File Handling in C

File Handling in "C"

In Computer, when we run programs, they use memory (RAM) to store the variables, which are temporary. Memory is volatile and its content will be in memory till the program is running, once the program get terminated, then values stored in parameter also erased. If we need data again then we need to use keyboard input. So this will be valid for small data, but for large data it’s not possible to enter data again and again when you require it and another way to get the values may be generate the values pragmatically. But both of the ways are very difficult to implement.

             So we need to store data on disk, which we can retrieve whenever we require. So here we are going to discuss how to store data on disk and perform various input and output operations.

Note: Data which we store on disk is always stored in form of binary data. Binary data which we store on disk may be varies from OS.

DATA ORGANIZATION

All DATA on the Disk store in the Binary form. How this binary data is stored on the Disk varies from one OS to another.

OS

C  Library functions

Our        Program

     However, this doesn’t affect programmer. He has to use only the library functions written for the particular OS to be able to perform input/output. It is compiler vendor’s responsibility to correctly implement these library functions by taking the help of OS. 

File:- A file represents a sequence of byte on the disk where a group of related data is stored. File is created for permanent storage of data. We use a structure pointer of file type to declare a file.

e.g. File *file;

File is named location of stream of bits. It may be stored at single place or different places but it represents a single stream.

   There are 2 types of files:-

Ø Text Files

Ø Binary Files

ASCII Text files:-

A text file can be a stream of characters that a computer can process sequentially. It is not only processed sequentially but only in forward direction. For this reason a text file is usually opened for only one kind of operation (reading, writing, or appending) at any given time.

Similarly, since text files only process characters, they can only read or write data one character at a time. 

Binary files:-

A binary file is no different to a text file. It is a collection of bytes. In C Programming Language a byte and a character are equivalent. Hence a binary file is also referred to as a character stream, but there are two essential differences.

1.     No special processing of the data occurs and each byte of data is transferred to or from the disk unprocessed.

2.     C Programming Language places no constructs on the file, and it may be read from, or written to, in any manner chosen by the programmer.

Binary files can be either processed sequentially or, depending on the needs of the application, they can be processed using random access techniques. In C Programming Language, processing a file using random access techniques involves moving the current file position to an appropriate place in the file before reading or writing data. This indicates a second characteristic of binary files.

Advantage of File:-

It will contain the data even after program exit. Normally we use variable or array to store data, but data is lost after program exit. Variables and arrays are non-permanent storage medium whereas file is permanent storage medium.

Stream:-

Stream is not a hardware it is linear queue which connect file to program and passes block of data in both direction .So it is independent of devices which we are using. We can also define stream as source of data. This source can be

(a) A file

(b) Hard disk or CD, DVD etc.

(c) I/O devices etc.

In c programming language there are two type of stream.

(a) Text streams

(b) Binary streams

Buffer :- Buffer is a technique which reduces number of I/O call.

Through file handling, one can perform operations like create, modify, delete etc on system files. 

File Handling:-

C language basically provides two mechanisms to handle the files:

• Low-Level File Handling
• High-Level File Handling

Low-Level File Handling:-
The low-level file handling mechanism requires the secondary storage devices to directly fetch/put a specific number of bytes for each read/write request. It provides no formatting facilities (i.e. data-types are irrelevant) as we are dealing directly with the bytes. In this approach the files are referred through integer numbers termed as file descriptors. As we handle the data in binary format (prescribing exactly how many bytes to read or write) this approach baffles a novice programmer.
High-Level File Handling:-
C provides a wrapper around the low-level file handling mechanism which uses buffers to communicate with secondary storage devices. A program’s read/write requests are transferred through a buffer. As this enables data management through available data-types, a programmer can perceive it in a normal sense.
As the high-level file handling approach simplifies involved process considerably, we will follow the same further. It is also referred as buffered input/output (I/O). 

High-Level File Handling in C Language:-

The involved buffers in high-level file handling are used through an abstraction termed as “streams”. The streams provide a portable way of writing/reading data to/from a file. A stream is considered as a file or a physical device (e.g.printer or monitor). The involved input/output (I/O) through a stream is buffered (i.e. a fixed chunk is read from or written to a file via some temporary storage area, the buffer). A pointer to a stream is used to manipulate the stream. It is termed as a file pointer and it actually points to involved buffer.
File Pointer:-

There exists an internal C data structure, FILE, which represents all streams. FILE is a structure containing the information about a file or text stream needed to perform input or output operations on it. It includes the following fields:

• a file descriptor
• the current stream position
• an end-of-file indicator
• an error indicator
• a pointer to the stream's buffer, if applicable

It is declared in the header file “stdio.h”. We simply need to refer to the FILE structure in C programs when performing I/O with streams. The declaration of the FILE structure is as follows:

typedef struct{
short level;
unsigned flags;
char fd;
unsigned char  hold;
short bsize;
unsigned char *buffer, *curp;
unsigned istemp;
short token;
}FILE;

A file pointer is declared as a pointer to this FILE structure.
FILE *fp; /* fp is declared as a FILE pointer */
The high-level file handling provides a set of functions involving file pointers to perform various operations on files. Generally all the functions in this set are prefixed with “f”.

