Unit-VI discusses files in C programming. A file is a collection of related records stored permanently on secondary storage devices like hard disks. There are several file operations in C - opening a file using fopen(), reading the file using fgetc(), and closing it using fclose(). Different text modes like w, r, a, w+, a+ are used for opening files for write, read, append, write and read, append and read operations respectively. Programs are provided to demonstrate opening, writing, reading and closing files in various modes.

1. Unit-VI
Files

2. Introduction of a File
 A file is nothing but collection of records.
 Record is group of related data items.
 All data items related to students, employees, customers etc is
nothing but records.
 In other words, file is a collection of numbers, symbols & text
placed onto the secondary devices like hard disk, pen drive,
compact disk etc.
 Files are stored permanently on to the disk and one can access
them for further monitoring/ processing if needed.

3. Introduction of a File(cont..)
 DEFINITION OF FILE:
A file can be considered as a stream of characters. A file can be a set
of records that can be accessed through the set of library functions.
These functions are available in stdio.h.

4. Introduction of a File(cont..)
 All data stored on the disk is in binary form.
 How this binary data is stored on the disk varies from one OS to
another.
 However, this does not affect the C programmer since she/he has to
use only the library functions written for the particular OS to be able
to perform input/output.

5. File Operations
i. Opening a File
ii. Reading a File
iii. Closing a File

6. Opening of File
 A file has to be opened before read and write operations.
 Opening of a file creates a link between the operating system
and the file functions.
 The name of the file and its mode of operation are to be
indicated to the operating system.
 This important task is carried out by the structure FILE that is
defined in stdio.h header file.
 So this file must be included.

7. Opening of File (cont..)
 When a request is made to the operating system for opening a
file, it does so by granting the request.
 If request is granted then the operating systems points to the
structure FILE.
 The following declaration before opening of the file is to be
made:
FILE *fp
where fp is the file pointer.

8. Opening of File (cont..)
 Each file that we open has its own FILE structure.
 The information that is there in the file may be its current size, and its
location in memory.
 The only one function to open a file is fopen().
 Syntax:
FILE *fp;
fp=fopen(“data.txt”,”r”);
 Here fp is a pointer variable that contains the address of the structure FILE
that has been defined in the header file stdio.h.

9. Opening of File (cont..)
 It is necessary to write FILE in the uppercase.
 The function fopen() will open a file ‘data.txt’ in read mode.
 The C compiler reads the contents of the file because it finds the
read mode (“r”).
 Here, “r” is a string and not a character. Hence, it is enclosed with
double quotes and not with single quotes.

10. fopen() performs the following important tasks:
 1. It searches the disk for opening of the file.
 2. In case the file exists, it loads the file from the disk into
memory. If the file is found with huge contents then it loads the
file part by part.
 3. If the file is not existing this function returns a NULL. NULL is
a macro defined in the header file ‘stdio.h‘. This indicates that it
is unable to open a file. There may be the following reasons for
failure of fopen() functions: (A) when the file is in protected or
hidden mode and (B) the file may be in use by another program.

11. fopen() performs the following important tasks: (cont..)
 4. It locates a character pointer, which points the first
character of the file. Whenever a file is opened the character
pointer points to the first character of the file.

12. Reading a File
 Once the file is opened using fopen(), its contents are loaded into
the memory (partly or wholly).
 The pointer points to the very first character of the file.
 The fgetc() function is used to read the contents of the file.
 The syntax for fgetc() is as follows:
ch=fgetc(fp);
 where fgetc() reads the character from current pointer position
and advances the pointer position so that the next character is
pointed.

13. Closing a File
 The file that is opened from the fopen() should be closed after
the work is over, i.e. we need to close the file after reading and
writing operations.
 In other words, the file must be closed after operations.
 Also, whenever an opened file needs to be reopened in other
mode in such a case also the opened file must be closed first.
 Closing the file enables to wash out all its contents from the RAM
buffer and further the link is disconnected from the file.

