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Date: Mon, 16 Apr 2012 14:28:50 +0200
From: Sebastian Huber <sebastian.huber@embedded-brains.de>
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Subject: XDR Library and Short Enums
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Hi,

the XDR library implementation of xdr_enum() is currently:

/*
  * XDR enumerations
  */
bool_t
xdr_enum(XDR *xdrs, enum_t *ep)
{
	enum sizecheck { SIZEVAL };	/* used to find the size of an enum */

	/*
	 * enums are treated as ints
	 */
	/* LINTED */ if (sizeof (enum sizecheck) == sizeof (long)) {
		return (xdr_long(xdrs, (long *)(void *)ep));
	} else /* LINTED */ if (sizeof (enum sizecheck) == sizeof (int)) {
		return (xdr_int(xdrs, (int *)(void *)ep));
	} else /* LINTED */ if (sizeof (enum sizecheck) == sizeof (short)) {
		return (xdr_short(xdrs, (short *)(void *)ep));
	} else {
		return (FALSE);
	}
}

The enum_t is defined as:

typedef int32_t enum_t;

This is problematic with short enums (variable sized enums).  I case of short 
enums sizeof (enum sizecheck) would be 1.  The ARM EABI lets you a choice 
between two alternatives described in the document issued by ARM.  See also 
section 7.1.3 "Enumerated Types"

http://infocenter.arm.com/help/topic/com.arm.doc.ihi0042d/IHI0042D_aapcs.pdf

How would you implement and use the XDR library with short enums?  The 
xdr_enum() can be easily changed to:

/*
  * XDR enumerations
  */
bool_t
xdr_enum(XDR *xdrs, enum_t *ep)
{
	/*
	 * enums are treated as ints
	 */
	/* LINTED */ if (sizeof (enum_t) == sizeof (long)) {
		return (xdr_long(xdrs, (long *)(void *)ep));
	} else /* LINTED */ if (sizeof (enum_t) == sizeof (int)) {
		return (xdr_int(xdrs, (int *)(void *)ep));
	} else /* LINTED */ if (sizeof (enum_t) == sizeof (short)) {
		return (xdr_short(xdrs, (short *)(void *)ep));
	} else {
		return (FALSE);
	}
}

The problem is in the XDR library usage.  An example is this (rpc_msg.h):

enum msg_type {
	CALL=0,
	REPLY=1
};

How would you fix this?  What about

enum msg_type {
	CALL=0,
	REPLY=1,
         _MSG_TYPE_INVALID = 0xffffffff
};

?

-- 
Sebastian Huber, embedded brains GmbH

Address : Obere Lagerstr. 30, D-82178 Puchheim, Germany
Phone   : +49 89 18 90 80 79-6
Fax     : +49 89 18 90 80 79-9
E-Mail  : sebastian.huber@embedded-brains.de
PGP     : Public key available on request.

Diese Nachricht ist keine geschäftliche Mitteilung im Sinne des EHUG.