Creating and Opening Files with the CreateFile Function

The CreateFile function can create a new file or open an existing file.

When an application uses CreateFile, it must specify whether it will read from the file, write to the file, or both. The application must also specify what action to take whether or not the file exists. For example, an application can specify that CreateFile always be used to create the file. As a result, the function creates the file if it does not exist and overwrites the file if it does exist.

CreateFile also enables an application to specify whether it wants to share the file for reading, writing, both, or neither. A file that is not shared cannot be opened more than once by the first application nor by another application until the first application has closed the file.

The operating system assigns a unique identifier, called a file handle, to each file that is opened or created. An application can use the file handle in functions that read from, write to, and describe the file. It is valid until the file is closed. When an application starts, it inherits all open file handles from the process that started it, if the handles are inheritable. For more information about processes, see Processes and Threads.

For information about the standard input, standard output, and standard error file handles, see Consoles and Character-Mode Support.

An application should check the return value of CreateFile before attempting to use the handle to access the file. If an error occurs, the application can use the GetLastError function to get extended error information.