The current Windows product contains a minimal set of file I/O functions that are just procedural versions of the corresponding DOS INT 21h functions. These functions are not portable and lack many of the necessary file system functions an application might need.
Lacking a set of file system functions, the current Windows environment requires that applications call DOS for some file I/O needs. Win32 has been extended in this area so that the DOS dependency can be removed from applications. The functions are similar to their DOS counterparts and provides the minimum services required to portably access the file system from a Win32 application.
When a process is created, it inherits the current directory from the application that starts it. An application can determine which directory is current by using the GetCurrentDirectory function. An application can change the current directory using the SetCurrentDirectory function. When accessing a file, filenames may be either absolute or relative to the current directory.
The file functions include:
| Function | Description |
| _lclose | Closes a file |
| _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 |
| CompareFileTime | Compares two 64-bit file times. |
| CopyFile | Copies a file |
| CreateDirectory | Creates a directory |
| CreateFile | Creates or opens a file |
| DeleteFile | Deletes a file |
| FindClose | Finishes a file enumeration |
| FindFirstFile | Finds the first file matching the specified name |
| FindNextFile | Finds the next file after a FindFirstFile |
| FlushFileBuffers | Flushes buffered file i/o |
| GetCurrentDirectory | Returns the current directory |
| GetDiskFreeSpace | Returns the current disk free space |
| GetDriveType | Determines whether a disk drive is removable, fixed, or remote |
| GetFileAttributes | Gets the attributes of the specified file |
| GetFileSize | Returns the size of a file. |
| GetFileTime | Gets the creation, last access, and last modified times of a file |
| GetFileType | Determines the file type (disk,pipe,char) |
| GetFullPathName | Returns the fully qualified pathname for the file |
| GetLogicalDrives | Returns a bitmask of available drives. |
| GetLogicalDriveStrings | Returns the names of available drives. |
| GetOverlappedResult | Returns the result of the previous overlapped i/o |
| GetStdHandle | Returns the handles for stdin, stdout, and stderr |
| GetSystemDirectory | Retrieves the pathname of the Windows system subdirectory |
| GetTempFileName | Creates a temporary filename |
| GetTempPath | Returns the pathname for the temp directory |
| GetVolumeInformation | Returns information for the specified volume |
| GetWindowsDirectory | Retrieves the pathname of the Windows directory |
| LockFile | Locks a file |
| MoveFile | Renames a file |
| OpenFile | Creates, opens, reopens, or deletes the specified file |
| OpenSystemFile | Opens a system file. |
| ReadFile | Reads from a file |
| RemoveDirectory | Deletes a directory |
| SearchPath | Searches for a file along the specified path |
| SetCurrentDirectory | Sets the current directory |
| SetEndOfFile | Sets the end of file location |
| SetFileAttributes | Sets the attributes of a file |
| SetFilePointer | Seeks to a new location in a file |
| SetFileTime | Sets the time for a file |
| SetHandleCount | Changes the number of file handles available to a task |
| SetStdHandle | Sets the handles for stdin, stdout, and stderr |
| UnlockFile | Unlocks a file |
| WriteFile | Writes to a file |