how to pass reference of a pointer with parallelism? - pointers

I'm writing a bot to run some commands in parallel and at the same time run the bots in parallel, but I'm having trouble starting and pausing functions.
Below I'll leave an example I set up. It was expected that one of the bots would continue to run and others would stop, but all would end up running.
Could someone explain to me why, when using the startbot() command, it does not get bool?
package main
import (
"log"
"time"
)
type botBase struct {
isEnabled bool
}
func (b *botBase) startFunctionX() {
b.isEnabled = true
}
func (b *botBase) pauseFunctionX() {
b.isEnabled = false
}
func (b botBase) runCommandX() {
for {
if b.isEnabled {
log.Print("running...")
} else {
log.Print("paused...")
}
time.Sleep(1 * time.Second)
}
}
type bot struct {
botBase
//other stuffs
}
func (b bot) runAllCommands() {
go b.runCommandX()
//wait parallels commands
for{
time.Sleep(10 * time.Hour)
}
}
type bots struct {
List []bot
}
func (b *bots) loadListDB() {
b1 := bot{}
b1.isEnabled = false
b2 := bot{}
b2.isEnabled = false
b.List = []bot{b1, b2}
}
var myBots bots
func main() {
myBots.loadListDB()
for _, b := range myBots.List {
b.startFunctionX()
go b.runAllCommands()
}
//control stop and start bots
log.Print("expected true = ", myBots.List[0].isEnabled)
myBots.List[0].pauseFunctionX()
log.Print("expected false = ", myBots.List[0].isEnabled)
//wait bots parallels
for {
time.Sleep(10 * time.Hour)
}
}

the range statement returns the value of a bot which is then changed so you're actually checking a different bot.. work with references -
for i := range myBots.List {
b := &myBots.List[i]
b.startFunctionX()
go b.runAllCommands()
}
https://play.golang.org/p/1V8tKx431QJ

Related

go - software panics for concurrent access despite being handled by mutexes

i have a map that is being read and written by 3 goroutines constantly, the program always ends up with a "fatal error: concurrent map iteration and map write" despite me setting up the mutex to protect it, I know I could use sync.Map or I could sync with a channel but I'd really like to understand what I am doing wrong. this is the code:
//book.go
type OrderbookMap map[float64]float64
type Orderbook struct {
Bids OrderbookMap
Asks OrderbookMap
Symbol string
IsInit bool
UpdateId int
mu sync.Mutex
}
func (book *Orderbook) Init() {
book.mu.Lock()
defer book.mu.Unlock()
if book.IsInit {
return
}
book.Asks = make(OrderbookMap)
book.Bids = make(OrderbookMap)
book.IsInit = true
}
//functions with mutexes
func DelBid2(b *Orderbook, price float64) {
b.mu.Lock()
defer b.mu.Unlock()
if _, ok := b.Bids[price]; ok {
delete(b.Bids, price)
} else {
fmt.Printf("VALUE NOT FOUND %v\n", price)
}
}
func AddBid2(b *Orderbook, price float64, qty float64) {
b.mu.Lock()
defer b.mu.Unlock()
b.Bids[price] = qty
}
func GetBids2(b *Orderbook) OrderbookMap {
b.mu.Lock()
defer b.mu.Unlock()
return b.Bids
}
//TesterFile.go
func TestBookRace(t *testing.T) {
var B Orderbook
B.Init()
//add
go func() {
for {
b, q := rFloat(), rFloat()
AddBid2(&B, b, q)
fmt.Printf("ADD %v NEW: %v\n", b, GetBids2(&B))
}
}()
//del
go func() {
for {
b := rFloat()
DelBid2(&B, b)
fmt.Printf("DEL %v NEW: %v\n", b, GetBids2(&B))
}
}()
//read
go func() {
for {
fmt.Printf("READ %v\n", GetBids2(&B))
}
}()
for { time.Sleep(10 * time.Second)}
}

