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From: dalegass@dalcs.UUCP
Newsgroups: comp.sys.ibm.pc
Subject: Re: stupid COMMAND.COM  (WAS:  DOS enviroment size)
Message-ID: <2438@dalcs.UUCP>
Date: Thu, 12-Mar-87 15:32:25 EST
Article-I.D.: dalcs.2438
Posted: Thu Mar 12 15:32:25 1987
Date-Received: Fri, 13-Mar-87 05:45:07 EST
References: <2091@ncoast.UUCP> <409500003@prism> <2986@iuvax.UUCP> <12572@watnot.UUCP>
Organization: Dept. of Math., Stats & C.S., Dalhousie University, Halifax, N.S., Canada
Lines: 37
Keywords: DOS environment substitutions
Summary: COMMAND.COM doesn't do the redirection

In article <12572@watnot.UUCP>, djfiander@watnot.UUCP writes:
> >>                                     it is physically impossible to
> >> pass certain characters to a program on the commandline, specifically
> >> " ' * ? | etc.
> >  ^ ^ ^ ^ ^
> >I wrote a quicky C program which contained just printf("%s\n", argv[1]);
> >I could pass all of those characters highlighted above and also < and >.
> >The only thing I had to do is enclose | < and > in double quotes.  ...
> > This is for PC-DOS 3.1 on an IBM PC XT.
> 
> Oops, * and ? are passed inside double quotes (sorry), but under DOS
> 3.0 the others are still not passed.  I know this because I tried to 
> use '|' in an egrep pattern.  DOS passed the first part of the pattern to
> egrep and tried to execute the second part.
> 

I'm pretty sure that DOS does not do all of the redirection.  Of course it
handles redirection for it's own built-in commands (COPY,ECHO,etc.) but
as far as executables go, they must do the work themselves.

Most (if not all) C compilers will automatically handle this for you in
their startup routine (which allocates memory, initializes channels, and so
on).  I've seen the source code for Aztec C, and redirection is taken care
of from the program which is executed, and is not a trapping at the DOS
level.

This means that different C compilers might parse the " character a bit
differently; hence the discrepancy between results.  The C compiler which
Microsoft used for their transient (.com .exe) commands could very likely
perform a bit differently from a program compiled by another version of C.

When DOS gives control to a .com or .exe file, it simply passes the whole
command line to it, and let's it do what it wishes with the info.
I think, however, that piping '|' *is* intercepted at the DOS level,
whereas the '>' and '<' redirections are not.

-dalegass@dalcs