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fread crashes on pointer

General and Gameplay Programming Programming

Started by flodihn March 25, 2012 02:18 AM

2 comments, last by Potatoman 12 years, 1 month ago

flodihn

292

Author

March 25, 2012 02:18 AM

Hello, I got this strange problem that perhaps someone with better knowledge of C++ than me could explain.
For some reasion the progrem crashes when trying to read data into a pointer to regionVoxel.
However, reading into a voxel allocated on the stack and then copy it over to the regionVoxel works...



// * This is how I call the read_region function

voxel regionVoxel;

...

result = read_region(region_filename.str(), block_dir.str(), &regionVoxel);



int read_region(string regionFile, string blockDir, voxel *regionVoxel)

{

	voxel tmp;

	if(file_exists(blockDir.c_str()))

		return ABYDOS_TERRAIN_MIXED_REGION;



	if(!file_exists(regionFile.c_str()))

		return ABYDOS_TERRAIN_READ_ERROR;



	FILE *file = openFile(regionFile.c_str());



	if(file == NULL)

		return ABYDOS_TERRAIN_READ_ERROR;



	memset(regionVoxel, 0, sizeof(voxel));

	memset(&tmp, 0, sizeof(voxel));



	// * In my world, &tmp and *regionVoxel is kind of the same, why a pointer

	// * to voxel can not be read, but a pointer when taken by the adress opererator can eludes me.

	//fread((void *) regionVoxel->density, 1, 1, file);

	//fread((void *) regionVoxel->material, 2, 1, file);

	read(&tmp.density, 1, 1, file);

	fread(&tmp.material, 2, 1, file);

	*regionVoxel = tmp;

	closeFile(file);



	regionVoxel->material = ntohs(regionVoxel->material);

	return ABYDOS_TERRAIN_SHARED_REGION;

}

https://www.abydosentertainment.com

alvaro

21,604

March 25, 2012 02:31 AM

You mean

  fread((void *) &regionVoxel->density, 1, 1, file);

  fread((void *) &regionVoxel->material, 2, 1, file);

The things you are converting to (void *) should be the addresses of the fields you want to fill out, not their values.

flodihn

292

Author

March 25, 2012 02:36 AM

Thanks, I actually just noticed that I wanted the address of density and material inside the struct

So I changed to this and it worked.



    fread((void*) &(regionVoxel->density), 1, 1, file);

    fread((void*) &(regionVoxel)->density, 1, 1, file);

https://www.abydosentertainment.com

Potatoman

108

March 25, 2012 03:28 AM

Thanks, I actually just noticed that I wanted the address of density and material inside the struct
So I changed to this and it worked.

fread((void*) &(regionVoxel->density), 1, 1, file); fread((void*) &(regionVoxel)->density, 1, 1, file);

To avoid similar issues in future consider using static_cast, this provides the compiler with context to confirm the cast is valid, so you don't have things like this creep in by accident



	unsigned int val = 0xcdcdcdcd;

	void* ptr1 = static_cast<void*>(val);	// error C2440: 'static_cast' : cannot convert from 'unsigned int' to 'void *'

	void* ptr2 = static_cast<void*>(&val);   // ok

	void* ptr3 = reinterpret_cast<void*>(val);	// if you insist..

fread crashes on pointer

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