Introduction
- A collection of data which is stored on a secondary device like a hard disk is known as a file.
- A file is generally used as real-life applications that contain a large amount of data.
There are two problems with such applications:
I) It is time-consuming and unmanageable for handling such huge amount of data,
II) When the I/O terminal is used the entire data is lost if the program is terminated or the computer is being turned off. So it is a compulsion for storing the data on a permanent device.
Files are divided into two types
1. Stream-oriented – they are standard or high-level files. They are easier to work with than the sytem-oriented data-files and are used more commonly.
2. System-oriented – they are low-level files.
The library functions which are used for operating the files are:
1. High-level file I/O functions – they do their own buffer management.
2. Low-level file I/O functions – the buffer management is done by the programmer.
Let us go through the file operations in detail one by one.
Opening a file
Before opening any file, a file pointer needs to be established.
Syntax : Establishing a file pointer
FILE *fptr;
Where,
FILE is the structure which is defined in the header file <stdio.h>.
- A file should be opened before any operation is being performed on it.
- The fopen() function is being used for opening the file.
Syntax:
FILE *fopen(const char *filename, const char *mode);
In the above syntax, filename is the literal which is used for naming the files.
They can be accessed by using the following modes:
| Mode | Description |
|---|
| “r” | It opens an existing file for reading only. |
| “w” | It opens a new file for writing. If the filename does not exist it will be created and if the file already exists then its contents are deleted. |
| “a” | It appends the existing file. If the filename does not exist it will be created. |
| “r+” | It opens an existing file for reading and writing. It indicates that the file is to be read before writing. |
| “w+” | It opens a new file for reading and writing. If a file with the current filename exists then it is destroyed and a new file name is created. |
| “a+” | It opens an existing file for reading and appending. Its stream is positioned at the end of the file content. |
Closing a file
- The fclose() function is used for closing a file.
- When this function is used the file pointer is disconnected from a file.
Syntax:
int fclose(FILE *fp);
Where,
fp is the file pointer that points to the file that has to be closed.
- An integer value is returned which will indicate if the function was successful or not.
- In addition to the fclose() function we even have the fcloseall() function which will close all the streams which are open currently except the standard streams (stdin, stdout and stderr).
Syntax:
intfcloseall(void);
- This function will flush any of the stream buffers and will return the number of streams which are closed.
Reading a file
Following are the list of functions which are used for reading a file:
| Functions | Syntax | Description |
|---|
| fscanf( ) | int fscanf (FILE *stream, const char *format,....); | It is used for reading the formatted data from the stream. |
| fgets( ) | char *fgets(char *str, int size, FILE *stream); | It stands for file get string. It is used for getting the string from a stream. |
| fgetc( ) | int fgetc (FILE *stream); | It will return the next character from the stream from the end of the file or an error. |
| fread( ) | int fread(void *str, size_t size, size_t num, FILE *stream); | It is used for reading data from a file. |
Writing a file
Following are the list of functions which are used for writing a file:
| Functions | Syntax | Description |
|---|
| fprintf() | int fprintf (FILE *stream, const char * format,...); | It is used for writing the formatted output of the stream. |
| fputs() | int fputs(const char *str, FILE *stream); | It is used for writing a line to a file. |
| fputc() | int fputc(int c, FILE *stream); | It is opposite to fgetc() and is used for writing a character to the stream. |
| fwrite() | int fwrite(const void *str, size_t size, size_t count, file *stream); | It is used for writing data to a file. |
Error handling in file operations
The function ferror() is used for checking the errors in the stream.
Syntax:
int ferror(FILE *stream);
This function returns 0 if there are no errors and a value if there are some errors.
Following are the functions which are used for detecting the errors:
| Functions | Syntax | Description |
|---|
| clearerr() | void clearerr(FILE *stream); | It is used for clearing the end-of-file and error indicators for the stream. |
| perror() | void perror(char *msg); | It stands for the print error. |
Accepting the command line arguments
The main() can accept two arguments
I) First argument will be an integer value that will specify the number of command-line arguments.
II) The second argument is a full list of all the command-line arguments.
Syntax:
int main (int arg c, char *argv[])
Where,
arg c will specify the number of arguments that are to be passed into the program from the command-line including the name of the program.
- argv will contain the list of all the arguments.
- Each element of the array argv is a pointer where each pointer points to a string.
- In main() the command line arguments are accepted by using the argc and argv.
Example
Write a program for reading a file character by character and display it on the screen.
#include <stdio.h>
#include <string.h>
void main()
{
FILE *fp;
int c;
char fnm[25];
printf("\n Enter a filename:");
scanf("%s",fnm);
fp = fopen(fnm, "r");
if (fp==NULL)
{
printf("\n Error in opening the file");
exit(1);
}
c=fgetc(fp);
while(c!=EOF)
{
putchar(c);
c =fgetc(fp);
}
fclose(fp);
}
Assuming that we are using a text file
hello.txt, that has the following content:
Welcome to TutorialRide
Now, the output is of the above problem will be:
Welcome to TutorialRide
Functions used for selecting a record randomly
Following are the functions which are used for selecting a record randomly:
| Functions | Syntax | Description |
|---|
| fseek() | int fseek(FILE *stream, long offset, int origin); | It is used to reposition a binary stream. |
| ftell() | long ftell (FILE *stream); | It is used to know the current position of the file pointer. |
| rewind() | void rewind (FILE *f); | It is used for adjusting the position of the file pointer to the next I/O operation and it will take place at the beginning of the file. |
| fgetpos() | int fgetpos (FILE *stream, fpos_t *pos); | It is used for determining the current position of the stream. |
remove()
The remove() function is used for erasing a file.
Syntax:
int remove (const char *filename);
- All the files which are specified by the filename will be erased.
- If this function is successful it will return a zero else it will return a value other than zero.
Example : Removing a file
#include <stdio.h>
void main()
{
remove(“hello.txt”);
}
Renaming the file
The rename() function is used for renaming a file.
Syntax:
int rename(const char *oldname, const char *newname);
Where,
oldname will be the pathname of the file that needs to be renamed.
newname will be the new pathname of the file.
- If this function is successful it will return a zero else it will return a value other than zero.
- If an error occurs neither the oldfile name nor the newfile name are renamed or changed.
Example : Renaming a file
#include <stdio.h>
void main()
{
int successful=0;
successful = rename("hello.txt", "hey.txt");
if (successful !=0)
printf("\nThe file is not renamed");
}
