File I/O functions create, open, read from, write to, and close files. The following list briefly describes each file I/O function:
| Function | Description | |
| GetDriveType | Determines whether a disk drive is removeable, fixed, or remote. | |
| GetSystemDirectory | Retrieves the pathname of the Windows system subdirectory. | |
| GetTempDrive | Returns the letter of the optimal drive for temporary file storage. | |
| GetTempFileName | Creates a temporary filename. | |
| GetWindowsDirectory | Retrieves the pathname of the Windows directory. | |
| _lclose | Closes a file. | |
| Function | Description | |
| _lcreat | Creates a new file or opens and truncates an existing file. | |
| _llseek | Positions the pointer to a file. | |
| _lopen | Opens an existing file. | |
| _lread | Reads data from a file. | |
| _lwrite | Writes data in a file. | |
| OpenFile | Creates, opens, reopens, or deletes the specified file. | |
| SetHandleCount | Changes the number of file handles available to a task. |