File I/O Stream:-

Ø All I/O is done with streams.

Ø Stream is nothing but sequence of byte of data.

Ø A sequence of byte flowing into stream is called Input Stream.

Ø A sequence of byte flowing out of stream is called Output Stream.

Ø Use of stream make I/O machine independent.

Predefined Streams:-

stdin	Standard Input
Stdout	Standard Output
Stderr	Standard Error

Standard Input Stream device:-

Ø stdin stands for standard input

Ø Keyboard is input stream device.

Ø Standard input is data(often text) going in to a program.

Ø The program requests data transfer by use of read operation.

Ø Not all program require input.

Standard Input Stream device:-

Ø stdout stands for standard output

Ø Screen(monitor) is output stream device.

Ø Standard output is data(often text) going out from a program.

Ø The program sends data to output device by using write operation.

Difference b/w Standard Input  & Standard Output device

Point	Standard Input Stream Device	Standard Output Stream Device
Stands for	Standard Input	Standard Output
Example	Keyboard	Screen(Monitor)
Data Flow	Data(often text) going into program	Data(often text) going out from the program
Operation	Read Operation	Write Operation

Some important Summary:-

Point	Input Stream	Output Stream
Standard Device 1	Keyboard	Screen
Standard Device 2	Scanner	Printer
IO Function	scanf & gets	printf & puts
IO Operation	Read	Write
Data	Data goes from Stream	Data comes into Stream

Text File Format:-

Ø Text File is also calld “Flat File”.

Ø Text File is simple sequence of ASCII characters.

Ø Each line is characterized by EOL character.

Text File Formats:-

Ø Text file have .txt Extension.

Ø Files with .txt extension can easily be read or opened by any program that reads text and, for that reason, are considered universal(or platform independent).

    File Operations:-

 There are different operations that we can carry out on a file.

These operations are general operations which we need to work on files.

a)     Creation of  a new file

b)    Opening an existing file

c)     Reading from a file

d)    Writing to a file

e)     Append content to file

f)      We can assign name to file

g)     We can rename to file name

h)    We can position to particular byte in the file

i)       We can check end of the line in file

j)       Can find size of the file

k)     We can delete the file

l)       We can return result of last file input/output operation from the file

m)   Copy a file to another file

n)     Closing a file

Functions for file handling:-

There are many functions in C library to open, read, write, search and close file.

Function	Syntax	Example
fopen - Open file	FILE * fopen(char * filename, char * mode) Filename – string filename with path Mode – mode of opening              r    read only             w   write only             a    append file             +    read and write (r+/w+/a+) Return value – FILE pointer on success NULL on failure	FILE * infile ; Infile = fopen(“a.txt”,”r”); FILE *outfile = fopen(“b.txt”,”w”); if(infile==NULL || outfile==NULL) { printf(“error”); }
fclose – close the open file	fclose(FILE *fptr) fptr – FILE * of the open file Returns void	fclose(infile);
fgetc – read one char from file	int fgetc(FILE *fptr) fptr – FILE * of the open file returns – character read             -1 on eof or error	while( (ch=fgetc())!=-1)       putchar(ch); //display content of file //on screen
fputc – write one character to the file	int fputc(int ch,FILE *fptr) ch – character to write fptr – open file pointer returns character written            EOF if there is error	for(ch=’A’;ch<=’Z’;ch++)        fputc(ch,fptr);
fscanf – reads data from file	int fscanf(FILE *ptr,char * fmt,…) ptr – FILE pointer fmt – format specifier for variables to be read Returns – number of values read successfully	fscanf(infile, “%d %d”,&n1,&n2);
fprintf – write data into file	int fprintf(FILE *ptr,char * fmt,…) ptr – FILE pointer fmt – format specifier for variables to be read Returns – number of characters  printed or negative number on error	fprintf(outfile,“Value is %d\n”,n1);
fread – read from binary file	int fread( void *buffer, size_t size, size_t num, FILE *ptr ) buffer – stores the value read size – size of the buffer num – number of blocks to be read ptr – file pointer Returns – number of values read	Char *str = malloc(50); fread(str,50,1,infile);
fwrite – write to a binary file	int fwrite( void *buffer, size_t size, size_t num, FILE *ptr ) buffer – stores the value read size – size of the buffer num – number of blocks to be read ptr – file pointer Returns – number of values written	for(i=0;i<10;i++)    fwrite(&i,sizeof(i),1,outfile); char a[]=”end”; fwrite(a,strlen(a)+1,1,outfile);
fseek – move the file pointer by given offset	int fseek(FILE*ptr,long offset,int whence) ptr – file pointer offset – offset in bytes from third parameter whence – SEEK_SET – from beginning of file SEEK_CUR – from current position SEEK_END – from end of file Returns – zero on success Non-zero on failure	if(fseek(infile,4L,SEEK_SET)==0) {      char ch=fgetc(infile);      printf(“The fourth char of the file is %c\n”,ch);  }
ftell – get the position of file pointer	int ftell(FILE *ptr) ptr – FILE pointer Returns – position of file pointer -1 on error	FILE *ptr = fopen(b[1],"r"); fseek(ptr,0L,SEEK_END);  int size_of_file = ftell(ptr);
rewind – moves the file pointer to the beginning of the file	void rewind(FILE *ptr) ptr – FILE pointer	rewind(infile); int n = fscanf(infile,”%d”,&n);
feof – detects end-of-file marker in a file.

