Question about file seeking position..

Question

My previous Question is about raw data reading and writing, but a new problem arised, it seems there is no ending....

The question is: the parameters of the functions like lseek() or fseek() are all 4 bytes. If i want to move a span over 4G, that is imposible. I know in Win32, there is a function SetPointer(...,Hign, Low,....), this pointers can generate 64 byte pointers, which is what i want.

But if i want to create an app in Linux or Unix (create a file or directly write the raw drive sectors), How can I move to a pointer over 4G?

Thanx, Waiting for your replies...

Answer 1

a 4 byte unsigned integer can represent a value up to 4294967295, which means if you want to move more than 4G, you need to use lseek64(). In addition, you can use fgetpos() and fsetpos() to change the position in the file.

Answer 2

The offset parameter of lseek is of type off_t. In 32-bit compilation environments, this type defaults to a 32-bit signed integer - however, if you compile with this macro defined before all system includes:

#define _FILE_OFFSET_BITS 64

...then off_t will be a 64-bit signed type.

For fseek, the fseeko function is identical except that it uses the off_t type for the offset, which allows the above solution to work with it too.

Answer 3

On Windows, use _lseeki64(), on Linux, lseek64().

I recommend to use lseek64() on both systems by doing something like this:

#ifdef _WIN32
#include <io.h>
#define lseek64 _lseeki64
#else
#include <unistd.h>
#endif

That's all you need.