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 don't 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
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 descriptor for given file
_dup2 Create second descriptor 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, _sopen_s, _wsopen_s Open file for file sharing
_tell, _telli64 Get current file-pointer position
_umask, _umask_s Set file-permission mask
_write Write data to file

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

See also

Input and output
Universal C runtime routines by category
System calls