Opening File:-

Concept:-  Before we read (or write) information from (to file) on a disk, we must open a file in particular mode. To open a file we use fopen() function. fopen() is a high level IO function. There are many high level IO function we use in file handling.

 fopen():-

It opens a file and associates a stream with it. It returns a file pointer that identifies the stream in subsequent operations.

Header File: - stdio.h
Syntax: - FILE *fopen(const char *filename, const char *mode);
Arguments:-

• filename: File that the function open
• mode: Defines the mode of the opened file

or

Syntax:-

FILE *fp;                fp= fopen(“filename”,”mode”);

FILE = it’s a data structure type. Its defined in standard I/O function definitions

fp = this is a file pointer of type FILE data structure.

fopen =  High level IO function

filename = valid filename which you want to open

mode = (read, write, append)

How to open file?

1.     fopen( ) opens a stream.

2.     fopen( ) opens a file & associate a stream with it.

3.     Function returns a pointer that identifies the stream in subsequent operations means fopen() will return the address of the structure that contains information needed for subsequent I/O on the file being opened(i.e. file pointer).

e.g.  here fopen() will open “file.txt” in the read mode.

fp=fopen(“file.txt”,”r”);

Before storing data on secondary storage, firstly we must specify following things:-

1.     File Name

2.     Data Structure

3.     Purpose/Mode

4.     Very first task in file handling is to open a file

File Name:-

1.     File name consist of 2 fields.

2.     First field is name field & second field is extension field.

3.     Extension field is optional.

4.     Both file name & extensions are separated by period or dot.

Data Structure:-

1.     Data Structure of file is defined as FILE in the library of standard I/O functions.

2.     In short we have to declare the pointer variable of type FILE.

Opening Mode:-

We can open file in different modes such as reading mode, writing mode, appending mode  depending on purpose of file.

Following are the different opening modes of File:-

MODE	DESCRIPTION
r	Opens an text file for reading mode       If file is opened successfuly fopen() loads it in to memory  & sets up a pointer which points to first character in it. If the file can’t be opened, fopen() returns NULL.
w	Opens or create a text file for writing mode        If file exists, its content are overwritten . If the file doesn’t exist, a new file is created, It returns NULL if unable to open a file.
a	Opens a text file in append mode means adding new contents at the end of file      If file is opened successfuly fopen() loads it in to memory  & sets up a pointer which points to last character in it. If the file doesn’t exist, a new file is created, It returns NULL if unable to open a file.
r+	Opens an text file in both reading & writing mode        If file is opened successfuly fopen() loads it in to memory  & sets up a pointer which points to first character in it. If the file can’t be opened, fopen() returns NULL.
w+	Opens an text file in both reading & writing mode      If file exists, its content are overwritten . If the file doesn’t exist, a new file is created, It returns NULL if unable to open a file.
a+	Opens an text file in both reading & writing mode but can’t modify existing content        If file is opened successfuly fopen() loads it in to memory  & sets up a pointer which points to first character in it. If the file doesn’t exist, a new file is created, It returns NULL if unable to open a file.
rb	Opens an binary file for reading mode
wb	Opens or create a binary file for writing mode
ab	Opens a binary file in append mode
rb+	Opens an binary file in both reading & writing mode
wb+	Opens an binary file in both reading & writing mode
ab+	Opens an binary file in both reading & writing mode

In fact fopen() performs three important tasks when you open the file in “r” mode:

a.     Firstly it searches on the disk the file to be opened.

b.     Then it loads the file from the disk into a place in memory called buffer.

c.      It sets up a character pointer that points to the first character of the buffer.

