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From: emery@linus.mitre.org (David Emery)
Newsgroups: comp.lang.ada
Subject: Re: Making EXECL Unix call from Ada
Message-ID: <EMERY.90Dec10105104@aries.linus.mitre.org>
Date: 10 Dec 90 15:51:04 GMT
References: <10048@jarthur.Claremont.EDU>
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Organization: The Mitre Corporation, Bedford, MA
In-reply-to: cstein@jarthur.claremont.edu's message of 9 Dec 90 19:38:55 GMT
List-Id: <comp.lang.ada>

I suspect the problem is that Unix is running off the end of your
string and/or your parameter list.  Try the following:

	with system;
	  -- assumed to provide the object "null_address"
	  -- (also known as "address_zero" or "no_addr" on some compilers)
	with text_io;
	procedure print
	    function execl (name : system.address;
			    arg0 : system.address;
			    arg1 : system.address)	-- all we need
	    return integer;	-- assumed to be the same size as C "int"
	    pragma interface (C, execl);
	    
	    program_name : constant string
			 := "/usr/local/bin/enscript" & Ascii.nul;
	    file_name    : constant string
		         := "/hmc2/guests/lsdada/cliffie/hello" & Ascii.nul;
	    status : integer;
	begin
	    status := execl (name => program_name(program_name'first)'address,
			     arg0 => file_name(file_name'first)'address,
			     arg1 => system.null_address);
	    if status/=0 then
		put_line("True.");
  	    else
		put_line("False");
	    end if;
	end print;

Remember, that in C strings are null-terminated, and represented by
the address of the first element.  Also, from the C definition of the
parameters for execl, the last parameter is of type "address", with
value 0, not the address of an object with value 0.  There's a subtle
difference, in that the former is "0", and the latter is not "0", but
some (non-zero) address.  

Here are the changes to print a user-supplied file.  Append a null to
the end of the user-supplied string, and pass this to C:

	procedure print (file_to_print : string) is
	    ...
	    file_name : constant string
		      := file_to_print & Ascii.nul;
	    ...
	end print;

				dave emery
				emery@aries.mitre.org

<for more tricks like this, see my paper in Tri-Ada on the POSIX Ada
binding prototype.>