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Rev

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Shortest code to throw SIGILL

Background

We already have a challenge about throwing SIGSEGV, so why not a challenge about throwing SIGILL?

What is SIGILL?

SIGILL is the signal for an illegal instruction at the processor, which happens very rarely. The default action after receiving SIGILL is terminating the program and writing a core dump. The signal ID of SIGILL is 4. You encounter SIGILL very rarely, and I have absolutely no idea how to generate it in your code except via sudo kill -s 4 <pid>.

Rules

You will have root in your programs, but if you don't want to for any reasons, you may also use a normal user. I'm on a Linux computer with German locale and I do not know the English text which is displayed after catching SIGILL, but I think it's something like 'Illegal instruction'. The shortest program which throws SIGILL wins.

Answer*

# PDP-11 Assembler (UNIX Sixth Edition), 1 byte

Instruction 9 is not a valid instruction on the PDP-11 (in octal, it would be `000011`, which does not appear on [the list of instructions](http://www.cdf.toronto.edu/~ajr/258/pdp11.pdf) (PDF)). The PDP-11 assembler that ships with UNIX Sixth Edition apparently echoes everything it doesn't understand into the file directly; in this case, 9 is a number, so it generates a literal instruction 9. It also has the odd property (unusual in assembly languages nowadays) that files start running from the start, so we don't need any declarations to make the program work.

You can test out the program using [this emulator](http://pdp11.aiju.de/), although you'll have to fight with it somewhat to input the program.

Here's how things end up once you've figured out how to use the filesystem, the editor, the terminal, and similar things that you thought you already knew how to use:

% a.out
 Illegal instruction -- Core dumped

I've confirmed with the documentation that this is a genuine `SIGILL` signal (and it even had the same signal number, 4, all the way back then!)

If this is an answer to a challenge…

…Be sure to follow the challenge specification. However, please refrain from exploiting obvious loopholes. Answers abusing any of the standard loopholes are considered invalid. If you think a specification is unclear or underspecified, comment on the question instead.
…Try to optimize your score. For instance, answers to code-golf challenges should attempt to be as short as possible. You can always include a readable version of the code in addition to the competitive one. Explanations of your answer make it more interesting to read and are very much encouraged.
…Include a short header which indicates the language(s) of your code and its score, as defined by the challenge.

More generally…

…Please make sure to answer the question and provide sufficient detail.
…Avoid asking for help, clarification or responding to other answers (use comments instead).

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1

\$\begingroup\$ It had the same signal number because POSIX and UNIX and the SUS are closely related :) \$\endgroup\$

cat
– cat

2016年11月20日 13:57:04 +00:00
Commented Nov 20, 2016 at 13:57
6

\$\begingroup\$ Almost all of the signal numbers in V6 still have the same meanings today; the mnemonics have actually been less stable than the numbers. Compare minnie.tuhs.org/cgi-bin/utree.pl?file=V6/usr/sys/param.h with github.com/freebsd/freebsd/blob/master/sys/sys/signal.h — identical semantics for 1 through 13, but only 1, 2, and 13 have exactly the same names. (SIGALRM/14 and SIGTERM/15 were only added in V7.) (The System V lineage has a couple of changes, notably moving SIGBUS from 10 to 7 (replacing the useless SIGEMT) and SIGSYS above 15, to make room for SIGUSR1 and SIGUSR2.) \$\endgroup\$

zwol
– zwol

2016年11月20日 19:35:44 +00:00
Commented Nov 20, 2016 at 19:35
4

\$\begingroup\$ @cat POSIX and SUS don't actually specify the values of signals - they do specify the meaning of some numbers when passed as arguments to the kill command, but SIGILL is not included. \$\endgroup\$

Random832
– Random832

2016年11月20日 23:16:57 +00:00
Commented Nov 20, 2016 at 23:16
10

\$\begingroup\$ a.out has multiple bytes in it, indeed (the 9 instruction compiles to two bytes, and the assembler also adds a header and footer to make the program executable). That's why I wrote the program in assembly language, not in machine code. The assembly language program has only one byte in, and compiles to a program with more bytes in; this is a code-golf problem (minimize the size of the source), not a sizecoding problem (minimize the size of the executable), so it's the 1-byte size of the source that matters. \$\endgroup\$

user62131
– user62131

2016年11月23日 02:39:34 +00:00
Commented Nov 23, 2016 at 2:39
2

\$\begingroup\$ Very clever abuse of an old but awesome system. \$\endgroup\$

Mast
– Mast

2016年11月23日 11:45:45 +00:00
Commented Nov 23, 2016 at 11:45

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