Why do we need a buffer at all? Imagine how efficient it would be to actually access the disk every time we want to read a character from it. Every time we read something from a disk, it takes some time for the disk drive to position the read/write head correctly. On a floppy disk system, the driver motor has to actually start rotating the disk from a standstill position every time the disk is accessed. If this were to be done for every character we read from the disk, it would take a long time to complete the reading operation. This is where a buffer comes in . It would be more sensible to read the contents of the file into the buffer while opening the file and then read the file character by character from  the buffer rather than from the disk.

 

fopen() returns this address of this structure, which is collected in the structure pointer called fp.We have declared fp as

File *fp;

Example 1 of opening the file:-

#include<stdio.h>

int main()

{

FILE *fp;

char ch;

fp = fopen("INPUT.txt","r")      // Open file in Read mode

fclose(fp);    // Close File after Reading

return(0);

}

Example 2 of opening the file:-

#include<stdio.h>

void main()

{

 FILE *fp;

 char ch;

 fp = fopen("INPUT.txt","r");           // Open file in Read mode

   while(1)

   {

   ch = fgetc(fp); // Read a Character

      if(ch == EOF )                            // Check for End of File

         break ;

   printf("%c",ch);

   }

 fclose(fp);                                        // Close File after Reading

}

Closing a File:-

Whenever we open a file in read or write mode then we perform appropriate operations on file and when file is no longer needed then we close the file. The only argument it[fclose() function] accepts is the file pointer.

FILE close will flush out all the entries from buffer.

Important:-

Ø After performing all operations, File is no longer needed.

Ø File is closed using fclose() function.

Ø All information associated with file is flushed out of buffer.

Ø Closing file will prevent misuse of FILE.

Syntax: fclose()

int  fclose (FILE *fp);

The fclose( ) function returns zero on success, or EOF if there is an error in closing the file. This function actually, flushes any data still pending in the buffer to the file, closes the file, and releases any memory used for the file. In case of failure, if the stream is accessed further then the behavior remains undefined. The EOF is a constant defined in the header file stdio.h.

Return Type : fclose()

Action	Return Value
On Success	0
On Error	NULL

If a program terminates, it automatically closes all opened files. But it is a good programming habit to close any file once it is no longer needed. This helps in better utilization of system resources, and is very useful when you are working on numerous files simultaneously. Some operating systems place a limit on the number of files that can be open at any given point in time.

Input and Output using file pointers

Writing a File

When a file is opened for writing, it will be created if it does not already exist and it will be reset if it does, resulting in the deletion of any data already there. Using the w indicates that the file is assumed to be a text file.

Character Output with File using putc() function:-

The functions putc() is equivalent to putchar() functions, except that these functions require an argument which is the file pointer.Function putc() writes a character to the file identified by its second argument. 

The format of putc() function is as follows :
       putc(c, out_file);

Note:-The second argument in the putc() function must be a file opened in either write or append mode.

Example 1

#include<stdio.h>

#include<string.h>

void writeToFile(char* fileName);

void main()

{

          char fileName[20];

          printf("Enter File Name \t:");

          scanf("%s",fileName);

          fflush(stdin);                 //  Clear the buffer from memory

          writeToFile(fileName);  

}

void writeToFile(char* fileName)

{

             FILE *fp;            // File Pointer

             char c;

             fp= fopen(fileName,"w");    // Opening file in writing mode

            printf("Enter the content , Enter ctrl+Z to exit   :\n\n");

             printf (“*****************************************\n\n”);

             while((c=getchar())!=EOF)

             {

                 putc(c,fp);        // Writing Characters in file using pointer

             }

           

             fclose(fp);            // Closing File Pointer

             printf (“\n\n*****************************************\n”);

             printf("%s File Written successfully\n",fileName);

}

Example 2

#include <stdio.h>

main ()

{

  char in_file[30], out_file[30];

  FILE *fpin, *fpout;

  int c;

  printf("This program copies the source file to the destination file

\n\n");

  printf("Enter name of the source file :");

  scanf("%30s", in_file);

  printf("Enter name of the destination file :");

  scanf("%30s", out_file);

  if((fpin=fopen(in_file, "r")) == NULL)

    printf("Error could not open source file for

reading\n");

  else if ((fpout=fopen(out_file, "w")) == NULL)

    printf("Error could not open destination file for

reading\n");

  else

   {

    while((c =getc(fpin)) != EOF)

      putc(c, fpout);

    printf("Destination file has been copied\n");

  }

}

Important Points:–

1.    When we open file in write mode, automatic a buffer area will assigned to that pointer.

2.    The characters which we want to save on file, first save in that buffer area only, once we finishes our writing, then it will copy into that file.

