?

#include <dos.h> #include<stdio.h> int main() {      int i=25;     int far*ptr=&i;     unsigned int s,o;     s=FP_SEG(ptr);     o=FP_OFF(ptr);     printf("%Fp",MK_FP(s,o));     return 0; }

Output: 8FD9:FFF4 (Assume)                         We cannot guess what will offset address, segment address and far address of any far pointer. These address are decided by operating system. Limitation of the far pointers: We cannot change or modify the segment address of given far address by applying any arithmetic operation on it. That is by using arithmetic operator we cannot jump from one segment to other segments. If you will increment the far address beyond the maximum value of its offset address instead of incrementing segment address, it will repeat its offset address in cyclic order.

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What Is Huge Pointer

Huge Pointer

The Pointer which can pointer total 16 segment of memory is called Huge pointer.
the size of the huge pointer is 4bytes ie 32 bits.

Example:

void main() {   int*p;   int near *np;   int far *fp;   int huge *hp;   printf("\n %d %d %d %d ", sizeof(p),sizeof(np),sizeof(fp),sizeof(hp)); }

Output:- 2 2 4 4

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What Is Pointer?

A Pointer is a derive data type in C, which is built from the fundamental data type in c a Pointer is a variable which stores always an address of another variable.

Advantage of Pointer:

By using Pointer we can use access a variable which is defined outside the function

By using Pointer, we can handle we can handle the database like graph, trees and linked list more efficiently.

Pointer can increase the execution speed.

In Pointer, we using two operators

& address of operator  * object \value of address \indirection operator \deference operator

void main() {    int i;     int* ptr;    ptr=&i; }

Here i is a variable of type integer.ptr is a variable of type int* and it is an address type.ptr can hold an address of an integer variable.

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What Is Wild Pointer

  Wild pointer:

           A pointer in C which has not been initialized is known as wild pointer.

    Example:

   What will be output of following c program?

#include<stdio.h>  int main()  {     int *ptr;     printf("%u\n",ptr);     printf("%d",*ptr);     return 0;    }

         Output:

Any address Garbage value

Here ptr is wild pointer because it has not been initialized.

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What Is Null Pointer in C Language?

It is always a good practice to assign a NULL value to a Pointer variable in case you do not have an exact address to be assigned. This is done at the time of variable declaration. A Pointer that is assigned NULL is called a null Pointer. The NULL Pointer is a constant at a value of zero defined in several standard libraries.
Consider the following program:

#include <stdio.h> int main () {    int  *ptr = NULL;    printf("The value of  ptr is : %x\n", &ptr  );    return 0; }

When the above code is compiled and executed, it produces following result:

The value of ptr is 0. On most of the operating systems, programs are not permitted to access memory at address 0 because that memory is reserved by the operating system. However, the memory address 0 has special significance; it signals that the Pointer is not intended to point to an accessible memory location. However, by convention, if a pointer contains the null (zero) value, it is assumed to point to nothing.
To check for a null pointer you can use an if statement as follows:

if(ptr)     /* succeeds if p is not null */ if(!ptr)    /* succeeds if p is null */

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