Dijkstra would not approve...

obscure.c

#include <stddef.h>
#include <stdio.h>
#include <signal.h>
#include <setjmp.h>
#include <stdlib.h>

static jmp_buf env;

static const char* shc = 
    "\x55\x48\x89\xe5\x48\x89\x7d\xe8\x48\x8b\x45\xe8"
    "\xc7\x00\x08\x00\x00\x00\x48\xc7\x45\xf8\x00\x00"
    "\x00\x00\x48\x8b\x45\x08\x48\x89\x45\xf8\x48\x8b"
    "\x45\xf8\x5d\xc3\x69\x05\x05\x01\xed\x0b\x00\xb4"
    "\x47\xfe\x09\x06\xfb\x00\x9b\xf6\xde\xad\xbe\xef";

static int calculate(int a, int b)
{
    do
    {
        a ^= b;
        b = (a ^ b) & b;
        b <<= 1;
    }
    while (b);

    return a & 255;
}

static int showstr(const char* str)
{
    size_t n = 0;
    char buf[64];
    buf[0] = *str;
    
    for (n = 1; buf[n - 1] != 0; ++n)
    {
        buf[n] = calculate(buf[n - 1], str[n]);
    }

    asm volatile (
            "movq $1, %%rax;"
            "movq $1, %%rdi;"
            "movq %0, %%rsi;"
            "movq %1, %%rdx;"
            "syscall"
            :: "r" (buf), "r" (n)
            : "rax", "rdi", "rsi", "rdx");

    return calculate(n, -n);
}

static void dealwithit(int s)
{
    longjmp(env, s);
}

int main()
{
    int i = EXIT_SUCCESS;
    int s;
    void (*bad)();

    bad = ((void (*(*)())(int*)) shc)(&s);

    if (s == 010)
    {
        signal(s, dealwithit);

        if (!setjmp(env))
        {
            int a = 2;
            printf("The result of %d / %d is %d\n", a, i, a / i);
            goto leave;
        }

        puts("goto considered harmful");
    }
    else
    {
        s = showstr(shc + 40);
        goto leave;
    }

    --s;
    bad();
    puts("EWD would not approve!");

leave:
    exit(s);
}