3.    If the buffer gets full, then it will copy the content on disk or file and make space for new content

4.    At the end of file it puts EOF character to indicate end of file.

5.    Buffer plays a important role in writing text file or appending text.

Formatted Output with File Pointers using fprintf() function

This function is the parent function of printf(). The normal printf() function writes the contents to the screen whereas fprintf() writes them to a file identified by a stream. On success it returns the number of bytes output. On error it will return EOF.
To work with text output, you use fprintf , both of which are similar to their friends printf except that you must pass the FILE pointer as first argument.

Header File: stdio.h
Syntax:  int fprintf(FILE *stream, const char *format [, argument, …]);
Arguments:

• stream: Stream where the function output the formatted data
• format: Normal format string as that of printf() function

For example:
FILE *fp;
fp=fopen("c:\\test.txt", "w");
fprintf(fp, "Testing...\n");
Here is the program to create a file and write some data into the file.

e.g. 1

#include <stdio.h>

int main()

{

 FILE *fp;

 file = fopen("file.txt","w");

 /*Create a file and add text*/

 fprintf(fp,"%s","This is just an example :)"); /*writes data to the file*/

 fclose(fp); /*done!*/

 return 0;

}

e.g. 2

#include<stdlib.h>

#include<stdio.h>

int main() {

    FILE *fp = fopen("/path/to/file/filename.txt", "w");

    if (fp == NULL) {

        fprintf(stderr, "Can't open input file!\n");

        exit(1);

    }

    fprintf(fp, "Whatever you want to write.\n");

    fprintf(fp, "Really, anything.\n");

    fclose(fp);

    return 0;

}

Formatted Output with Strings

This is the third set of the printf families. This is called sprintf.

sprintf

puts formatted data into a string which must have sufficient space allocated to hold it. This can be done by declaring it as an array of char. The data is formatted according to a control string of the same form as that for printf.

Reading a File

If you want to read a file you have to open it for reading in the read (r) mode. Using the r indicates that the file is assumed to be a text file. Opening a file for reading requires that the file already exist. If it does not exist, the file pointer will be set to NULL and can be checked by the program.

Character Input with Files using getc() function

Ø we use getc function to read from file. This function reads character from current pointer location and returns character, which we can save in a character variable and display on screen.

Ø To read using this function we need a loop which will iterate through all characters and display on screen or perform any operation.

Ø We use while loop to iterate through the file, but before using any loop we need a condition to break the rule.

Ø In files, usually it puts a character at end of each file, we can check for this character and break the loop. This character has ASCII value of 26 and we can also define it as EOF. So we can check for this variable and break the rule.

Ø EOF is defined in stdio.h header file.

The functions getc() is equivalent to getchar() function, except that this functions require an argument which is the file pointer. Function getc() reads a single character from the file which has previously been opened using a function like fopen(). 

Example

#include<stdio.h>

#include<string.h>

void readFromFile(char * fileName);

void main()

{

          char fileName[20];

          printf("Enter File Name \t:");

          scanf("%s",fileName);

          fflush(stdin);                 //  Clear the buffer from memory

          readFromFile(fileName);

}

void readFromFile(char* fileName)

{

       FILE *fp;            // File Pointer

             char c;

             fp= fopen(fileName,"r");    // Opening file in reading mode

             if(fp==NULL)

             {

                 printf("File not found \n");

                       printf("Please check file Name\n");

                       exit();

             }

             printf("File Reading   :\n\n");

             printf("*************************************************\n\n");

             while((c=getc(fp))!=EOF)

             {

                 printf("%c",c);       // Display Characters on screen

             }

             printf("\n\n***********************************************\n");

             fclose(fp);            // Closing File Pointer

             printf("\n\n   %s File read successfully\n",fileName);

}
      

Reading a file character by character

#include <stdlib.h>

#include <stdio.h>

int main() {

    FILE *fp = fopen("/path/to/file/filename.txt", "r");

    if (fp == NULL) {

        fprintf(stderr, "Can't open input file!\n");

        exit(1);

    }

    int ch;

    while ((ch = fgetc(fp)) != EOF) {

        char character = (char) ch;

        // do something with character

        printf("%c", character);

    }

    fclose(fp);

    return 0;

}

Formatted Input with File Pointers using fscanf() function

fscanf() is the parent function of scanf() function. It takes input from the specified stream. It returns the number of input fields successfully scanned, converted and stored. It returns 0 (zero) when no field is stored or EOF when it attempts to read at end-of-file.

