These functions invoke the operating system directly for lower-level operation than that provided by stream I/O. Low-level input and output calls do not buffer or format data.
Low-level routines can access the standard streams opened at program startup using the following predefined handles:
| Stream | Handle |
| stdin | 0 |
| stdout | 1 |
| stderr | 2 |
Low-level I/O routines set the errno global variable when an error occurs. You must include STDIO.H when you use low-level functions only if your program requires a constant that is defined in STDIO.H, such as the end-of-file indicator (EOF).
Low-Level I/O Functions
| Function | Use |
| _close | Close file |
| _commit | Flush file to disk |
| _creat, _wcreat | Create file |
| _dup | Return next available file handle for given file |
| _dup2 | Create second handle for given file |
| _eof | Test for end of file |
| _lseek, _lseeki64 | Reposition file pointer to given location |
| _open, _wopen | Open file |
| _read | Read data from file |
| _sopen, _wsopen | Open file for file sharing |
| _tell, _telli64 | Get current file-pointer position |
| _umask | Set file-permission mask |
| _write | Write data to file |
_dup and _dup2 are typically used to associate the predefined file handles with different files.