C fopen() function with Examples

Pre-requisite: Basics of File Handling in C

The fopen() method in C is a library function that is used to open a file to perform various operations which include reading, writing etc. along with various modes. If the file exists then the particular file is opened else a new file is created.

Syntax:

FILE *fopen(const char *file_name, const char *mode_of_operation);

Parameters: The method accepts two parameters of character type:

file_name: This is of C string type and accepts the name of the file that is needed to be opened.
mode_of_operation: This is also of C string type and refers to the mode of the file access. Below are the file access modes for C:
1. “r” – Searches file. If the file is opened successfully fopen() loads it into memory and sets up a pointer which points to the first character in it. If the file cannot be opened fopen() returns NULL.
2. “w” – Searches file. If the file exists, its contents are overwritten. If the file doesn’t exist, a new file is created. Returns NULL, if unable to open the file.
3. “a” – Searches file. If the file is opened successfully fopen() loads it into memory and sets up a pointer that points to the last character in it. If the file doesn’t exist, a new file is created. Returns NULL, if unable to open the file.
4. “r+” – Searches file. If opened successfully, fopen() loads it into memory and sets up a pointer which points to the first character in it. Returns NULL, if unable to open the file.
5. “w+” – Searches file. If the file exists, its contents are overwritten. If the file doesn’t exist a new file is created. Returns NULL, if unable to open the file.
6. “a+” – Searches file. If the file is opened successfully fopen( ) loads it into memory and sets up a pointer which points to the last character in it. If the file doesn’t exist, a new file is created. Returns NULL, if unable to open the file.

Return Value: The function is used to return a pointer to FILE if the execution succeeds else NULL is returned.

Example 1:

// C program to illustrate fopen() 
  
#include <stdio.h> 
#include <stdlib.h> 
  
int main() 
{ 
  
    // pointer demo to FILE 
    FILE* demo; 
  
    // Creates a file "demo_file" 
    // with file acccess as write-plus mode 
    demo = fopen("demo_file.txt", "w+"); 
  
    // adds content to the file 
    fprintf(demo, "%s %s %s", "Welcome", 
            "to", "GeeksforGeeks"); 
  
    // closes the file pointed by demo 
    fclose(demo); 
  
    return 0; 
} 

On running the following command, a new file will be created by the name “demo_file” and with the following content:

Welcome to GeeksforGeeks

Example 2: Now if we wish to look into the file then we need to run the following code, which will open the file and display its content.

// C program to illustrate fopen() 
  
#include <stdio.h> 
  
int main() 
{ 
  
    // pointer demo to FILE 
    FILE* demo; 
    int display; 
  
    // Creates a file "demo_file" 
    // with file acccess as read mode 
    demo = fopen("demo_file.txt", "r"); 
  
    // loop to extract every characters 
    while (1) { 
        // reading file 
        display = fgetc(demo); 
  
        // end of file indicator 
        if (feof(demo)) 
            break; 
  
        // displaying every characters 
        printf("%c", display); 
    } 
  
    // closes the file pointed by demo 
    fclose(demo); 
  
    return 0; 
} 

Output:

Welcome to GeeksforGeeks

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