Thread: How to use fstream or istream to read a binary file and write out what's in it?

1) I am trying to open a binary file.
2) With a pointer value, I am trying to read the value in memory set by the pointer. But the first value I get I want to start the pointer from 0 to a specified location.
3) I want to get that value in memory and be able to use it (i.e. int, double) so I can do math with it!!

4) I'm having trouble!!!! HELP!!!!!

Code:

		
	fstream data;
		
	data.open(filename, ios::in |ios::out | ios::binary);
	
	if(!data.is_open())
	{
		cout << "Error opening file. Please try again." << endl;
		exit(1);
	}


       ptr5 = user_def_beg_pix_ptr = (given_line - 1)*numsamples + (given_col - 1);
	
	ptr1 = ptr2 - 1;
	ptr2 = ptr5 - numsamples;
	ptr3 = ptr2 + 1;
	ptr4 = ptr5 - 1;
	ptr6 = ptr5 + 1;
	ptr7 = ptr8 - 1;
	ptr8 = ptr5 + numsamples;
	ptr9 = ptr8 +1;
	
	ptr5 = ptr5*sizeof(float);
	ptr6 = ptr6*sizeof(float);
	ptr4 = ptr4*sizeof(float);

//here's the important part giving me trouble 

	data.seekg(0,ios::beg);
	size = data.tellg();
	cout<< "This is the first size:" << size << endl;
			   
	value[0] = 0; //clearing first array just to test things.
	value[1] = 0;
	value[2] = 0;
	//may erase things later
	
	data.seekg(ptr1, ios::beg);
	size = data.tellg();
	cout<< "this is the second size: " << size << endl;
	
	data.write((char*) &value, sizeof value);
	data.read((char*) &value, sizeof value);
	cout << value[0] << " -next value- " << value[1]
	<< " -next value- " << value[2] << endl;

Output:




This is the first size:-1
this is the second size: -1
0 -next value- 0 -next value- 0
this is what's in ptr 96876
This is the pixel val 5: 96876
96872
96880

You are calculating results before you know what they are: the value of ptr1 depends on ptr2, but it is calculated before the value of ptr2 is known.

This doesn't seem to make any sense. What are you doing with all those pointers, and what are their types? Where and to what have they been initialised? Why are you using the read position returned by tellg() as a size value?