Shortest code that raises a SIGSEGV

Question

Write the shortest code that raises a Segmentation Fault (SIGSEGV) in any programming language.

Answer 1

Phooey, Interpreter Bug, 2 bytes

<?

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This interpreter is so buggy it is beautiful. I can cause a bad free() with one byte of code in two ways. But that, unfortunately, is a SIGABRT. 😂

A quick check of the source code will tell you that the interpreter does not support wrapping. However, the bug isn't "lul I read backwards and it crashes", it is a little more in depth.

The Phooey code translates to these function calls:

< tape.left(1) // move left one cell
? debug() // print debug info

So let's step through it:

First, we call tape.left(1).

    void left(int64_t amount) {
        right(-amount);
    }

This turns into tape.right(-1):

    void right(int64_t amount = -1) {
        pointer += -1;

Adding -1 to pointer results it being -1, since the initial value was zero.

However, let's double check what type pointer is:

    size_t pointer = 0;
    size_t furthest = 0;

pointer is a size_t, which is an unsigned type, meaning that this -1 turns into a REALLY big number. Specifically, SIZE_MAX, the largest object size C++ is allowed to handle.

        if(pointer > furthest)
            furthest = pointer;

We assign pointer to furthest, because SIZE_MAX > 0.

Now, we call debug():

void Phooey::debug() {
    std::cout << "Tape: " << tape << std::endl;
}

And this calls the operator overload function for std::ostream << Tape, and here is our bug:

std::ostream& operator<<(std::ostream& stream, Tape& tape) {
    for(size_t i = 0; i <= tape.furthest; i++) {
        stream << tape[i];
    }
}

Let's substitute tape.furthest, if the bug wasn't clear enough:

    for(size_t i = 0; i <= SIZE_MAX; i++) {
        stream << tape[i];
    }

This loop will keep reading data from the tape pointer and printing it, until it segfaults. It is actually an infinite loop otherwise: Since tape.furthest is SIZE_MAX, it will always be true, as SIZE_MAX + 1 == 0.

How ironic that the bug occurs in the debug() function.

A simple way to fix this would be to add wrapping support, making sure that the pointer is always in range. As a matter of fact, almost every bug in this program can be fixed with range checks.

Bonus!

debug() is the buggiest function in this program. It segfaults even without putting any ?s in your code!

<3-509

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You may be saying, "How are you calling debug() without ??

And the answer is: ROP. 😂

Specifically, on this exact build of Phooey, the tape is stored on the stack, and cells[-3] contains the return address from call Phooey::run(), 0x403957.

509 bytes before that is just after Phooey::debug() sets up its stack frame. (If I set the pointer to the start of the function, it crashes too early to be interesting since the stack is unaligned). That is why I subtract 509.

It segfaults because it is definitely NOT called how the code expects it to be, and rdi doesn't have the this pointer, so it crashes.

This is OBVIOUSLY going to be dependent on the system and binary itself.

Answer 2

J (6)

memf 1

memf means free memory, 1 is interpreted as a pointer.

Answer 3

Unix PDP-11 assembly, 18 bytes binary, 7 bytes source

(this is becoming a theme with me, maybe because it's the only language I sort of know that no-one else here does.)

inc(r0)

Increments the single byte addressed by the initial value of r0 [which happens to be 05162 according to the simh debugger] as of program start.

0000000 000407 000002 000000 000000 000000 000000 000000 000000
0000020 005210 000000

And, as always, the extraneous bytes at the end can be removed with strip.

I made a few attempts to get the source shorter, but always ended up getting either a syntax error or SIGBUS.

Answer 4

x86 and x86_64 machine language, 3 bytes

0:       50                      push   %eax
1:       eb fd                   jmp    0

This pushes the value of the EAX (or RAX in long mode) register to the stack in a loop until the stack overflows.

To try this, compile and run the following C program.

const char main[]="\x50\xeb\xfd";

To try it on Windows, prepend the following to mark main as executable.

#pragma section("foo", execute)
__declspec(allocate("foo"))

Answer 5

Squire commit 93d3bf1, 6 bytes

" "*-1

A quite intriguing blunder.

My initial attempt was to allocateth \$\text{II}^\text{LXIV}-\text{I}\$ bytes of memory, but alas, Squire thwarteth me by limiting string lengths to \$\text{XXXII}\$-bit integers, and even my peasant computer with a mere \$\text{VIII}\$ gigabytes of RAM was impervious to said attacks, but lo! Even if malloc rewardeth null, Squire containeth a defense stronger than steel:

void *xmalloc(size_t length) {
        void *ptr = malloc(length);

        if (ptr == NULL) {
                fprintf(stderr, "error allocating %zu bytes of memory\n", length);
                // Alas, thwarted again with SIGABRT!
                abort();
        }

        return ptr;
}

Instead, I ventured into value.c and spied this:

        case SQ_TSTRING: {
                sq_number amnt = sq_value_to_number(rhs);

                struct sq_string *result = sq_string_alloc(AS_STRING(lhs)->length * amnt + 1);
                // ...
        }

As with most strings in C, Squire terminateth all strings with \N, so Sampersand addeth \$I\$ whilst calculating the length.