14. Closing a File (cont..)
Example:
The function to close a file is
fclose(fp);
This statement closes the file associated with file pointer fp. This
function closes one file at a time.
In order to close all files function syntax used is as follows:
fcloseall();
This function closes all the opened files and returns the number of
files closed. It does not require any argument.

15. Text Modes
1. w(write)
 This mode opens a new file on the disk for writing.
 If the file already exists, its contents will be over-written without
confirmation.
 If the concerned file is not found, a new file is created.
 Syntax:
fp=fopen (“data.txt”,“w”);
 Here data.txt is the file name and “w” is the mode.

16. Text Modes (cont..)
2. r(read)
 This mode searches a file and if it is found the same is loaded into the memory
for reading from the first character of the file.
 The file pointer points to the first character and reading operation begins.
 If the file does not exist then compiler returns NULL to the file pointer.
 Using pointer with the if statement we can prompt the user regarding failure
of operation.
 The syntax of read mode is as follows:
 Syntax:
fp=fopen(“data.txt”,“r”);
if(fp==NULL)
printf(“File does not exist”);

17. Text Modes (cont..)
 3. a(append)
 This mode opens a pre-existing file for appending data.
 The data appending process starts at the end of the opened file.
 The file pointer points to the last character of the file.
 If the file does not exist, then a new file is opened, i.e. if the file
does not exist then the mode of “a” is the same as “w”.
 Due to some or other reasons if file is not opened in such a case
NULL is returned.
 File opening may be impossible due to insufficient space on to
the disk and some other reasons.

18. Text Modes (cont..)
3. a(append)(cont…)
 Syntax:
fp=fopen(“data.txt”,“a”);
 Here, if data.txt file already exists, it will be opened. Otherwise a
new file will be opened with the same name.

19. Text Modes (cont..)
4. w+ (Write + read)
 This mode starts for file search operation on the disk.
 In case the file is found, its contents are destroyed.
 If the file is not found, a new file is created. It returns NULL if it fails to open
the file.
 In this file mode, new contents can be written and thereafter reading
operation can be done.
 Example:
fp=fopen(“data.txt”,“w+”);
 In the above example, data.txt file is open for reading and writing operations.

20. Text Modes (cont..)
5. a+ (append + read)
 In this file operation mode the contents of the file can be read
and records can be added at the end of file.
 A new file is created in case the concerned file does not exist.
 Due to some or the other reasons if a file is unable to open
then NULL is returned.
 Example:
fp=fopen(“data.txt”,“a+”);
 Here data.txt is opened and records are added at the end of
file without affecting the previous contents.

21. Text Modes (cont..)
6. r+ (read + write)
 This mode is used for both reading and writing.
 We can read and write the record in the file.
 If the file does not exist, the compiler returns NULL to the file
pointer. It can be written as follows:
 Example:
fp=fopen(“data.dat”,“r+”);
if(fp==NULL)
printf(“File not found”);

22. Write a program to write data to text file and read it.
# include <stdio.h>
# include <conio.h>
void main()
{ FILE *fp;
char c=‘ ’;
clrscr();
fp=fopen(“data.txt”,“w”);
if(fp==NULL)
{
printf(“Can not open file”);
exit(1);
}
printf(“Write data & to stop press ‘.’ :”);
while(c!=‘.’)
{ c=getche();
fputc(c,fp);
}
fclose(fp);
printf(“n Contents read :”);
fp=fopen(“data.txt”,”r”);
while(!feof(fp))
printf(“%c”,getc(fp));
getch();
}

23. Cont…
 In the above program, the file named “data.txt” is opened in
write mode.
 The characters are read from the keyboard and stored in
variable ‘c’.
 Using fputc() the characters are written to a file until ‘.’ (dot) is
pressed.
 The same file is closed and then it is reopened in read mode.
 On reopening of the file, character pointer sets to the beginning
of the file.
 The contents of the file will be displayed on the screen using
getc().

