> ## Documentation Index
> Fetch the complete documentation index at: https://learn.acadlink.app/llms.txt
> Use this file to discover all available pages before exploring further.

# File Handling in C

> Introduction to file handling — opening, reading, writing, and closing files

# File Handling in C

**File handling** allows you to store and access data permanently on disk. Unlike variables (which lose their values when the program ends), a file retains its contents even after the program closes.

***

## Steps in File Handling

1. Declare a file pointer
2. Open the file
3. Perform read/write operations
4. Close the file

```c theme={null}
FILE *fp;                         // Step 1: declare file pointer
fp = fopen("file.txt", "r");      // Step 2: open file
// Step 3: read/write operations
fclose(fp);                       // Step 4: close file
```

***

## File Opening Modes

| Mode   | Description                                      |
| ------ | ------------------------------------------------ |
| `"r"`  | Read — file must already exist                   |
| `"w"`  | Write — creates new file, or overwrites existing |
| `"a"`  | Append — adds data at the end of the file        |
| `"r+"` | Read + Write — file must exist                   |
| `"w+"` | Read + Write — creates new or clears existing    |
| `"a+"` | Read + Append — read anywhere, write at end only |

***

## Common File Functions

| Function    | Purpose                         |
| ----------- | ------------------------------- |
| `fopen()`   | Open a file                     |
| `fclose()`  | Close a file                    |
| `fprintf()` | Write formatted data            |
| `fscanf()`  | Read formatted data             |
| `fputc()`   | Write a single character        |
| `fgetc()`   | Read a single character         |
| `fputs()`   | Write a string                  |
| `fgets()`   | Read a string (line by line)    |
| `feof()`    | Check if end of file is reached |

***

## Writing to a File

```c theme={null}
FILE *fp = fopen("data.txt", "w");

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

fprintf(fp, "Hello World!\n");
fputs("C Programming\n", fp);
fputc('A', fp);

fclose(fp);
```

***

## Reading from a File

```c theme={null}
FILE *fp = fopen("data.txt", "r");

if (fp == NULL) {
    printf("File not found!");
    return 1;
}

char ch  = fgetc(fp);       // read one character
char str[50];
fgets(str, 50, fp);         // read one line

int num;
fscanf(fp, "%d", &num);     // read formatted value

fclose(fp);
```

***

## Appending to a File

```c theme={null}
FILE *fp = fopen("data.txt", "a");

fprintf(fp, "New line added!\n");
fputs("More text\n", fp);

fclose(fp);
```

***

## Checking for NULL

Always check if `fopen()` returned `NULL` — it means the file could not be opened (wrong path, no permissions, etc.):

```c theme={null}
FILE *fp = fopen("test.txt", "r");
if (fp == NULL) {
    printf("Cannot open file.\n");
    return 1;
}
```