Intriguingly, \$\text{I} \times -{\text{I}} + \text{I} == \text{N}\$. Therefore, this rewardeth &sq_string_empty.

struct sq_string *sq_string_alloc(unsigned length) {
        if (length == 0)
                return &sq_string_empty;
        // ...
}

To be more swift, Squire containeth some "static" strings for common values like "false" or "".

struct sq_string {
        char *ptr;
        int refcount;
        unsigned length;
        bool borrowed;
};

struct sq_string sq_string_empty = {
    .ptr = "",
    .length = 0,
    .refcount = -1,
    .borrowed = false,
};

Curiously, we have a string literal treateth as char *, which, despite its mutable appearances, is actually a member of the evil .rodata clan. These sneaky impostors are indistinguishable from normal strings, unless one encanteth -Wwrite-strings.

Returning to value.c, the next line triggers the .rodata impostor:

                // Lo! Sampersand hath summoned nasal demons!
                *result->ptr = '\0';

In order to make thine computer repent from nasal demons, thy great and powerful Linux exorcises Squire with a SIGSEGV.

Answer 6

Ruby 3.x, 10 bytes

->x=(1in^x

Ruby doesn't like a circular pin reference in proc argument for some reason, this results in a null pointer dereference. Since this is a parser bug the program doesn't even have to be syntactically valid.

Answer 7

JavaScript Shell, 7 bytes

clear()

Clears absolutely everything, not just the current scope which obviously causes lots of borks which result in JS blowing up and segfaulting

Answer 8

Java (OpenJDK 9), 311 227 223 bytes

import sun.misc.*;import java.lang.reflect.*;class M{public static void main(String[]args) throws Exception{Constructor<Unsafe> c=Unsafe.class.getDeclaredConstructor();c.setAccessible(true);c.newInstance().getAddress(0);}}

Try it online!

Ungolfed:

import sun.misc.Unsafe;

import java.lang.reflect.Constructor;

public class SegFault {

    public static void main(String[] args) throws Exception {
        Constructor<Unsafe> unsafeConstructor = Unsafe.class.getDeclaredConstructor();
        unsafeConstructor.setAccessible(true);
        Unsafe unsafe = unsafeConstructor.newInstance();
        System.out.println(unsafe.getAddress(0));
    }
}

Saved 84 Bytes thanks to Mistah Figg

Answer 9

Befunge-98 (FBBI), 1 byte

=

Try it online! (expand the Debug section to view the segfault)

This only works in the FBBI implementation of Befunge, exploiting a bug in its string handling code. Any attempt to read a string from an empty stack will result in a null pointer dereference. You can achieve the same result with the i (Input File) and o (Output File) instructions, which also expect a string on the stack.

Note that this error wouldn't occur if the stack was simply full of zeros, which in Befunge should be semantically equivalent to an empty stack. For it to crash the stack must be genuinely empty, as is the case on startup.

Answer 10

Ruby, 15 bytes

eval a='eval a'

Segfaults (Ruby 2.3 on Ubuntu xenial)

Answer 11

Lua (luajit), 52 48 bytes

function f()c=coroutine;c.resume(c.create(f))end

An attempt to compete for the bounty that was recently set on an answer in Lua. This is a function submission (basically because I had to define a function anyway, so not including the code to run it saves bytes). Now with 4 bytes saved due to a suggestion by @ceilingcat to avoid repeating the word coroutine.

The program works by creating infinitely many coroutines, suspending each in turn to create and start the next. The most commonly used Lua interpreter, lua, thought of this case and starts causing coroutine creation to fail after a while. luajit, however, segfaults. (Valgrind reports the issue as a failure to grow the OS-defined stack; this is believable, seeing as one common coroutine implementation gives them each separate parts of the stack.)

Answer 12

Lua 5.3.2 PUC-RIO (the "official") interpreter - 57 bytes

local t={}t.__newindex=t local y=setmetatable({},t)y[1]=1

Note: does not work on all machines, and was fixed in Lua 5.3.3.

Answer 13

Tcl, 60 bytes

set a a;while {[incr i]<999999999} {set a [list $a]};puts $a

Not short, but crashes with a segfault.

This builds a deeply nested list (each with only one element), and when trying to serialize it, Tcl will crash with a stack overflow.

I have reported this bug here

Answer 14

Clean, 19 12 bytes

Start=code{}

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Every function always returns something in Clean, including Start. Because we haven't specified the type of Start, the compiler assumes (and it can only assume when you inline ABC) that it takes no arguments. Since it takes no arguments, there's nothing on either stack when the function resolves, and so the runtime tries to evaluate the first node in the spine of a graph with... Zero nodes.

