Re: Trouble with malloc().

From: Kevin Goodsell (usenet1.spamfree.fusion_at_neverbox.com)
Date: 11/23/03 

Date: Sun, 23 Nov 2003 22:23:50 GMT

Pegboy wrote:
> I am having trouble with malloc() again for a PC app I am developing. The
> method of the suspicious line of code seems to be Ok on a embedded platform,
> but not with the PC platform. The embedded platform uses a different
> compiler. I feel like I'm overlooking a very simple problem, but I can't
> see it. I would appreciate any help. Thank you.
>
> I am trying to allocate memory for a structure of type NAT_S which contains
> a pointer to a structure of type NAT_ENTRY_S (see code below).
>
> The compiler reports warning 'Suspicious pointer conversion' as indicated
> below in the code. This is the only warning or error I get.
>
> This code appears to work but the next call to malloc() following this, the
> app crashes. In some instances, ReadNat() is not called and the app works
> Ok. So the problem points to within ReadNat(), which is where the warning
> is.
>
> I examined/watched the address given to 'nat' and 'nat->entries' and they
> are different by 24 bytes when they should be different by 6. The sizeof(
> NAT_S ) is 6.
>
> /************************************************************/
> typedef struct
> {
> short a;
> short b;
> long c;
> long d;
> } NAT_ENTRY_S;
>
> typedef struct
> {
> short num_entries;
> NAT_ENTRY_S *entries;
> } NAT_S;
>
> NAT_S *ReadNat( FILE *file, long offset )
> {
> NAT_S *nat;
> short i, num_entries;
>
> fseek( file, offset, SEEK_SET );
> num_entries = ReadShort( file );
>
> if( (nat = malloc( sizeof( NAT_S ) + (num_entries * sizeof(
> NAT_ENTRY_S )) )) != NULL )
> {
> nat->num_entries = num_entries;
> nat->entries = nat + sizeof( NAT_S ); /* COMPILER WARNING:
> Suspicious pointer conversion */

I'll say it's suspicious. It's also most likely the cause of your
problem. What makes you think that nat + sizeof(NAT_S) yields a memory
address that will be properly aligned for a NAT_ENTRY_S? You cannot
simply place an object anywhere in memory that is convenient. Some
objects in some implementations must be aligned on certain boundaries
(e.g., a 4-byte integer might be required to start on an address that is
divisible by 4).

malloc() is required to return a pointer to memory that is properly
aligned for any type, but that address plus some arbitrary value is NOT
required to yield a pointer to memory that is properly aligned for some
random type.

-Kevin 

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