File I/O
All our programs so far have had a temporary existence. When the program ends, any data it was holding in variables, arrays, or structs is lost forever.
To make our programs truly useful, we need PERSISTENCE–the ability to save data to a storage device (like a hard drive) and load it back later. This is achieved through FILE I/O (Input/Output).
The Core Concepts of File I/O
-
The
FILEPointer: When you work with a file in C, you use a special pointer type calledFILE *. This is a struct defined in<stdio.h>that holds all the information C needs to manage the connection to a file (its location, your current position in it, etc.). You never need to know what’s inside theFILEstruct; you just use a pointer to it. -
fopen()- Opening a File: This function establishes a connection, or “stream,” to a file on your disk. It takes two arguments: the filename and a “mode” string.- “w”: WRITE mode. Creates a new file. If the file already exists, its contents are completely ERASED and overwritten.
- “r”: READ mode. Opens an existing file for reading. If the file doesn’t
exist,
fopen()will fail. - “a”: APPEND mode. Opens a file to add new data to the END of it. If the
file doesn’t exist, it will be created.
fopen()returns aFILE *on success orNULLon failure. YOU MUST ALWAYS CHECK FORNULL!
-
fclose()- Closing a File: This is the essential cleanup step. It finalizes any pending writes to the disk and tells the operating system you are done with the file, releasing resources. You MUST close every file you open. -
Writing and Reading Functions:
fprintf(): Works just likeprintf(), but its first argument is aFILE *. It writes formatted text TO a file.fgets(): Reads a line of text FROM a file. It’s safer than other functions because you specify the maximum number of characters to read, preventing buffer overflows.
Full Source
/**
* @file 14_file_io.c
* @brief Part 2, Lesson 14: File I/O
* @author dunamismax
* @date 06-15-2025
*
* This file serves as the lesson and demonstration for File I/O (Input/Output).
* It explains how to write data to files and read data from them, allowing
* programs to save and load information permanently.
*/
/*
* =====================================================================================
* | - LESSON START - |
* =====================================================================================
*
* All our programs so far have had a temporary existence. When the program ends,
* any data it was holding in variables, arrays, or structs is lost forever.
*
* To make our programs truly useful, we need PERSISTENCE--the ability to save
* data to a storage device (like a hard drive) and load it back later. This is
* achieved through FILE I/O (Input/Output).
*
* --- The Core Concepts of File I/O ---
*
* 1. The `FILE` Pointer:
* When you work with a file in C, you use a special pointer type called `FILE *`.
* This is a struct defined in `<stdio.h>` that holds all the information C
* needs to manage the connection to a file (its location, your current
* position in it, etc.). You never need to know what's inside the `FILE`
* struct; you just use a pointer to it.
*
* 2. `fopen()` - Opening a File:
* This function establishes a connection, or "stream," to a file on your disk.
* It takes two arguments: the filename and a "mode" string.
* - "w": WRITE mode. Creates a new file. If the file already exists, its
* contents are completely ERASED and overwritten.
* - "r": READ mode. Opens an existing file for reading. If the file doesn't
* exist, `fopen()` will fail.
* - "a": APPEND mode. Opens a file to add new data to the END of it. If the
* file doesn't exist, it will be created.
* `fopen()` returns a `FILE *` on success or `NULL` on failure. YOU MUST
* ALWAYS CHECK FOR `NULL`!
*
* 3. `fclose()` - Closing a File:
* This is the essential cleanup step. It finalizes any pending writes to the
* disk and tells the operating system you are done with the file, releasing
* resources. You MUST close every file you open.
*
* 4. Writing and Reading Functions:
* - `fprintf()`: Works just like `printf()`, but its first argument is a
* `FILE *`. It writes formatted text TO a file.
* - `fgets()`: Reads a line of text FROM a file. It's safer than other
* functions because you specify the maximum number of characters to read,
* preventing buffer overflows.
*/
#include <stdio.h>
#include <stdlib.h> // For exit()
int main(void)
{
// A FILE pointer, always initialize to NULL.
FILE *file_pointer = NULL;
char *filename = "my_first_file.txt"; // The name of the file we will work with.
// --- DEMONSTRATION 1: Writing to a File ---
printf("--- Part 1: Writing to a file ---\n");
// Step 1: Open the file in WRITE ("w") mode.
file_pointer = fopen(filename, "w");
// Step 2: ALWAYS check if the file opened successfully.
if (file_pointer == NULL)
{
// stderr is the "standard error" stream, a good place for error messages.
fprintf(stderr, "Error: Could not open file '%s' for writing.\n", filename);
exit(1); // exit() terminates the program immediately.
}
// Step 3: Write to the file using fprintf().
printf("Writing data to %s...\n", filename);
fprintf(file_pointer, "Hello, File!\n");
fprintf(file_pointer, "This is the second line.\n");
fprintf(file_pointer, "Player: %s, Score: %d\n", "Max", 100);
// Step 4: Close the file.
fclose(file_pointer);
file_pointer = NULL; // Good practice to nullify the pointer after closing.
printf("Successfully wrote to and closed the file.\n\n");
// --- DEMONSTRATION 2: Reading from a File ---
printf("--- Part 2: Reading from a file ---\n");
// This buffer will hold the lines we read from the file.
char line_buffer[256];
// Step 1: Open the same file, but now in READ ("r") mode.
file_pointer = fopen(filename, "r");
// Step 2: Check for errors again. The file might have been deleted, or
// we may not have permission to read it.
if (file_pointer == NULL)
{
fprintf(stderr, "Error: Could not open file '%s' for reading.\n", filename);
exit(1);
}
// Step 3: Read from the file line-by-line using fgets().
// `fgets()` reads until it hits a newline, the end of the file, or the
// buffer size limit is reached. It returns NULL when there is nothing left to read.
printf("Reading data from %s:\n", filename);
while (fgets(line_buffer, sizeof(line_buffer), file_pointer) != NULL)
{
// Print the line we just read from the file to the console.
printf("%s", line_buffer);
}
// Step 4: Close the file.
fclose(file_pointer);
printf("\nSuccessfully read and closed the file.\n");
return 0;
}
/*
* =====================================================================================
* | - LESSON END - |
* =====================================================================================
*
* Key Takeaways:
*
* 1. File I/O allows your program's data to be PERSISTENT.
* 2. The `FILE *` is your handle to an open file.
* 3. The basic workflow is always: OPEN -> VALIDATE -> READ/WRITE -> CLOSE.
* 4. `fopen()` opens a file with a specific mode (`"w"`, `"r"`, `"a"`). You MUST
* check its return value for `NULL` to handle errors.
* 5. `fprintf()` is like `printf()` but writes to a file stream.
* 6. `fgets()` is a safe and reliable way to read text line-by-line from a file.
* 7. `fclose()` is mandatory to save all changes and release system resources.
*
* HOW TO COMPILE AND RUN THIS CODE:
*
* 1. Open a terminal or command prompt.
* 2. Navigate to the directory where you saved this file.
* 3. Use the GCC compiler to create an executable file:
* `gcc -Wall -Wextra -std=c11 -o 14_file_io 14_file_io.c`
* 4. Run the executable:
* - On Linux/macOS: `./14_file_io`
* - On Windows: `14_file_io.exe`
*
* After running, check the directory! You will find a new file named
* `my_first_file.txt` containing the text from the program.
*/
How to Compile and Run
cc -Wall -Wextra -std=c11 -o 14_file_io 14_file_io.c
./14_file_io