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From: des@frogland.inmos.co.uk (David Shepherd)
Newsgroups: comp.os.minix
Subject: Re: The continuing saga of Minix 'ar' on the SPARC
Keywords: Almost there....
Message-ID: <12887@wraxall.inmos.co.uk>
Date: 10 Jan 91 11:08:58 GMT
References: <1991Jan9.151828@mobius.Viewlogic.COM>
Sender: news@inmos.co.uk
Reply-To: des@frogland.inmos.co.uk (David Shepherd)
Organization: INMOS architecture group
Lines: 50

In article <1991Jan9.151828@mobius.Viewlogic.COM>, greg@mobius.Viewlogic.COM (Gregory Larkin) writes:
|> Well, the fight continues to get a SPARC executable to read a Minix
|> .a file.
|>
|> However, when the first header structure is read out of the .a file,
|> most everything in it does not come in correctly.
|> Here's the struct for .a header:

.... let me guess .... the header gets read in by a command like

	fread(f, &hdr, sizeof (struct ar_hdr), 1);

if so this is your problem. This is *NON-PORTABLE*. The Sparc
C compiler does (clever?) things with the positioning of elements
inside a struct .... in particular it will align entries to word
boundaries if there are word lengthed items in it. So in

|> 	struct ar_hdr {
|> 		char ar_name[14];
|> 		long ar_date;
|> 		char ar_uid;
|> 		char ar_gid;
|> 		int ar_mode;
|> 		long ar_size;
|> 	};


The Sparc C compiler sees the struct as being

14 bytes of ar_name
 2 bytes padding
 4 bytes of ar_date
 1 byte  of ar_uid
 1 byte  of ar_gid
 2 bytes padding
 4 bytes of ar_mode  <- ints are 32 bits here
 4 bytes of ar_size

Hence reading in a struct which probably doesn't align components (and
probably has 16 bit ints) causes rubbish to be seen in the struct 
components.

The fix is probably to read in the struct as an array of bytes and assign 
the components by hand :-(

-- 
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david shepherd: des@inmos.co.uk or des@inmos.com    tel: 0454-616616 x 529
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