Reading specific memory blocks with C++

genderbender · 12-01-2005, 02:03 PM

Hello, Ive been reading this tutorial : http://www.cplusplus.com/doc/tutorial/files.html (just to prove i have done some research).

It demonstrates a way of copying a file to a specific memory block. I'd like to know if theres a way of retrieving the contents of a specific memory location, in my case the contents of the memory block is an integer which id like to be returned to stdout. Any ideas?

Thanks

Mara · 12-01-2005, 03:25 PM

Well..yes. If you don't care about values beeing big or little-endian you can simply cast the pointer to int or long or whatever type and read it.

Code:

char memblock[something];
int *myval_ptr;
int myval;
myval_ptr = (int *)(memblock+someoffset);
myval = *myval_ptr;

genderbender · 12-01-2005, 06:06 PM

Thats brilliant, exactly what I'm after! Thanks, i presume its the same for arrays?

Thanks for your quick reply, I really appreciate it.

Mara · 12-03-2005, 02:55 PM

Yep..Array is just a block of memory starting from pointer arr when you have something like char arr[something].

Note that you should be careful when using this method, because one small mistake and you get a segmentation fault. Nothing will stop you from accessing memory your program doesn't own (well.. the system will stop you...).

bamdiedwergie · 12-23-2007, 05:24 PM

I have tried the example of reading the memory blocks in the binary file reading,using pointers as in the example . i used a loop just to seeeach value , the strang thing is that all the numbers are the same.I have tried with all data types possible .

the code is
....
file.seekg (0, ios::beg);
file.read (memblock, size);
file.close();

cout << "the complete file content is in memory " <<"\n";

unsigned char *myval_ptr;
unsigned char myval;
for (int i = 0;i < size;i++)
{
myval_ptr = (unsigned char *)(memblock+i);
cout << myval <<"\t";
};

The file is an image file
there is a header with name , surname , date , and aquisition parameters
followed by the image data .