When parsing NES roms, the first four bytes are a 'magic' number:
78/0x4E (N)
69/0x45 (E)
83/0x53 (S)
26/0x1A (DOS end of file character)
What purpose does this, or any other examples, provide?
So-called "magic numbers" are often specified by binary file formats as a kind of "signature" of the file format. Programs that read the file format can check for the magic number and reject the file as invalid if it doesn't match. This provides an easy way to catch garbage data early. It also allows programs that understand multiple formats to figure out what type of file they're reading, so it can be processed appropriately.
In the case of iNES format ROM images, it's no different. The purpose of the magic number in the header is to tell the emulator (or other program) that this is an iNES format image and that it should be parsed as such.
Related
I have a large binary file (~ GB size) generated from a Fortran 90 program. I want to modify something in the head part of the file. The structure of the file is very complicated and contains many different variables, which I want to avoid going into. After reading and re-writing the head, is it possible to "copy and paste" the reminder of the file without knowing its detailed structure? Or even better, can I avoid re-writing the whole file altogether and just make changes on the original file? (Not sure if it matters, but the length of the header will be changed.)
Since you are changing the length of the header, I think that you have to write a new, revised file. You could avoid having to "understand" the records after the header by opening the file with stream access and just reading bytes (or perhaps four byte words if the file is a multiple of four bytes) until you reach EOF and copying them to the new file. But if the file was originally created as sequential access and you want to access it that way in the future, you will have to handle the record length information for the header record(s), including altering the value(s) to be consistent with the changed the length of the record(s). This record length information is typically a four-byte integer at beginning and end of each record, but it depends on the compiler.
I have a mainframe data file in binary format, with variable records. No copybook works in this case, nor do I know end of line. How do I read such a file?
Assuming you're reading this file in a COBOL program running on the Mainframe, this is really no problem. COBOL doesn't write null-delimited output. It writes variable length records with the length embedded in the first two bytes of 4-byte prefix area called a (R)ecord (D)escriptor (W)ord, which is NOT included in the record layout copybook. To read such a record back into another COBOL, you just need a properly coded copybook.
Is there any C# way to check an ISO file is valid or not i.e. valid Iso format or any other check possible or not.
The scenario is like, if any text file(or any other format file) is renamed to ISO and given it for further processing. I want to check weather this ISO file is a valid ISO file or not? Is there any way exist programmatically like to check any property of the file or file header or any other things
Thanks for any reply in advance
To quote the wiki gods:
There is no standard definition for ISO image files. ISO disc images
are uncompressed and do not use a particular container format; they
are a sector-by-sector copy of the data on an optical disc, stored
inside a binary file. ISO images are expected to contain the binary
image of an optical media file system (usually ISO 9660 and its
extensions or UDF), including the data in its files in binary format,
copied exactly as they were stored on the disc. The data inside the
ISO image will be structured according to the file system that was
used on the optical disc from which it was created.
reference
So you basically want to detect whether a file is an ISO file or not, and not so much check the file, to see if it's valid (e.g. incomplete, corrupted, ...) ?
There's no easy way to do that and there certainly is not a C# function (that I know of) that can do this.
The best way to approach this is to guess the amount of bytes per block stored in the ISO.
Guess, or simply try all possible situations one by one, unless you have an associated CUE file that actually stores this information. PS. If the ISO is accompanied by a same-name .CUE file then you can be 99.99% sure that it's an ISO file anyway.
Sizes would be 2048 (user data) or 2352 (raw or audio) bytes per block. Other sizes are possible as well !!!! I just mentioned the two most common ones. In case of 2352 bytes per block the user data starts at an offset in this block. Usually 16 or 24 depending on the Mode.
Next I would try to detect the CD/DVD file-systems. Assume that the image starts at sector 0 (although you could for safety implement a scan that assumes -150 to 16 for instance).
You'll need to look into specifics of ISO9660 and UDF for that. Sectors 16, 256 etc. will be interesting sectors to check !!
