Question

Access violation while writing to const array

Asked by: jhshukla

I am working on a library to deal with physical quantities like Displacement, Velocity, etc. This project produces a .LIB file.
There is a native DLL project that uses this .LIB; and then there is a .Net exe that dynamically loads this DLL.

There is a const array static member in class DimQty. it contains combinations of dimensions for known types (speed = length/time, density = mass/length^3, etc). the array is not initialized to desired values for two reasons:
(1) it would require **many** inline function calls in the initializer list which is plain ugly. e.g. array = { func1(), func2()/func829() };
(2) it would require reordering enums based on dependency causing readability problems;
Since the array is not initialized to desired values, I have a function, aptly named initialize(), that will take care of filling appropriate values. I get access violation in initialize(), when I try to write to the array.

The code compiles fine without warnings around the troublesome location. there are other minor warnings like comparing int and bool etc. which I ignored.

I have two possible ideas as to why this is happening (there may be other reasons that I don't know of):
(1) since the array is a class static const variable it is not on the stack (stack segment), but somewhere in a section marked "read-only." this would cause the hardware to signal an interrupt.
(2) .Net environment is messing up something. unlikely.

I need to know why this is happening and how to fix it. If my reasoning is correct, then the only way to fix it may be to change from static array to dynamically allocated array.

Help appreciated. Thanks.

/*=========== class/variable definitions in .LIB: =============*/
namespace Physics {
	struct Dimensions {
		float Current, Length, LuminousIntensity,
			Mass, Quantity, Temperature, Time;
	};
 
	class DimQty {			// abstract class
	public:
		enum Type {
			Null,			// dimensionless quantity
			/* SI base types */
			Current,	...
			/* SI derived types */
			...
			/* ------------------------ */
			Unknown // wildcard for any combo not listed above
		};
 
		/// this is the array that is mentioned in title
		static const Dimensions DefinedTypes[Unknown]; 
 
		static void initialize();
 
		// ctor, dtor & other funcs go here
	protected:
		static bool inited;
		Dimensions * dims;
	};
}
// ------------- src ------------
const Dimensions DimQty::DefinedTypes[DimQty::Unknown] = {{0}};
bool DimQty::inited = false;
void DimQty::initialize()
{
	Dimensions * def = const_cast<Dimensions*>(DefinedTypes);
	def[Current].Current				= 1;<< access violation here
	def[Length].Length				= 1;<< and here
	def[LuminousIntensity].LuminousIntensity	= 1;<< and here
	def[Mass].Mass					= 1;<< etc
	def[Quantity].Quantity				= 1;<<
	def[Temperature].Temperature			= 1;<<
	def[Time].Time					= 1;<<	
	...
}
 
DimQty::DimQty(const Dimensions *_dims)
{
	if (!inited)
		initialize();
	...
}

                                  
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Asked On
2008-06-29 at 13:10:44ID23525313
Tags

C++, C#

,

.net, dll, const, access violation, pointer

Topics

C++ Programming Language

,

.NET

,

Windows XP Operating System

Participating Experts
2
Points
355
Comments
4

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Answers

 

by: Infinity08Posted on 2008-06-29 at 13:18:49ID: 21895324

const means that you cannot modify the contents of the array ... You are trying to do exactly that in your initialize function.

It's true that that won't necessarily cause an access violation, but the possibility exists (depending on where the compiler decided to place the data), so you shouldn't try to modify const data. In fact, your compiler probably threw an error when you tried to.

Why not just remove the const (since you obviously don't want it to be const) ?

 

by: evilrixPosted on 2008-06-29 at 14:01:24ID: 21895458

Since it's const I presume you only really want to initialize it once and then use it read only? If so, initialize it when you define it. Simplified example below.

namespace Physics {
	struct Dimensions {
		float Current, Length, LuminousIntensity,
			Mass, Quantity, Temperature, Time;
	};
 
	class DimQty {			// abstract class
	public:
		static const Dimensions DefinedTypes[]; 
	};
}
 
// Initialize DefinedTypes (in .cpp file)
namespace Physics {
	Dimensions const DimQty::DefinedTypes[] = {
		{1.0f, 2.0f, 3.0f, 4.0f, 5.0f, 6.0f, 7.0f},
		{1.1f, 2.1f, 3.1f, 4.1f, 5.1f, 6.1f, 7.1f}
	};
}
                                              
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by: evilrixPosted on 2008-06-29 at 14:09:59ID: 21895482

If you want to stick to using your initalise function, do as Infinity08 suggests and make it non-const. To protect it from unexpected mutation make it private and access it using an accessor that returns a const pointer to the start of the array..

namespace Physics {
	struct Dimensions {
		float Current, Length, LuminousIntensity,
			Mass, Quantity, Temperature, Time;
	};
 
	class DimQty {			// abstract class
	public:
		static Dimensions const * get_DefinedTypes() // Return const pointer to start of array
		{
			return DefinedTypes;
		}
	private:
		static Dimensions DefinedTypes[]; // Private
	};
}

                                              
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by: jhshuklaPosted on 2008-06-29 at 23:25:37ID: 21897000

thanks inifinity & evilrix for responses.

@infinity,
the compiler did not give me an error or even a warning when I compiled it. but as you said the compiler can place it wherever it deems appropriate and that's what I too thought was happening. so it is definitely not a reliable way to modify const data. also as evilrix said, I want to initialize it once and then use it as read-only. so making it non-const it not an option.

@evilrix,
I will have to go with your second comment because I don't trust the compiler. They way you suggested to initialize may cause me some problems. I did not include entire initialize function for the sake of brevity but further down I use statements that use values from previous array elements (see the attached code). I don't know which element will be constructed first (0th or Nth). If it is the reverse order all my values will be messed up. (correct me if C++ spec says otherwise)

My other option is to hard code numbers as in your first comment but it is too error prone and unmaintainable.

I modified it a little so DefinedTypes is now declared as
public: static const Dimensions * const DefinedTypes;

and initialized as
const Dimensions * const DimQty::DefinedTypes = initialize();

Thanks again for you input.

Dimensions * DimQty::initialize()
{
	Dimensions * def = new Dimensions[Unknown];
	memset(def, 0, sizeof(Dimensions) * Unknown);
 
	def[Current].Current				= 1;
	def[Length].Length				= 1;
	def[LuminousIntensity].LuminousIntensity	= 1;
	def[Mass].Mass					= 1;
	def[Quantity].Quantity				= 1;
	def[Temperature].Temperature			= 1;
	def[Time].Time					= 1;
 
	def[Angle] = def[Null];
	def[SolidAngle] = def[Null];
 
	def[Area] = def[Length] * def[Length];
	def[Volume] = def[Area] * def[Length];
 
	def[Frequency] = def[Null] / def[Time];
 
	def[Velocity] = def[Length] / def[Time];
	def[Acceleration] = def[Velocity] / def[Time];
	def[Jerk] = def[Acceleration] / def[Time];
	def[Snap] = def[Jerk] / def[Time];
	...

                                              
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