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From: kaiser@ananke.stgt.sub.org (Andreas Kaiser)
Newsgroups: comp.lang.c
Subject: Handling of untagged structures by the C compiler
Message-ID: <14.26FB1164@ananke.stgt.sub.org>
Date: 22 Sep 90 07:08:05 GMT
Organization: Ananke, Stuttgart, FRG
Lines: 33

In a message of <Sep 20 21:57>,  Vox Populi ) (ravim@gtenmc.UUCP (Ravi K Mandava ) writes: 

 VP)>     4   #define ST struct { int i; }
 VP)>     8           ST *a;
 VP)>    12           if ((a = (ST *)malloc(sizeof(ST))) == NULL)

 VP)>    It appears that for every untagged structure declaration/typecast, the
 VP)>    compiler generates a unique tag internally and does not bother to
 VP)>    check for equivalence of the previously encountered structures.

According to ANSI, two structures are only compatible, if they have the same 
"tagged type". Two identically declared structures are incompatible.

 VP)>    [As for myself, I always try to use typedefs or tags for structures.
 VP)>     I happened to come across this problem while bug-fixing someone
 VP)>     else's code.  So please no chidings for writing bad code :-) ]

 VP)>    Or is it that this limitation is only with this particular compiler?

Some very old C compilers (the original Ritchie compiler for PDP-11) do not care 
about structure types at all. It was possible to write code like this (found in 
UNIX Version 6):

        register int i;
        struct { char high, low; };
        .... i.high ....

 

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