Answer 15

Malbolge, 3046 bytes

Behold, an answer in Malbolge that you can embed inside an answer!

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Try it online!

Answer 16

Rust, 38 33 bytes

unsafe{*(std::ptr::null_mut())=0}

Try it online!

Full program is fn main(){...}. Segfaults as setting a null pointer to a type is undefined behavior (hence the unsafe block that is required). Old version:

unsafe{std::ptr::null::<u8>().read();}

Answer 17

Jelly, 2 bytes

ßß

Try it online!

oh

Since ß isn't actually coded as a recursive function call in the underlying Python, it isn't affected by the recursion limit. I don't actually know how Jelly's quicks work under the hood, but this appears to just create a nested structure that grows until it can't anymore. Any atom or atom-like quick works after the first ß.

Answer 18

NASM, 3 characters.

any of the following:

ret
nop
cbw
cwd
cdq
clc
cld
cli
cmc
aaa
aas
aad
aam
stc
std
sti

ret causes stack underflow. cli and sti trigger SIGSEGV because they are privileged instructions. All the other opcodes trigger a SIGSEGV simply because there's no exit point.

Answer 19

C++ (clang), compiler crash, 36 28 bytes

class{operator*(...){*this*1

Try it online!

Unlike the other compiler crash which I found on my own, I totally didn't look this one up on bugs.llvm.org..

Assembly (as, x64, Linux), 17 10 bytes

Unfortunately, this was not a segfault, but an assertion. Leaving it up with another compiler segfault as a bonus.

I wrote code that raises a SIGSEGV in assembler.

Literally. 😏

Replace \0 with a literal 0x00 byte.

"\0.asciz "

Try it online!

Found this bug when golfing the assembly here

Note that this bug seems to have been fixed as of v2.34.

Answer 20

Rust, 17 bytes

fn main(){main()}

Try it online!

Well... This just calls itself. It may be the only way to do it without unsafe.

Answer 21

Lua (LuaJIT), 47 43 bytes

f=require"ffi"f.cdef"int puts()"f.C.puts()

Try it online!

Uses FFI in LuaJIT to call puts() with no (valid) argument.

Answer 22

Perl 6, 22

shell "kill -11 $*PID"

Just shelling to whatever shell you have.

Answer 23

Dyvil, 42 bytes

dyvil.reflect.ReflectUtils.UNSAFE.getInt 0

Explanation:

dyvil.reflect.ReflectUtils // qualified type name
     .UNSAFE               // accesses the static field UNSAFE in class
                           // dyvil.reflect.ReflectUtils, of type sun.misc.Unsafe
     .getInt 0             // calls the method sun.misc.Unsafe.getInt(long),
                           // which tries to read a 4-byte integer from 
                           // the memory address 0

Answer 24

Whispers, 5 bytes

>> 1!

Try it online!

How it works

>> 1  - Call this line repeated
    ! - Prevent tail call

Answer 25

Haskell, 15 bytes

Causes a stackoverflow:

main=main>>main

Try it online!

Answer 26

Common Lisp (SBCL), 79 bytes.

SBCL captures pretty much every exception and signal, but we can cause an "Unhandled memory exception" which is the result of a SIGSEGV. We must tell SBCL to not consider type safety and just add a fixnum to a float, which ends up disastrous.

(defun f(x)(declare (optimize (safety 0))(fixnum x))(the fixnum (1+ x)))(f 0.0)

My SBCL image errors with:

Unhandled memory fault at #x14.
   [Condition of type SB-SYS:MEMORY-FAULT-ERROR]

Evaluating (f '(1 5)) returned a garbage object, then (gc) threw Lisp into the low-level debugger after it tried to GC that object presumably. I don't see the difference in results since it is possible to jump back into Lisp from this state, and I imagine this is 100% platform dependent behavior.

Answer 27

x86 .COM, 2 Bytes

66 61 POPAD

x86 .COM, 5 3 Bytes

A3 FF FF

Write to the segment border

Answer 28

WebAssembly (WaWrapper), 12 bytes

(func $main)

Try it online!

Answer 29

x86 machine code - 1 byte

08048060 <_start>:
 8048060:       5c                       pop    esp

This is change value of address esp into value 0x1, and then raises a SIGSEGV.

(gdb) disassemble _start
Dump of assembler code for function _start:
   0x08048060 : pop    esp
End of assembler dump.
(gdb) b *0x08048060
Breakpoint 1 at 0x8048060
(gdb) r
Starting program: /home/user/programming/assembly/pop 

Breakpoint 1, 0x08048060 in _start ()
(gdb) i r $esp
esp            0xbfffed40   0xbfffed40
(gdb) c
Continuing.

Program received signal SIGSEGV, Segmentation fault.
0x08048061 in ?? ()
(gdb) i r $esp
esp            0x1      0x1
(gdb) 

Answer 30

Zig, 26 bytes

pub fn main()void{main();}