C fopen() function with Examples

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 fopen() function opens the particular file else a new file is created.

Syntax

The syntax of C fopen() is:

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

Parameters

The method accepts two parameters of character pointer 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.

The below table lists valid mode_of_operation values in C with their meaning:

Opening Modes	Description
r	Searches file. Opens the file for reading only. If the file is opened successfully fopen() loads it into memory and sets up a pointer that points to the first character in it. If the file cannot be opened fopen() returns NULL.
w	Searches file. If the file exists already, its contents are overwritten. If the file doesn’t exist, a new file is created. Returns NULL, if unable to open the file. It creates a new file for writing only(no reading).
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. The file is opened only for appending(writing at the end of the file).
r+	Searches file. Opens the file for both reading and writing. If opened successfully, fopen() loads it into memory and sets up a pointer that points to the first character in it. Returns NULL, if unable to open the file.
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. The difference between w and w+ is that we can also read the file created using w+.
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. The file is opened for reading and appending(writing at the end of the file).
rb	Open the binary file in read mode. If the file does not exist, the open() function returns NULL.
wb	Open the binary file in write mode. As the pointer is set to the start of the file, the contents are overwritten. If the file does not exist, a new file is created.
ab	Open the binary file in append mode. The file pointer is set after the last character in the file. A new file is created if no file exists with the name.
rb+	Open the binary file in read and write mode. If the file does not exist, the open() function returns NULL.
wb+	Open the binary file in read and write mode. Contents are overwritten if the file exists. It will be created if the file does not exist.
ab+	Open the binary file in read and append mode. A file will be created if the file does not exist.

Return Value

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

Example of fopen()

C

// 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 access 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.txt” with the following content: 

Welcome to GeeksforGeeks

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

// 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 access 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

More articles on File Handling in C:

1. Basics of File Handling in C
2. fopen() for an existing file in write mode
3. EOF, getc() and feof() in C
4. File opening modes(r versus r+)