reading binary file stuff

[nbo10]

Hi all,
I'm reading a binary file. I come to a point where 64 bits is a pointer to a position in the file, say 9C 18 00 00. and the doc says that the order fo the bytes is little endian. How do I go to that position in the file? Thanks

[dwhitney67]

Specify your programming language; also, provide your apps requirements. Your OP was a little difficult to digest.

P.S. 9C 18 00 00 is represented using 4 bytes (32 bits).

[jpkotta]

Hi all,
I'm reading a binary file. I come to a point where 64 bits is a pointer to a position in the file, say 9C 18 00 00. and the doc says that the order fo the bytes is little endian. How do I go to that position in the file? Thanks

Little endian means it's written backwards, but (IMHO) is actually in the correct order*. You can think of it as being stored in an array: [0x9C, 0x18, 0x00, 0x00] where the least significant byte (LSB, the "little end") comes first. Written as a 32-bit hex number, it would be 0x0000189C.

Many languages use the fseek() and related functions to set the position in a file handle. E.g. in python:


import os
fd = open(filename_str, "a+b")
fd.seek(0x189C, os.SEEK_SET) # SEEK_SET means offset from beginning of file


* I say correct because it's always the same pattern. If you have a 16-bit value and want to convert it to 32-bit, you can just append 0s (or 1s). With big endian, you have to move bytes around. Neither big nor little endian is really correct in any absolute sense, though.

[nbo10]

ahh... I forgot to say I'm using c++, or I could use python. I don't know why I can't write code to read binary files. This is the second project that I've had to read some binary data files. The last project I crashed and burned.

[Can+~]

Binary files are nothing special. If you need to visualize the content of the file, download ghex.


sudo apt-get install ghex

A little something on binary files:

In C, using the fread(...) function, you can load directly the content of the binary file into a struct or data type, like:


struct mystruct
{
int number;
char str[20];
}

Then, if the file looks like:


0f 00 00 00 48 65 6c 6c 6f 20 57 6f 72 6c 64 0a 00 25 c7 bf a9 85 04 08

It will load the data trying to fit it directly on the struct, just like a memcpy.



|--number--||------------------------char[20]--------------------------|
0f 00 00 00 48 65 6c 6c 6f 20 57 6f 72 6c 64 0a 00 25 c7 bf a9 85 04 08

(In this case, this is 15 and Hello World\n\0).

In python, you do a similar thing, although it uses a perl-like (and php-like) struct format string (http://docs.python.org/library/struct.html):


"i20s"

(integer and 20-byte string)

And for the original question, you just read the number into an int, and use fseek() to that position (SEEK_SET or SEEK_CUR, depending on the specification).

[jpkotta]

ahh... I forgot to say I'm using c++, or I could use python. I don't know why I can't write code to read binary files. This is the second project that I've had to read some binary data files. The last project I crashed and burned.

Yes, but that Python code is not much more than a wrapper around the C library. In C, it would be more like


#include <stdio.h>

...

FILE fh = fopen(filename_str, "a+b");
fseek(fh, 0x0000189C, SEEK_SET);


There is probably a more C++-y way of doing it, but I don't really work with C++.

If this is a popular binary format, there is probably a library you can use to access the file.

[dwhitney67]

Yes, but that Python code is not much more than a wrapper around the C library. In C, it would be more like


#include <stdio.h>

...

FILE fh = open(filename_str, "a+b");
fseek(fh, 0x0000189C, SEEK_SET);


There is probably a more C++-y way of doing it, but I don't really work with C++.

If this is a popular binary format, there is probably a library you can use to access the file.

Yes, that is right... you need to include a header file, introduce a typo in the C syntax, and basically accomplish the same thing as posted in the Python example.

Jeez...

[nbo10]

Thanks. The code is coming along now. reading binary stuff is pretty easy in python.