> ## 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 Operations

> Complete programs for writing to and reading from files in C

# File Operations in C

File operations follow four steps every time:

1. Declare a file pointer
2. Open the file with the appropriate mode
3. Perform operations (read/write)
4. Close the file

***

## 1. Writing to a File

```c theme={null}
#include <stdio.h>

int main() {
    FILE *fp;

    fp = fopen("write.txt", "w");   // open in write mode
    if (fp == NULL) {
        printf("Error opening file!\n");
        return 1;
    }

    fprintf(fp, "This is a text file.\n");
    fprintf(fp, "File handling in C.\n");

    fclose(fp);   // always close after use

    printf("Data written successfully!");
    return 0;
}
```

After running this, a file named `write.txt` is created in the same directory containing the two lines.

***

## 2. Reading from a File

```c theme={null}
#include <stdio.h>

int main() {
    FILE *fp;
    char line[100];

    fp = fopen("write.txt", "r");   // open the file we just wrote
    if (fp == NULL) {
        printf("File not found!\n");
        return 1;
    }

    printf("Contents of File:\n");

    while (fgets(line, sizeof(line), fp)) {
        printf("%s", line);
    }

    fclose(fp);
    return 0;
}
```

**Output:**

```
Contents of File:
This is a text file.
File handling in C.
```

***

## 3. Appending to a File

```c theme={null}
#include <stdio.h>

int main() {
    FILE *fp = fopen("write.txt", "a");
    if (fp == NULL) {
        printf("Error!\n");
        return 1;
    }

    fprintf(fp, "Appended line.\n");
    fclose(fp);

    printf("Data appended successfully!");
    return 0;
}
```

***

## 4. Character-by-Character Read

```c theme={null}
#include <stdio.h>

int main() {
    FILE *fp = fopen("write.txt", "r");
    if (fp == NULL) return 1;

    char ch;
    while ((ch = fgetc(fp)) != EOF) {
        putchar(ch);   // print each character
    }

    fclose(fp);
    return 0;
}
```

`EOF` (End Of File) is a constant that signals no more data is available.

***

## Summary of Modes

| Need                          | Mode   |
| ----------------------------- | ------ |
| Only read an existing file    | `"r"`  |
| Create/overwrite a file       | `"w"`  |
| Add data to end of file       | `"a"`  |
| Read and write existing file  | `"r+"` |
| Read and write (create/clear) | `"w+"` |