[mdlug] C Pointer Problem

[Aaron Kulkis]

A. Zimmer wrote:
> Hello,
>
> I am attempting to convert a relatively ancient C program (ca. 1999)
> into 64-bit clean code that will run on a pure 64-bit Linux system.
> The program performs a certain type of digital image processing and it
> will run on a 32-bit or multilib (32/64-bit) system, but it will seg fault
> on pure 64-bit.
>
> So far I've converted all integer types and corrected all GCC compiler
> warnings and errors concerning 32-bit pointers.  The program will now
> compile without any problems but will always seg fault during run time.
> Using a debugger I've traced the seg faults to some unusual pointer
> casts that somehow cause trouble in 64-bit.
>
> Here are two places where segmentation faults occur.  These two functions
> are defined in a header file and are used to insert in-line functions
> at various places in the code:
>
> #define peiGetFlags(image)	(((imageType*)(image))->flags)
> #define peiGetConvKer(image)	(*(imageType**)&(image)->clientData[1])
>
> In these functions, "imageType" is a structure and "image" is a pointer
> to that structure.  The gcc compiler won't complain, but when called
> during run time the program seg faults.
>
> I cannot decipher the meaning of the constructs on the right-hand side.
>
> The first seems to cast a pointer into a pointer to imageType, which should
> not really be necessary since "image" is already of that type.  In fact,
> I can get past the seg fault by rewriting this as "(*image).flags."  But
> then why would the original author use this more complicated method?
>
> The second one has me totally baffled.  I can't even guess what that
> strange bit-AND is doing.

It's not a bit-AND, it's an "address of"

>
> Can someone shed some light on the meaning of these strange constructs?
>
> Here are two more examples:
>
> #define peiGetConvInt32Factor(image)	(*(int32_t*)&(image)->clientData[3])
> #define peiGetConvFloat64Factor(image)	(*(double*)&(image)->clientData[3])
>
> Also, for reference, here is the structure for which these functions are
> intended (most of the fields are int32_t):
>
> struct imageType {
> 	imageOpsType *ops;
> 	coordType width;
> 	coordType height;
> 	int32_t depth;			/* 1..32			*/
> 	int32_t linebytes;		/* bytes per line		*/
> 	int32_t allocated;		/* bytes allocated		*/
> 	pixelType fill;			/* color to fill with		*/
> 	int8_t sign;
> 	imageType *primary;		/* for secondary, assoc primary	*/
> 	coordType primaryX;		/* for secondary, offset in primary */
> 	coordType primaryY;		/* for secondary, offset in primary */
> 	int8_t *data;
> 	uint32_t flags;			/* image access mode(s)		*/
> 	int32_t verbose;
> 	int32_t page;			/* number of the current page	*/
> 	double resX;			/* pixels per unit in X		*/
> 	double resY;			/* pixels per unit in Y		*/
> 	int32_t resUnit;			/* 0=nores 1=unk 2=inch 3=cm	*/
> 	char modes[PEI_MAX_MODES];	/* modes used with open		*/
> 	char name[PEI_MAX_NAME];
> 	int32_t clientData[PEI_CLIENT_SIZE]; };
>
>
> I can always run this code on a multi-lib (32/64-bit) system but I'd like
> to use pure 64-bit only.
>
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