Header File: stdio.h 
Syntax:  int fscanff(FILE *stream, const char *format [, argument, …]);
Arguments:

• stream: Stream from where the function takes the formatted data
• format: Normal format string as that of scanf() function

The function fscanf() is similar to scanf() except that this function operates on files and require one additional and first argument to be a file pointer.

Reading a file word by word

#include <stdlib.h>

#include <stdio.h>

int main() {

    FILE *fp = fopen("/path/to/file/filename.txt", "r");

    if (fp == NULL) {

        fprintf(stderr, "Can't open input file!\n");

        exit(1);

    }

    char word[64];

    // assuming no word in the file is longer than 64 characters.

    while (fscanf(fp, "%64s", word) != EOF) {

        // do something with word

        printf("%s\n", word);

    }

    fclose(fp);

    return 0;

}

Formatted Input Output with Strings

This is the third set of the scanf families. This is called sscanf.

sscanf

takes data from a string and stores it in other variables as specified by the control string. This is done in the same way that scanf reads input data into variables. sscanf is very useful for converting strings into numeric v values.

Whole Line Input and Output using File Pointers

Predictably, equivalents to gets and puts exist called fgets and fputs. The programmer should be careful in using them, since they are incompatible with gets and puts. gets requires the programmer to specify the maximum number of characters to be read. fgets and fputs retain the trailing newline character on the line they read or write, wheras gets and puts discard the newline.
When transferring data from files to standard input / output channels, the simplest way to avoid incompatibility with the newline is to use fgets and fputs for files and standard channels too.

For Example, read a line from the keyboard using

  fgets(data_string, 80, stdin);

and write a line to the screen using

  fputs(data_string, stdout);

fgets() Function

fgets library functions can be used to read the contents of the file. (It is also possible to make use of the library function fscanf. But you have to be sure that the file is perfectly formatted or fscanf will not handle it correctly). Let’s take a look at an example:

#include<stdio.h>

int main()

{

FILE *file;

char buf[1000];

file=fopen(input.txt,”r”);

if(!file)

return 1;

while(fgets(buf,1000,file)!=NULL)

printf(“%s”,buf);

fclose(file);

return 0;

}

Note:-The printf statement does not have the new-line (\n) in the format string. This is not necessary because the library function fgets adds the \n to the end of each line it reads.

A file “input.txt” is opened for reading using the function fopen en the mode read (r). The library function fgets will read each line (with a maximum of 1000 characters per line.) If the end-of-file (EOF) is reached the fgets function will return a NULL value. Each line will be printed on stdout (normally your screen) until the EOF is reached. The file is then closed and the program will end.

Program to write and read data from a file

# include <stdio.h>

# include <conio.h>

void main()

{

char c ;

FILE *fptr1 ;

clrscr() ;

printf("Enter the text to be stored in the file.\n") ;

printf("Use ^Z or F6 at the end of the text and press

ENTER: \n\n") ;

fptr1 = fopen("COURSES.DAT","w") ;

while((c = getc(stdin)) != EOF)

fputc(c, fptr1) ;

fclose(fptr1) ;

printf("\nThe content of the file is : \n\n") ;

fptr1 = fopen("COURSES.DAT", "r") ;

do

{

c = fgetc(fptr1) ;

putchar(c) ;

} while(c != EOF) ;

fclose(fptr1) ;

getch() ;

}

Example of fprintf() & fscanf() Function

#include <stdio.h>

main ()

{

  FILE *fp;

  float total;

  fp = fopen("data.txt", "w+");

  if (fp == NULL) {

    printf("data.txt does not exist, please check!\n");

    exit (1);

  }

  fprintf(fp, 100);

  fscanf(fp, "%f", &total);

  fclose(fp);

  printf("Value of total is %f\n", total);

}

Binary stream input and output

The functions fread() and fwrite() are a somwhat complex file handling functions used for reading or writing chunks of data containing NULL characters ('\0') terminating strings.

The declarations for each are similar:

         size_t fread(void *ptr, size_t size_of_elements, size_t number_of_elements, FILE *a_file);

             

         size_t fwrite(const void *ptr, size_t size_of_elements, size_t number_of_elements, FILE *a_file);

Both of these functions deal with blocks of memories - usually arrays. Because they accept pointers, you can also use these functions with other data structures; you can even write structs to a file or a read struct into memory.

