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From: bet@ecsvax.UUCP
Newsgroups: comp.lang.c,comp.sys.ibm.pc
Subject: Re: Unbuffered I/O using MicroSoft C 3.0
Message-ID: <2609@ecsvax.UUCP>
Date: Wed, 28-Jan-87 19:25:09 EST
Article-I.D.: ecsvax.2609
Posted: Wed Jan 28 19:25:09 1987
Date-Received: Sat, 31-Jan-87 05:47:22 EST
References: <1867@sunybcs.UUCP> <645@uw-warp.UUCP> <650@uw-warp.UUCP>
Reply-To: bet@ecsvax.UUCP (Bennett E. Todd III)
Organization: Duke User Services
Lines: 53
Xref: watmath comp.lang.c:886 comp.sys.ibm.pc:1267


While setbuf will still be needed to get unbuffered I/O, there is a
wondrously simple way to ensure that I/O doesn't get redirected without
mucking about with DOS interrupts! Like so:

	FILE *fopen(), *console;

	console = fopen("\\dev\\con", "r");

The '\' characters are doubled because '\' is the quoting character in C
strings; you can simplify the above to the more obvious:

	FILE *fopen(), *console;

	console = fopen("con", "r");

(or "con:", or whatever floats your boat) if you are guaranteed that
your program will never be run on a system using the undocumented DOS
AVAILDEV function to hide devices.

-Bennett

P.S. AVAILDEV:

Issue DOS function call INT 21H with AH=37H to access SWITCHAR and
AVAILDEV:

	AL=0	read SWITCHAR value into DL
	AL=1	set SWITCHAR from DL
	AL=2	read AVAILDEV into DL
	AL=3	set AVAILDEV from DL

The SWITCHAR is the character used for "switches" in DOS command
arguments (defaults to '/', as in "DIR /P"). '-' is popular to make a
system look more like UNIX; if the SWITCHAR is anything other than '/',
then '/' may be used instead of '\' for pathnames (even more like
UNIX!). AVAILDEV=0 means than devices must be referenced in an imaginary
subdirectory "\dev" (shades of UNIX's /dev/*); a filename "prn.dat" can
be created on disk and manipulated like any other. If AVAILDEV != 0 then
device names are recognized anywhere (this is the default): "prn.dat"
is synonymous with "prn:".

NOTE: These functions are undocumented, whicn means they are
unsupported, which means they are liable to go away whenever Microsoft
is feeling frisky. Use at your own risk! However, PLEASE program around
them to make life nicer for the daring souls who take the risk.

-Bennett
-- 
Bennett Todd -- Duke Computation Center, Durham, NC 27706-7756; (919) 684-3695
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