Path: utzoo!dciem!dretor!nrcaer!sce!cognos!brunog
From: brunog@cognos.uucp (Bruno Godbout)
Newsgroups: comp.std.c
Subject: Constant expressions in initializers
Message-ID: <5716@cognos.UUCP>
Date: 29 Mar 89 00:05:11 GMT
Reply-To: brunog@cognos.UUCP (Bruno Godbout)
Organization: Cognos Inc., Ottawa, Canada
Lines: 48


	I need some help with the ANSI definition of a "constant
expression" inside an initializer.  I am trying to compile a
piece of code that is supposed to initialize a structure member with
the value of the "offsetof" macro and the compiler gives me an error.
The error message says that "the initializer must be a constant 
expression".

	I am using a C compiler which is said to be consistent with the
Draft Proposed ANSI standard for C.  The compiler defines the "offsetof"
macro (in <stddef.h>) as follows:

--------

#define offsetof(structype,field) \
		( (size_t)(char *) &(((structype *)0)->field) )

--------

	Here is a simplified example of what I am trying to do:

--------

struct dummy {
	size_t s;
	float f;
	};

struct dummy Dummy = { offsetof(struct dummy,f),
                       2.0 };

--------

	The compiler tells me that "the initializer must be a constant 
expression" and flags the element initialized by offsetof.  I compiled
this code successfully with other compilers.  Thinking that this might
be relevant to ANSI compilers only, I tried it with "gcc -ansi" and with
"gcc -ansi -pedantic" and it still compiled fine.

	I would like to know if this code is rejected by the ANSI
definition or if it is only a restriction of the compiler I am using.

	Thank you for your help.
-- 
Bruno Godbout        3755 Riverside Dr.  VOICE: (613) 738-1338 ext.5086
Cognos, Inc          P.O. Box 9707         FAX: (613) 738-0002
                     Ottawa, Ontario       
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