Let's look at one function to see how the notation works.

fread takes four arguments. Don't be confused by the declaration of a void *ptr; void means that it is a pointer(that can be used for any type variable) to the buffer that is used to reading/writing the data. The first argument is the name of the array or the address of the structure you want to write to the file. The second argument is the size of each element of the array; it is in bytes. For example, if you have an array of characters, you would want to read it in one byte chunks, so size_of_elements is one. You can use the sizeof operator to get the size of the various datatypes; for example, if you have a variable int x; you can get the size of x with sizeof(x);. This usage works even for structs or arrays. E.g., if you have a variable of a struct type with the name a_struct, you can use sizeof(a_struct) to find out how much memory it is taking up.

e.g.,

sizeof(int);

The third argument is simply how many elements you want to read or write; for example, if you pass a 100 element array, you want to read no more than 100 elements, so you pass in 100.

The final argument is simply the file pointer we've been using. When fread is used, after being passed an array, fread will read from the file until it has filled the array, and it will return the number of elements actually read. If the file, for example, is only 30 bytes, but you try to read 100 bytes, it will return that it read 30 bytes. To check to ensure the end of file was reached, use the feof function, which accepts a FILE pointer and returns true if the end of the file has been reached.

fwrite is similar in usage, except instead of reading into the memory you write from memory into a file.

For example,

FILE *fp;

fp=fopen("c:\\test.bin", "wb");

char x[10]="ABCDEFGHIJ";

fwrite(x, sizeof(x[0]), sizeof(x)/sizeof(x[0]), fp);

Function fread() reads it as a stream of bytes and advances the file pointer by the number of bytes read. If it encounters an error or end-of-file, it returns a zero, you have to use feof() or ferror() to distinguish between these two.

Let us rewrite program  using fread() and fwrite() functions .

Program

#include <stdio.h>

#define MAX_SIZE 1024

main ()

{

  FILE *fp, *gp;

  char buf[MAX_SIZE];

  int i, total = 0;

  if ((fp = fopen("data1.txt", "r") ) == NULL)

    printf("Error in data1.txt file \n");

  else if ((gp=fopen("data2.txt", "w")) == NULL)

    printf("Error in data2.txt file \n");

  else

   {

    while(i=fread(buf, 1, MAX_SIZE, fp))

    {

     fwrite(buf, 1, MAX_SIZE, gp);

     total +=i;

    }

    printf("Total is %d\n", total);

  }

  fclose(fp);

  fclose(gp);

}

Appending(a) a File

When a file is opened for appending, it will be created if it does not already exist and it will be initially empty. If it does exist, the data input point will be positioned at the end of the present data so that any new data will be added to any data that already exists in the file. Using the a indicates that the file is assumed to be a text file.

We open the file in append mode by following way

          fp = fopen(fileName,”a”);

fp = File pointer

filename = name of file in which we need to append text

#include<stdio.h>

#include<string.h>

void appendToFile(char* fileName);

void main()

{

          char fileName[20];

          printf("Enter File Name \t:");

          scanf("%s",fileName);

          fflush(stdin);                 //  Clear the buffer from memory

         appendToFile(fileName); 

}

void appendToFile(char* fileName)

{

            FILE *fp;            // File Pointer

             char c;

             fp= fopen(fileName,"a");    // Opening file in appending mode

           

             printf("Enter content which you want to append   :\n\n");

             printf("******************************************\n\n");

             while((c=getchar())!=EOF)

             {

                 putc(c,fp);       // saving Characters in file

             }

             fclose(fp);            // Closing File Pointer

             printf("\n\n*************************************\n\n");

             printf("\n\n   %s  :  Content saved successfully\n",fileName);

}

fseek()

This function repositions the file pointer of a stream. It sets the file pointer associated with a stream to a new position. fseek() returns a 0 (zero) on success, otherwise it returns a non-zero value on error. 

Header File: stdio.h
Syntax: int fseek(FILE *stream, long offset, int whence);
Arguments:

• stream: Stream whose file pointer fseek sets
• offset: tells the amount of bytes to seek.

or

Difference in bytes between whence (a file pointer position) and new position.

• whence: tells from where the seek of ‘offset’ number of bytes is to be done. The available values for whence are:-

Table 3 lists the three SEEK modes for “whence” argument:

SEEK Mode	File Location
SEEK_SET	Seeks from beginning of file
SEEK_CUR	Seeks from current position
SEEK_END	Seeks from end of file

EOF, The End of File Marker

EOF is a character which indicates the end of a file. It is returned by read commands of the getc and scanf families when they try to read beyond the end of a file.