24. Write a program to open a pre-existing file and add information at the
end of file. Display the contents of the file before and after appending.
# include <stdio.h>
# include <conio.h>
void main()
{
FILE *fp;
char c;
clrscr();
printf(“Contents of file before appending :n”);
fp=fopen(“data.txt”,“r”);
while(!feof(fp))
{
c=fgetc(fp);
printf(“%c”,c);
}
fp=fopen(“data.txt”,“a”);
if(fp==NULL)
{ printf(“File can not appended”);
exit(1); }
printf(“n Enter string to append :”);
while(c!=‘.’)
{ c=getche();
fputc(c,fp); }

25. Cont…
fclose(fp);
printf(“n Contents of file After appending :n”);
fp=fopen(“data.txt”,“r”);
while(!feof(fp))
{
c=fgetc(fp);
printf(“%c”,c);
}
getch();
}

26. Write a program to use w+ mode for writing and reading of a
file.
# include <stdio.h>
# include <conio.h>
void main()
{
FILE *fp;
char c=’ ‘;
clrscr();
fp=fopen(“data.txt”,“w+”);
if(fp==NULL)
{
printf(“Can not open file”);
exit(1);
}
printf(“Write data & to stop press ‘.’ :”);
while(c!=‘.’)
{
c=getche();
fputc(c,fp);
}
rewind(fp);
printf(“n Contents read :”);
while(!feof(fp))
printf(“%c”,getc(fp));
getch();
}

27. Cont..
 Instead of using separate read and write modes, we can use w+
mode to perform both the operations.
 At first writing operation is done. It is not essential to close the
file for reading the contents of it.
 We need to set the character pointer at the beginning.
 Hence, rewind() function is used. The advantage of using w+ is to
reduce the number of statements.

28. Write a program to open a file in append mode and add new
records in it.
# include <stdio.h>
# include <conio.h>
void main()
{ FILE *fp;
char c=‘ ’;
clrscr();
fp=fopen(“data.txt”,“a+”);
if(fp==NULL)
{
printf(“Can not open file”);
exit(1);
}
printf(“Write data & to stop press ‘.’ :”);
while(c!=‘.’)
{
c=getche();
fputc(c,fp);
}
printf(“n Contents read :”);
rewind(fp);
while(!feof(fp))
printf(“%c”,getc(fp));
getch();
}

29. Cont…
 In the above program, a file named “data.txt” is opened in read
and append mode (a+).
 If a file does not exist, a new file is created.
 Write operation is performed first and the contents are read
thereafter.
 Before reading character pointer is set at the beginning of file
using rewind().

30. Write a program to open a file in read/write mode in it. Read and write
new information in the file.
# include <stdio.h>
# include <conio.h>
void main()
{
FILE *fp;
char c=‘ ’;
clrscr();
fp=fopen(“data.txt”,“r+”);
if(fp==NULL)
{
printf(“Can not open file”);
exit(1);
}
printf(“n Contents read :”);
while(!feof(fp))
printf(“%c”,getc(fp));
printf(“Write data & to stop press ‘.’ :”);
while(c!=‘.’)
{
c=getche();
fputc(c,fp);
}
getch(); }

31. Cont..
 In the above example, file is opened in read and write mode (r+).
 The getc() function reads the contents of file which is printed
through printf() function.
 The getche() function reads characters from the keyboard and
the read characters are written to the file using fputc() function.

32. Binary Modes
 A text file contains only textual information like alphabets, digits and
special symbols. In actuality the ASCII codes of these characters are
stored in text files.
 As against this, a binary file is merely a collection of bytes
 When numerical data is to be transferred to disk from RAM, the data
occupies more memory space on disk.
 For example, a number 234567 needs 3 bytes memory space in RAM
and when transferred to disk requires 6 bytes memory space.
 For each numerical digit one byte space is needed.
 Hence, total requirement for the number 234567 would be 6 bytes.

33. Binary Modes (cont..)
 Thus, text mode is inefficient for storing large amount of
numerical data because space occupation by it is large.
 Only solution to this inefficient memory use is to open a file in
binary mode, which takes lesser space than text mode.