Daniel Mangum hasheddan

Division By Zero Exceptions

Setup

On an embedded project, I recently had to debug a crash where the root cause was a division by zero. The offending code in question reduced to something like function foo() presented here.

To validate that it actually was a division by zero problem, I added the if block that printed "GOTCHA". After this change, I saw that the processor status register (CFSR) had the UNDEFINST bit set instead of the exepected DIVBYZERO bit. What?

Using godbolt we can see the compiler emitting an instruction 0xdeff in main... with no other code (suggesting that the compiler realized that the code will not work and simply stopped -- without warning, I might add).

How to use PlantUML with Markdown

PlantUML is a really awesome way to create diagrams by writing code instead of drawing and dragging visual elements. Markdown is a really nice documentation tool.

Here's how I combine the two, to create docs with embedded diagrams.

Step 0: Setup

Get the command-line PlantUML from the download page or your relevant package manager.

Swift Concurrency Manifesto

Author: Chris Lattner

Chinese Translation by Jason Yu

	// -- 调用者（操作系统内核）

	extern "sbicall" fn sbi_call(param: [usize; 6], a6: usize, a7: usize) -> (usize, usize);
	// 最后两个参数必须是两个usize类型，a6: usize和a7: usize
	// 除了最后两个参数，前面的参数必须是usize、[usize; N]或(usize, usize, ..)类型
	// 依次会被填写到a0, a1, ..., a5寄存器中。

	// 返回值可以是(usize, usize), [usize; 2]或者SbiRet（包含两个usize的结构体）

	fn kernel() {

	#!/usr/bin/env bash

	set -e

	CONTEXT="$1"

	if [[ -z ${CONTEXT} ]]; then
	echo "Usage: $0 KUBE-CONTEXT"
	exit 1
	fi

	#include <iostream>
	#include <stdio.h>
	#include <stdlib.h>
	#include <linux/ioctl.h>
	#include <linux/types.h>
	#include <linux/v4l2-common.h>
	#include <linux/v4l2-controls.h>
	#include <linux/videodev2.h>
	#include <fcntl.h>
	#include <unistd.h>

	package main

	import (
	"encoding/json"
	"fmt"
	"log"
	"net"
	"os"
	"time"
	)

	section .text
	global _start
	_start:
	mov eax, 2 ; SYS_FORK Op Code
	int 0x80
	cmp eax, 0 ;If the return value is 0, we are in the child process
	jz child

	parent:
	mov edx, len ;Move msg length to edx