Magic Numbers

[Brandon Allbery]

Expires:

Quoted from <416 at ihdev.UUCP> ["Re: magic numbers? (teach me, please)"], by pdg at ihdev.UUCP (P. D. Guthrie)...
+---------------
| In article <124 at rexago1.UUCP> rich at rexago1.UUCP (K. Richard Magill) writes:
| >	1)  How does the shell (exec?) know whether the command I just typed
| >		is a shell script or one of several possible types of
| >		executable?
| 
| The shell doesn't know.  The shell merely tells the kernel to exec the
| file, after doing a fork.  The kernel determines if a file is a binary
| executable by the magic number, which is obtained by reading an a.out.h
| structure (4.1,4.2) or filehdr.h (sys 5) and comparing it against
| hardcoded numbers in the kernel. In 4.1 for instance only 407,413 and
| 410 are legal.  This also tells the kernel the specific type of
| executable, and in some cases can set emulation modes. The kernel also
| recognizes 
| #! /your/shellname
| at the beginning of a file and execs off the appropriate shell instead.
+---------------

In 4.2, the #! is recognized.  In all other Unices, the exec will fail, and the
shell will decide that the file must be a shell script; it proceeds to fork off
a copy of itself to run the script.  (Csh on non-4.2 systems checks for a # as
the first character of the file, and forks itself if it sees it; if not, it
forks a /bin/sh.)

+---------------
| >	2)  Presuming the answer to #1 above has something to do with
| >		magic numbers, who issues them?  is there a common
| >		(definitive) base of them or does each
| >		manufacturer/environment make up their own set?
| 
| The magic number is issued by the linker/loader.  Pretty much the magic
| number is decided by the manufacturer, but from what I have seen, is
| kept constant over machines. Forgive me if this is wrong, but I do not
| have any method of checking, but the magic numbers for say plain
| executable 4.x Vax and plain executable SysV.x Vax are the same, but
| SysV.x Vax and SysV.x 3B20 are different.  Could someone comfirm this?
+---------------

Executables using ``standard'' binary formats, i.e. a.out (PDP-11, Z8000)
and b.out (MC68000) use the standard magic numbers 0405, 0407, 0410, 0411.
Non-standard formats, like Xenix x.out (0x0206) and COFF (flames to /dev/null;
most systems are [ab].out) use distinctive magic numbers.

There are other magic numbers.  Old-style archives (ar) have 0177545 as a
magic number; again, the loader knows about this, since a library is an
archive.  System V archives begin with the magic ``number'' "!<arch>\n".
Cpio archives also have magic numbers in them, but at the archive-member
level.
-- 

			Lord Charteris (thurb)

ncoast!allbery at Case.CSNet (ncoast!allbery%Case.CSNet at CSNet-Relay.ARPA)
..decvax!cwruecmp!ncoast!allbery (..ncoast!tdi2!root for business)
6615 Center St., Mentor, OH 44060 (I moved) --Phone: +01 216 974 9210
CIS 74106,1032 -- MCI MAIL BALLBERY (WARNING: I am only a part-time denizen...)