Defined in rt_sys.h, the _sys_open() function opens a file.
The _sys_open() function is required by fopen() and freopen(). These functions in turn are required if any file input/output function is to be used.
The openmode parameter is a bitmap whose bits mostly correspond directly to the ISO mode specification. Target-dependent extensions are possible, but freopen() must also be extended.
The return value is –1 if an error occurs.