File Access Modes

Understanding file access modes is crucial for interacting with the file system in Windows. These modes define how a file is opened and what operations are permitted.

Common Access Modes

When opening a file, you specify an access mode to control read, write, or execute permissions, as well as how the file should be created or overwritten.

Read Access

Allows reading data from the file. The file must exist.

• GENERIC_READ: Standard flag for read operations.

Write Access

Allows writing data to the file. The file does not necessarily need to exist.

• GENERIC_WRITE: Standard flag for write operations.

Read/Write Access

Combines both read and write capabilities.

• GENERIC_READ | GENERIC_WRITE: Opens the file for both reading and writing.

Execute Access

Allows executing the file as a program. This is less common for general data files.

• GENERIC_EXECUTE: Standard flag for execute operations.

Creation Disposition

These flags determine what happens if the file exists or does not exist when the `CreateFile` function is called.

Create New

Creates a new file. If the file already exists, the operation fails.

• CREATE_NEW: Creates a new file.

Create Always

Creates a new file. If the file already exists, it is overwritten.

• CREATE_ALWAYS: Creates a new file, overwriting if it exists.

Open Existing

Opens the file. If the file does not exist, the operation fails.

• OPEN_EXISTING: Opens the file, fails if it doesn't exist.

Open Always

Opens the file. If the file does not exist, it is created.

• OPEN_ALWAYS: Opens the file, creates it if it doesn't exist.

Truncate Existing

Opens the file and truncates its size to zero bytes. If the file does not exist, the operation fails.

• TRUNCATE_EXISTING: Opens the file and truncates it to zero length, fails if it doesn't exist.

Example Usage (Conceptual C++):

#include <windows.h>

// ...

HANDLE hFile;

// Open an existing file for reading and writing
hFile = CreateFile(
    L"my_data.txt",                // File name
    GENERIC_READ | GENERIC_WRITE,  // Read and write access
    0,                             // No sharing
    NULL,                          // Default security attributes
    OPEN_ALWAYS,                   // Open if it exists, create if not
    FILE_ATTRIBUTE_NORMAL,         // Normal file attributes
    NULL                           // No template file
);

if (hFile == INVALID_HANDLE_VALUE) {
    // Handle error
    DWORD error = GetLastError();
    // ...
}

// ... operations on hFile ...

CloseHandle(hFile);
            

Sharing Modes

Sharing modes determine whether other processes can access the file while it is open.

• FILE_SHARE_READ: Allows subsequent opens for reading.
• FILE_SHARE_WRITE: Allows subsequent opens for writing.
• FILE_SHARE_DELETE: Allows subsequent delete operations.
• 0: Prevents other processes from accessing the file.

File Attributes

Common attributes include:

• FILE_ATTRIBUTE_NORMAL: Default attribute for regular files.
• FILE_ATTRIBUTE_ARCHIVE: The file is an archive.
• FILE_ATTRIBUTE_COMPRESSED: The file is compressed.
• FILE_ATTRIBUTE_ENCRYPTED: The file is encrypted.