How to Read Files in C: A Guide to File Operations

Learn how to read files in C programming, building on our previous chapter about writing to files using w and a modes with the fopen() function. This chapter will focus on the r mode, enabling you to effectively retrieve data from files and utilize it in your programs.

Read Files in C

Reading a File

In the previous chapter, we learned how to write to a file using the w and a modes in the fopen() function. Now, we will explore how to read from a file using the r mode.

Example


#include 

int main() {
    FILE *fptr;

    // Open a file in read mode
    fptr = fopen("example.txt", "r");
    return 0;
}

This code opens example.txt for reading. Now, let's proceed step-by-step to read the content of this file.

Creating a String to Store Content

We need to create a string that is large enough to hold the content of the file. For instance, let's make a string that can store up to 100 characters:

Example


#include 

int main() {
    FILE *fptr;

    // Open a file in read mode
    fptr = fopen("example.txt", "r");

    // Store the content of the file
    char myString[100];
    return 0;
}

Reading the File Content

To read the content of example.txt, we can use the fgets() function, which reads a line of text from the file:

Example


#include 

int main() {
    FILE *fptr;

    // Open a file in read mode
    fptr = fopen("example.txt", "r");

    // Store the content of the file
    char myString[100];

    // Read the content and store it inside myString
    fgets(myString, 100, fptr);

    // Print the file content
    printf("%s", myString);

    // Close the file
    fclose(fptr);
    return 0;
}

Output

If example.txt contains "Hello Folks!", the output will be:


Hello Folks!

Reading All Lines in a File

To read every line in the file, you can use a while loop:

Example


#include 

int main() {
    FILE *fptr;

    // Open a file in read mode
    fptr = fopen("example.txt", "r");

    // Store the content of the file
    char myString[100];

    // Read the content and print it
    while(fgets(myString, 100, fptr)) {
        printf("%s", myString);
    }

    // Close the file
    fclose(fptr);
    return 0;
}

Output

If example.txt contains:


Hello Folks!
How are you?

Good Practice

If you try to open a file for reading that does not exist, the fopen() function will return NULL. To handle this gracefully, you can use an if statement to check for NULL and print a message if the file does not exist:

Example


#include 

int main() {
    FILE *fptr;

    // Open a file in read mode
    fptr = fopen("nonexistent.txt", "r");

    // Print some text if the file does not exist
    if (fptr == NULL) {
        printf("Not able to open the file.");
    }

    // Close the file
    fclose(fptr);
    return 0;
}

Output

If the file does not exist, the output will be:


Not able to open the file.

Combining Everything into One Example

We can combine the file reading logic with error handling as follows:

Example


#include 

int main() {
    FILE *fptr;

    // Open a file in read mode
    fptr = fopen("example.txt", "r");

    // Store the content of the file
    char myString[100];

    // If the file exists
    if (fptr != NULL) {
        // Read the content and print it
        while (fgets(myString, 100, fptr)) {
            printf("%s", myString);
        }
    // If the file does not exist
    } else {
        printf("Not able to open the file.");
    }

    // Close the file
    fclose(fptr);
    return 0;
}

Output

If example.txt contains:


Hello Folks!
How are you?