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Date: 21 Jul 93 18:50:16 GMT
From: emery@mitre-bedford.arpa  (David Emery)
Subject: Re: SunAda question
Message-ID: <EMERY.93Jul21135016@goldfinger.mitre.org>
List-Id: <comp.lang.ada>

>procedure gethost is 
>        namelen : integer := 20;
>        hostname : string (1..namelen);
>        stat : unix.status_code;
>begin
>        stat := unix.gethostname(hostname'address,namelen);
>        text_io.put_line ("Hostname is " & hostname);
>end gethost;

First, this code is not portable across compilers.  In particular, it
'just happens to work' on Verdix that string'address is the same as
string(string'first)'address.  The address passed to C must be the
address of the first element of the string.  String'address is not
well-defined, but string(string'first)'address -is- defined to be the
address of the first element of the string.  On some compilers,
string'address may return the address of the 'dope vector' for the
string, or something else besides the address of the first element.

Another problem with this code is that it presumes the length of the
hostname is -exactly- 20 characters.  If the hostname is greater than
20, it will be truncated by the C/Unix operation gethostname().  If it
is less than 20, then you'll have garbage after the end of the string,
marked by Ascii.nul.  Furthermore, this code perpetuates the C
tradition by completely ignoring the error return, which means that
the data in hostname will be undefined if the function failed.  

Try this:

with System;
function Hostname 
    return string	-- will return "" on any error
is

        maxnamelen : integer := 2048;	-- much safer value than 20!
        buffer : string (1..namelen) := (others => Ascii.nul);
					-- essentially a Zero-length C string

	subtype int is integer;		-- assuming C's int = integer
        status : int;
	stringlength : int;

	-- C function prototype for gethostname: 
	--     int gethostname(name, namelen)
	--     char *name;
	--     int namelen;
	-- returns 0 on success, -1 on failure.
	function gethostname (addr : System.address; length : int)
            return int;
	pragma interface (C, gethostname);      

	-- C function prototype for strlen:
	--     int strlen (str);
	--     char * addr;
	-- returns the length of the string, not including the null at the
	-- end of the string.  Can return 0.
	function strlen (addr : System.address)
	    return int;
        pragma interface (C, strlen);

begin
        status := gethostname(buffer(buffer)'address, maxnamelen);
	if status = 0 then
		stringlength := strlen (buffer(buffer)'address);
	       	return buffer(buffer'first 
			      .. buffer'first + stringlength - 1 );
	else
		return "";	-- empty string on any error
	end if;
end Hostname;

				dave