Background procedure of a C program

A program in C language involves into different processes.  Below diagram will help you to understand all the processes that a C program comes across.

Preprocessor directive in C

The C Preprocessor is not part of the compiler, but is a separate step in the compilation process. In simplistic terms, a C Preprocessor is just a text substitution tool and they instruct compiler to do required pre-processing before actual compilation. We'll refer to the C Preprocessor as the CPP.

All preprocessor commands begin with a pound symbol (#). It must be the first nonblank character, and for readability, a preprocessor directive should begin in first column. Following section lists down all important preprocessor directives:

Directive	Description
#define	Substitutes a preprocessor macro
#include	Inserts a particular header from another file
#undef	Undefines a preprocessor macro
#ifdef	Returns true if this macro is defined
#ifndef	Returns true if this macro is not defined
#if	Tests if a compile time condition is true
#else	The alternative for #if
#elif	#else an #if in one statement
#endif	Ends preprocessor conditional
#error	Prints error message on stderr
#pragma	Issues special commands to the compiler, using a standardized method

File Handling in C Language

A file represents a sequence of bytes on the disk where a group of related data is stored. File is created for permanent storage of data. It is a readymade structure.

In C language, we use a structure pointer of file type to declare a file.

FILE *fp;

C provides a number of functions that helps to perform basic file operations. Following are the functions,

Function	description
fopen()	create a new file or open a existing file
fclose()	closes a file
getc()	reads a character from a file
putc()	writes a character to a file
fscanf()	reads a set of data from a file
fprintf()	writes a set of data to a file
getw()	reads a integer from a file
putw()	writes a integer to a file
fseek()	set the position to desire point
ftell()	gives current position in the file
rewind()	set the position to the begining point

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Opening a File or Creating a File

The fopen() function is used to create a new file or to open an existing file.

General Syntax :

*fp = FILE *fopen(const char *filename, const char *mode);

Here filename is the name of the file to be opened and mode specifies the purpose of opening the file. Mode can be of following types,

*fp is the FILE pointer (FILE *fp), which will hold the reference to the opened(or created) file.

mode	description
r	opens a text file in reading mode
w	opens or create a text file in writing mode.
a	opens a text file in append mode
r+	opens a text file in both reading and writing mode
w+	opens a text file in both reading and writing mode
a+	opens a text file in both reading and writing mode
rb	opens a binary file in reading mode
wb	opens or create a binary file in writing mode
ab	opens a binary file in append mode
rb+	opens a binary file in both reading and writing mode
wb+	opens a binary file in both reading and writing mode
ab+	opens a binary file in both reading and writing mode

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Closing a File

The fclose() function is used to close an already opened file.

General Syntax :

int fclose( FILE *fp );

Here fclose() function closes the file and returns zero on success, or EOF if there is an error in closing the file. This EOF is a constant defined in the header file stdio.h.

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Input/Output operation on File

In the above table we have discussed about various file I/O functions to perform reading and writing on file.getc() and putc() are simplest functions used to read and write individual characters to a file.

#include<stdio.h>

#include<conio.h>

main()

{

 FILE *fp;

 char ch;

 fp = fopen("one.txt", "w");

 printf("Enter data");

 while( (ch = getchar()) != EOF) {

    putc(ch,fp);

 }

 fclose(fp);

 fp = fopen("one.txt", "r");

 while( (ch = getc()) != EOF)

    printf("%c",ch);

 fclose(fp);

}

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Reading and Writing from File using fprintf() and fscanf()

#include<stdio.h>

#include<conio.h>

struct emp

{

   char name[10];

   int age;

};

void main()

{

   struct emp e;

   FILE *p,*q;

   p = fopen("one.txt", "a");

   q = fopen("one.txt", "r");

   printf("Enter Name and Age");

   scanf("%s %d", e.name, &e.age);

   fprintf(p,"%s %d", e.name, e.age);

   fclose(p);

   do

   {

      fscanf(q,"%s %d", e.name, e.age);

      printf("%s %d", e.name, e.age);

   }

   while( !feof(q) );

   getch();

}

In this program, we have create two FILE pointers and both are refering to the same file but in different modes. fprintf() function directly writes into the file, while fscanf() reads from the file, which can then be printed on console usinf standard printf() function.

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Difference between Append and Write Mode

Write (w) mode and Append (a) mode, while opening a file are almost the same. Both are used to write in a file. In both the modes, new file is created if it doesn't exists already.

The only difference they have is, when you open a file in the write mode, the file is reset, resulting in deletion of any data already present in the file. While in append mode this will not happen. Append mode is used to append or add data to the existing data of file(if any). Hence, when you open a file in Append(a) mode, the cursor is positioned at the end of the present data in the file.

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Reading and Writing in a Binary File

A Binary file is similar to the text file, but it contains only large numerical data. The Opening modes are mentioned in the table for opening modes above.

fread() and fwrite() functions are used to read and write is a binary file.

fwrite(data-element-to-be-written, size_of_elements,

                                              number_of_elements, pointer-to-file);

fread() is also used in the same way, with the same arguments like fwrite() function. Below mentioned is a simple example of writing into a binary file

const char *mytext = "The quick brown fox jumps over the lazy dog";  

FILE *bfp= fopen("test.txt", "wb");  

if (bfp) {    

   fwrite(mytext, sizeof(char), strlen(mytext), bfp) ;    

   fclose(bfp) ;  

}

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fseek(), ftell() and rewind() functions

·         fseek() - It is used to move the reading control to different positions using fseek function.

·         ftell() - It tells the byte location of current position of cursor in file pointer.

·         rewind() - It moves the control to beginning of the file.