lclint-interest message 62

From evs Sun Mar 24 14:00:57 1996
Date: Sun, 24 Mar 96 13:33:31 -0500
From: evs (David Evans)
In-Reply-To: Eric Bloodworth's message of Fri, 22 Mar 1996 20:32:57 -0500 <>
Subject: can lclint detect this leak? 


The reason lclint doesn't detect the error in the code example you sent
if because it doesn't know anything about the semantics of memset.  

For example, for 

> int frob(booga_s *stuff)
> {
> 	/* 
>		How do I say storage->bar isn't allocated after the 	
>		memset? 
>	*/
> 	(void)memset(stuff, 0, sizeof(*stuff));  
>	stuff->bar = malloc(100); 

lclint is (incorrectly) reporting a memory leak when stuff->bar is
assigned, since it does not know that the memset only assigned all the
bits in stuff 0 and did not allocate anything.  In this example, of
course, it would make more sense to just do stuff->foo = 0 (although I
can understand why you might want to use the memset in some other
cases).  Then, you would need to use /*@-mustfree@*/ comments around the
assignment to suppress the error message.

The free_booga_s reveals a limitation of what can be expressed with
lclint annotations.  If I understand correctly, the intent of this
function is to deallocate all storage in fields from a struct pointer,
but not the structure itself.  If the structure were deallocated also,
you would use /*@only@*/ annotation on the parameter (and an error would
be reported for the existing implementation since it does not free the
storage the parameter points to).  

For the storage leak, lclint is unable to detect this since it doesn't
check storage contained in fields of local structures.  This should
probably be considered a bug, that will be fixed in an update to lclint

In general, lclint will not be able to correctly analyze code that
depends on functions that deallocate some pointer-to-structure fields
without deallocating the structure since there is no way to express this
with the annotations.  So, short of restructuring the code to avoid
this, the only thing I can recommend is to use syntactic comments to
suppress the relevant messages.

--- Dave

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University of Virginia, Computer Science