File Input & Output (I/O)

🔵 C Programming Lesson 14 Advanced

C communicates with storage drives via file streams. Files are opened, modified, and closed using standard stream declarations.

1 File pointers and Modes (`FILE*`)

C represents file streams using the `FILE` structure pointer. Common access modes include:

`"r"`: Read mode. Fails if the file does not exist.
`"w"`: Write mode. Overwrites the file contents or creates a new file.
`"a"`: Append mode. Appends new data to the end of the existing file.

2 Writing and Reading Files

Let's run a program that writes data to a text file and then reads it back to display on the screen:

C — File Handling ▶ Run Code

#include <stdio.h>

int main() {
    // Open stream in write mode
    FILE *file = fopen("example.txt", "w");

    if (file == NULL) {
        printf("Error opening file!\n");
        return 1;
    }

    // Write text to file stream
    fprintf(file, "Learning C File Operations!\n");
    fclose(file); // Always close the stream

    // Open stream in read mode
    file = fopen("example.txt", "r");
    if (file == NULL) {
        printf("Error opening file!\n");
        return 1;
    }

    char buffer[100];
    // Read formatted lines from stream
    while (fgets(buffer, sizeof(buffer), file) != NULL) {
        printf("File Content: %s", buffer);
    }
    
    fclose(file);
    return 0;
}

3 Code Challenge

Challenge: Write a program that writes three lines containing the numbers `100`, `200`, and `300` to a text file. Open the file in read mode, parse the integers, compute their sum, and print the final sum to the console.