Different Operations on Files in C Language

A file is a collection of data stored in secondary memory. The data is entered directly from the keyboard, files are used to store both information and programs. The following operations can be performed on files in the C language ?

• Naming the file.
• Opening the file.
• Reading from the file.
• Writing into the file.
• Closing the file.

File Naming and Opening

The syntax for opening and naming file is as follows ?

FILE *File pointer;

For example, FILE * fptr; opens a file in a specified mode and returns a file pointer.

File pointer = fopen ("File name", "mode");

For example, fptr = fopen ("sample.txt", "r") opens a file named "sample.txt" in read mode and assigns the file pointer to fp.

FILE *fp;
fp = fopen ("sample.txt", "w");

File Opening Modes

The modes of opening the file in C language are explained below ?

Opening a File in Writing Mode

Writes a specified number of objects from an array to an output stream, this produces the file position indicator.

The syntax is as follows ?

FILE *fp;
fp =fopen ("sample.txt", "w");

If the file does not exist, a new file is created. If the file exists, the old content is erased and the new content is stored.

Opening a File in a Read Mode

Reads a specified number of objects into an array from an input stream, that determines the file position indicator.

The syntax is as follows ?

FILE *fp
fp =fopen ("sample.txt", "r");

If the file does not exist, the fopen function returns a NULL value. If the file exists, data is read successfully from the file.

Opening a File in Append Mode

Append mode is used to add content to the existing file. When we open a file in Append mode, then the cursor is placed at the end of the current data in the file.

The syntax is as follows ?

FILE *fp;
fp =fopen ("sample.txt", "a");

If the file does not exist, a new file will be created. If the file exists, the new content will be added to the old content.

Closing the File

Closes the given file and redirects the data to the OS and deletes the unread data. Using the pointer after fclose returns an undefined behavior.

The syntax is as follows ?

int fclose(FILE *stream);

Example

Here is a C program for file operations. This program copies the content of one file to another. It opens the source file for reading and writing the files.

#include <stdio.h>
#include <stdlib.h> 
int main(){
    FILE *fptr1, *fptr2;
    char filename[100], c;
    printf("Enter the filename to open for reading 
");
    scanf("%s",filename);
    // Open one file for reading
    fptr1 = fopen(filename, "r");
    if (fptr1 == NULL){
        printf("Cannot open file %s 
", filename);
        exit(0);
    }
    printf("Enter the filename to open for writing 
");
    scanf("%s", filename);
    // Open another file for writing
    fptr2 = fopen(filename, "w");
    if (fptr2 == NULL){
        printf("Cannot open file %s 
", filename);
        exit(0);
    }
    // Read contents from file
    c = fgetc(fptr1);
    while (c != EOF){
        fputc(c, fptr2);
        c = fgetc(fptr1);
    }
    printf("
Contents copied to %s", filename);
    fclose(fptr1);
    fclose(fptr2);
    return 0;
}

Output

When the above program is executed, it produces the following result ?

Enter the filename to open for reading
file2.txt
Enter the filename to open for writing
file1.txt
Contents copied to file1.txt