This file discusses the file primitives
for the software tools.


file open routine


READ, WRITE, READWRITE, and APPEND are global constants known
to all programs.


WRITE access implies erasing the file first.
READWRITE and APPEND access are currently illegal.


If name = TERMINAL  then open()
must assign the I/O to the user's console.
If name is PRINTER then open() must assign the I/O to the
line printer.
Note that because BDS C truncates names at 8 characters,
the name TERMINAL is the same as the names TERMINAL_IN and
TERMINAL_OUT.


open() returns unit numbers to the user.
Units may be assigned to disk files, the console, or the
line printer.
open() never returns unit < SYS_STD.


Unit 1 is reserved for standard input.
Unit 2 is reserved for standard output.
Unit 3 is reserved for error output.
Unit 4 is reserved for the console.
Unit 5 is reserved for the line printer.



open()  might save the file name for the software tools
error routines.
At present it does not do so.


The system maintains the following file information:

	int sys_spec [SYS_STD + MAXOFILES];

	int sys_bufp [SYS_STD + MAXOFILES];

	int sys_bufn [SYS_STD + MAXOFILES];


As you can see, there is one entry in each array for each unit.
If sys_bufp [unit] != ERR then I/O is assigned to the file
whose buffer is sys_bufp [unit].
Otherwise, if sys_spec [unit] != ERR then I/O is assigned to
either special unit 3 (the console) or 4 (the line printer).
If sys_bufp [unit] == sys_spec [unit] == ERR then the
unit is not open.


Note that both sys_bufp [unit] and sys_spec [unit] may be
non-ERR.
In this case the unit is NOT a special file, it is a
disk file.
In other words, sys_bufp [] is checked before sys_spec []
in getch1() and putch1() and close() and flush().
This is somewhat of a kludge, but it makes open() simpler.


The array sys_bufn [unit] is used only by open() and close().
sys_bufn [] keeps track of which disk files
(0 -- (MAXOFILES - 1))
have been assigned.
Close() uses sys_bufn [] to deassign disk files.
Sys_bufn [] may not really be needed, but without it
close() would have to do some hairy pointer subtraction
and division on sys_bufp [] in order to figure out what
disk file was being closed.
It seems simpler to use sys_bufn [].


system initialization routine


initst() initializes all system data areas.
It puts the command line arguments in sys_args[] and
puts the number of command line arguments in sys_narg.
It redirects I/O if requested on the command line.
This is done by calling _assign() which in turn updates
the arrays sys_spec [], sys_bufp [] and sys_bufn [].