When you are reading from a file it is useful to be able to check whether or not you have reached the end, without just failing to read. The function feof can be used to find out if you have hit the end :

if ( feof (input_file ) ) printf ( "BANG!\n" ) ;

This function returns the value 0 if the end of the file has not been reached, and another value if it has. It takes a FILE pointer as a parameter.

Copy a file to another file

1.    To copy a file to another file, we need two file pointers.

2.    File which we need to copy should be open in read mode (file A), as we don’t need to perform any write operation on that file. Another file we need to open in write mode(file B).

3.    We get character by character from file A and put that in file B.

Example :-

#include<stdio.h>

#include<string.h>

void copyFile(char* oldFileName, char* newFileName);

void main()

{

          char oldFileName[20],newFileName[20];

          printf("Enter the File Name which we need to copy \t:");

          scanf("%s",oldFileName);

          fflush(stdin);                 //  Clear the buffer from memory

          printf("Enter the new File Name \t:");

          scanf("%s",newFileName);

          fflush(stdin);                 //  Clear the buffer from memory

         copyFile(oldFileName,newFileName);          

}

void copyFile(char* oldFileName, char* newFileName)

{

            FILE *oldP,*newP;            // File Pointer

             char c;

             oldP= fopen(oldFileName,"r");    // Opening file in reading mode

             newP= fopen(newFileName,"w");

             if(oldP==NULL)

             {

                 printf("File not found \n");

                       printf("Please check file Name\n");

                       exit();

             }

             while((c=getc(oldP))!=EOF)

             {

                 putc(c,newP);       // Display Characters on screen

             }

             fclose(oldP);            // Closing File Pointer

             fclose(newP);

             printf("\n\n    File copied successfully\n");

}

Function int fstat (char *, struct stat *) store the information of open file in form of structure struct stat Structure struct stat has been defined in sys\stat.h as

struct stat {

    short  st_dev,   st_ino;

    short  st_mode,  st_nlink;

    int    st_uid,   st_gid;

    short  st_rdev;

    long   st_size,  st_atime;

    long   st_mtime, st_ctime;

};

Here

(a)st_dev: It describe file has stored in which drive of your computer  ,it returns a number.

(b)st_mode:  It describes various modes of file like file is read only, write only, folder, character file etc.

(c)st_size: It tells the size of file in byte.

(d)st_ctime:It tells last data of modification of the file in date format.

Note: 65 is ASCII value of A .So after adding status.st_dev with 65 it will return appropriate drvie name as in your computer.

There are some macro has been defined in sys\stat.h

 Name          Meaning

 S_IFMT       File type mask

 S_IFDIR      Directory

 S_IFIFO      FIFO special

 S_IFCHR      Character special

 S_IFBLK      Block special

 S_IFREG      Regular file

 S_IREAD      Owner can read

 S_IWRITE     Owner can write

 S_IEXEC      Owner can execute

So masking or bit wise and operation between status.st_mode and S_IREAD return true you have read permission and so on. 

WRITE A C PROGRAM TO FIND SIZE OF A FILE

#include <time.h>

#include <sys\stat.h>

#include <stdio.h>

void main()

{

    struct stat status;

    FILE *fp;

    fp=fopen("test.txt","r");

    fstat(fileno(fp),&status);

    clrscr();

    printf("Size of file : %d",status.st_size);

    printf("Drive name   : %c",65+status.st_dev);

    getch();

}

Write a c program to find out the last date of modification

#include <time.h>

#include <sys\stat.h>

#include <stdio.h>

void main(){

    struct stat status;

    FILE *fp;

    fp=fopen("test.txt","r");

    fstat(fileno(fp),&status);

    clrscr();

    printf("Last date of modification : %s",ctime(&status.st_ctime));

    getch();

}

Write a c program to know read/write permission of given file

#include "time.h"

#include "sys\stat.h"

#include "stdio.h"

void main(){

    struct stat status;

    FILE *fp;

    stat("test.txt",&status);

    clrscr();

    if (status.st_mode & S_IREAD)

         printf("You have read permission.\n");

    if (status.st_mode & S_IWRITE)

         printf("You have write permission.");

    getch();

}

What is difference between file opening mode r+ and w+ in c?

Both r+ and w+ we can read ,write on file but r+ does not truncate  (delete) the content of file  as well it doesn’t create a new file if such file doesn’t exists while in w+ truncate the content of file as well as create a new file if such file doesn’t exists.