Bottom line, it's not an easy task to do and you will need to familiarize yourself with optical disc layouts and optical disc file-systems (ISO9660, UDF but possibly also HFS and even FAT on BD).
If you're digging into this I strongly suggest to get IsoBuster (www.isobuster.com) to help you see what the size per block is, what file systems there are, to inspect the different key blocks etc.
In addition to the answers above (and especially #peter's answer): I recently made a very simple Python tool for the detection of truncated/incomplete ISO images. Definitely not validation (which as #Jake1164 correctly points out is impossible), but possibly useful for some scenarios nevertheless. It also supports ISO images that contain Apple (HFS) partitions. For more details see the following blog post:
Detecting broken ISO images: introducing Isolyzer
And the software's Github repo is here:
Isolyzer
You may run md5sum command to check the integrity of an image
For example, here's a list of ISO: http://mirrors.usc.edu/pub/linux/distributions/centos/5.4/isos/x86_64/
You may run:
md5sum CentOS-5.4-x86_64-LiveCD.iso
The output is supposed to be the same as 1805b320aba665db3e8b1fe5bd5a14cc, which you may find from here:
http://mirrors.usc.edu/pub/linux/distributions/centos/5.4/isos/x86_64/md5sum.txt
I want to write unformatted (binary) data to STDOUT in a Fortran 90 program. I am using AIX Unix and unfortunately it won't let me open unit 6 as "unformatted". I thought I would try and open /dev/stdout instead under a different unit number, but /dev/stdout does not exist in AIX (although this method worked under Linux).
Basically, I want to pipe my programs output directly into another program, thus avoiding having an intermediate file, a bit like gzip -c does. Is there some other way I can achieve this, considering the two problems I have encountered above?
I would try to convert the data by TRANSFER() to a long character and print it with nonadvancing i/o. The problem will be your processors' limit for the record length. If it is too short you will end up having an unexpected end of record sign somewhere. Also your processor may not write the unprintable characters the way you would like.
i.e., something like
character(len=max_length) :: buffer
buffer = transfer(data,buffer)
write(*,'(a)',advance='no') trim(buffer)
The largest problem I see in the unprintable characters. See also A suprise with non-advancing I/O
---EDIT---
Another possibility, try to use file /proc/self/fd/1 or /dev/fd/1
test:
open(11,file='/proc/self/fd/1',access='stream',action='write')
write(11) 11
write(11) 1.1
close(11)
end
This is more of a comment/addition to #VladimirF than a new answer, but I can't add those yet. You can first inquire about the location of the preconnected I/O units and then open the unformatted connection:
character(1024) :: stdout
inquire(6, name = stdout)
open(11, file = stdout, access = 'stream', action = 'write')
This is probably the most convenient way, but it uses stream access, a Fortran 2003 feature. Without this, you can only use sequential access (which adds header data to each record) or direct access (which does not add headers but requires a fixed record length).
From what I understand, \r\n are special characters (correct?). Is there any possibility that an audio file can contain line breaks?
What I'm trying to do is send both line broken strings and full audio files to my socket and I'm just curious if detecting line breaks will ever stop me in the middle of the file.
An audio file may contain any byte or byte sequence, so trying to detect any specific sequence in the middle of the stream is bound to fail eventually. If we assume the audio is essentially random, the odds of any 2-byte sequence equaling \r\n is about 1/65536.
What's a "special character"? There is no such thing except in some relative sense you specify. Your audio stream might well contain bytes with value 10 (\n) or 13 (\r). You need some way to distinguish text from audio data in your stream other than looking for newlines.
A line break is just chr(11) (or some other number). Your data will likely contain a byte with a value of 11. However, you shouldn't be converting it to characters anyway, just read it as a byte.
But yeah, injecting line breaks would corrupt the file, but if you just read is as a stream of bytes treated as bytes you'll be fine.
Interpretation of line break bytes only applies when processing textual data. Audio is binary data instead. You should not be processing audio as text.