Modifying file hex values

[squarey]

Hey Guys

I am working on an app which requires the modification of hex values of files. I have two questions:

1. Does anyone know where I can find a list of hex file markers E.g. each segament in a JPEG file starts with 0xFF followed by a byte indicating the type of marker. So 0FFD8 is the beginning of a JPEG image.

2. I have noticed that whenever I modify some files like RIFF, when I run then they need to be 'repaired' (in VLC player) before they can be run. Anyone know why this happens and how I can overcome this problem ?

[Salem]

> 1. Does anyone know where I can find a list of hex file markers
A search engine?
Pre-digested results:
http://www.softpedia.com/get/System/File-Management/File-identifier.shtml
http://www.forensicswiki.org/wiki/File_Format_Identification

> 2. I have noticed that whenever I modify some files like RIFF, when I run then they need to be 'repaired'
You didn't do a good enough job.
Many parameters are inter-dependent. For example, changing the width and height may mean you also have to change the file length as well.
If you do a half-job, then some utilities may gloss over the defects, some may "repair" and some may just say "invalid file".

[jwenting]

http://www.wotsit.org has file format information for hundreds if not thousands of formats.

[squarey]

> 1. Does anyone know where I can find a list of hex file markers
A search engine?
Pre-digested results:
http://www.softpedia.com/get/System/File-Management/File-identifier.shtml
http://www.forensicswiki.org/wiki/File_Format_Identification

Nice apps but I am looking more for theory. I can find file markers for image file formats but not many others outside that scope.

> 2. I have noticed that whenever I modify some files like RIFF, when I run then they need to be 'repaired'
You didn't do a good enough job.
Many parameters are inter-dependent. For example, changing the width and height may mean you also have to change the file length as well.
If you do a half-job, then some utilities may gloss over the defects, some may "repair" and some may just say "invalid file".

Yeh, its alittle tedious trying to do that but I guess it needs to be done. If you have any good reads on file segment dependancy then please do share.

[squarey]

http://www.wotsit.org has file format information for hundreds if not thousands of formats.

Brilliant!! Top stuff, thanks!