PDP-11/40 videos

Andrew Burton aliensrcooluk at yahoo.co.uk
Thu Mar 15 17:00:50 CDT 2007

Dave McGuire <mcguire at neurotica.com> wrote:*>> snip <<*

   Nope, there is no such header in any UNIX implementation that I'm  
aware of.  The "magic number" you speak of isn't part of any  
header...the "file" program opens the target file, looks at the first  
few bytes, and then looks up the pattern in its database to arrive at  
an *educated guess* as to the type of file it's looking at...for  
example, if bytes 7-10 of the file are 0x4a464946 (ascii "JFIF"), it  
is most likely (but not definitely!) a JPEG image file.  Similarly,  
if bytes 1-6 are 0x474946383961 (ascii "GIF89a") the file is most  
likely a v89a GIF image file, and if bytes 1-8 are 0xfeedface, it's a  
Mach-O executable from a MacOS X system.

    It is important to understand, though, that this has nothing at  
all to do with the operating system, and there is no common header  
format of any sort.  It just so happens that many types of files are  
consistent in what their first few bytes contain.  

Is the 0xfeedface ascii or hex?! ;) Was that deliberate or purely accidental, and who was behind it?

I agree with what you are saying. When i was writing a program that involved using images on my Amiga I discovered that the .IFF files always had the same ASCII within the first 20 bytes (well bytes 1-8 and 16-20, or thereabouts). I never realised that the same was true about most file types.

Andrew B
aliensrcooluk at yahoo.co.uk

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