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Low-Level I/O 

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 file descriptors.

Stream File Descriptor







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 file


Flush file to disk

_creat, _wcreat

Create file


Return next available file descriptor for given file


Create second descriptor for given file


Test for end of file

_lseek, _lseeki64

Reposition file pointer to given location

_open, _wopen

Open file


Read data from file

_sopen, _wsopen, _sopen_s, _wsopen_s

Open file for file sharing

_tell, _telli64

Get current file-pointer position

_umask, _umask_s

Set file-permission mask


Write data to file

_dup and _dup2 are typically used to associate the predefined file descriptors with different files.