goroutine soon blocked the http server when it was requested

goroutine sooblocked the http server when it was reqn uested
The following code will soon be blocked
In a device management function, by visiting the http REST ful interface to determine whether the device is online, 30s access to 1000 devices, the current program is roughly as follows to see the number of goroutine is not very high, but soon the program will not Move, cpu, memory is not occupied too high
package main
import (
"fmt"
"net/http"
"runtime"
"time"
)
func a() {
b()
//.....
}
var bb = 0
func b() {
fmt.Printf("b:%d\n", bb)
bb++
resp, err := http.Get("http://www.baidu.com")
if err == nil {
resp.Body.Close()
}
//...
}
func c() {
t := time.NewTicker(time.Second * 30)
for {
fmt.Printf("start time:%s\n", time.Now().Format("15:04:05"))
bb = 0
for i := 0; i < 1000; i++ {
go a()
if i%11 == 0 {
time.Sleep(time.Millisecond * 300)
fmt.Printf("i:%d go:%d\n", i, runtime.NumGoroutine())
}
}
<-t.C
fmt.Printf("over time:%s\n", time.Now().Format("15:04:05"))
}
}
func main() {
go c()
for {
}
}
block
The following code will not blockļ¼ŒThis is why, hope to give me some advice, thank you
package main
import (
"fmt"
"net/http"
"runtime"
"time"
)
func a() {
b()
}
var bb = 0
func b() {
fmt.Printf("b:%d\n", bb)
bb++
resp, err := http.Get("http://www.baidu.com")
if err == nil {
resp.Body.Close()
}
}
func main() {
for {
for {
go b()
time.Sleep(time.Millisecond * 10)
fmt.Printf("go:%d\n", runtime.NumGoroutine())
}
}
no-block
I think there is no switching point.
the Go scheduler is non preemptive. (cooperative)
all goroutines must be cooperative of scheduling
func main() {
go c()
for {
// it is not cooperative
}
}
the Go scheduler can switch only at specific points.
specific points is I/O, chan, Sleep, Gosched
try below code on block example
func main() {
go c()
for {
runtime.Gosched() // or time.Sleep(any)
}
}
I hope this would help you

Cannot Call pointer method on and Cannot take address of [maps golang] [duplicate]

I'm learning Go currently and I made this simple and crude inventory program just to tinker with structs and methods to understand how they work. In the driver file I try to call a method from and item type from the items map of the Cashier type. My method have pointer reciever to use the structs directly instead of making copies. When I run the program I get this error .\driver.go:11: cannot call pointer method on f[0]
.\driver.go:11: cannot take the address of f[0]
Inventory.go:
package inventory
type item struct{
itemName string
amount int
}
type Cashier struct{
items map[int]item
cash int
}
func (c *Cashier) Buy(itemNum int){
item, pass := c.items[itemNum]
if pass{
if item.amount == 1{
delete(c.items, itemNum)
} else{
item.amount--
c.items[itemNum] = item
}
c.cash++
}
}
func (c *Cashier) AddItem(name string, amount int){
if c.items == nil{
c.items = make(map[int]item)
}
temp := item{name, amount}
index := len(c.items)
c.items[index] = temp
}
func (c *Cashier) GetItems() map[int]item{
return c.items;
}
func (i *item) GetName() string{
return i.itemName
}
func (i *item) GetAmount() int{
return i.amount
}
Driver.go:
package main
import "fmt"
import "inventory"
func main() {
x := inventory.Cashier{}
x.AddItem("item1", 13)
f := x.GetItems()
fmt.Println(f[0].GetAmount())
}
The part of the code that really pertains to my problem is the GetAmount function in inventory.go and print statement in the driver.go
A map entry cannot be addressed (as its address might change during map growth/shrink), so you cannot call pointer receiver methods on them.
Detail here: https://groups.google.com/forum/?fromgroups=#!topic/golang-nuts/4_pabWnsMp0
As Volker said in his answer - you can't get address of an item in the map. What you should do - is to store pointers to items in your map, instead of storing item values:
package main
import "fmt"
type item struct {
itemName string
amount int
}
type Cashier struct {
items map[int]*item
cash int
}
func (c *Cashier) Buy(itemNum int) {
item, pass := c.items[itemNum]
if pass {
if item.amount == 1 {
delete(c.items, itemNum)
} else {
item.amount--
}
c.cash++
}
}
func (c *Cashier) AddItem(name string, amount int) {
if c.items == nil {
c.items = make(map[int]*item)
}
temp := &item{name, amount}
index := len(c.items)
c.items[index] = temp
}
func (c *Cashier) GetItems() map[int]*item {
return c.items
}
func (i *item) GetName() string {
return i.itemName
}
func (i *item) GetAmount() int {
return i.amount
}
func main() {
x := Cashier{}
x.AddItem("item1", 13)
f := x.GetItems()
fmt.Println(f[0].GetAmount()) // 13
x.Buy(0)
f = x.GetItems()
fmt.Println(f[0].GetAmount()) // 12
}
http://play.golang.org/p/HkIg668fjN
While the other answers are useful, I think in this case it is best just to make non-mutating functions not take a pointer:
func (i item) GetName() string{
return i.itemName
}
func (i item) GetAmount() int{
return i.amount
}

Go improper struct initialization?

While coding I encountered a problem. When I use method of inner struct in goroutine, I can't see inner state like in this code.
package main
import (
"fmt"
"time"
)
type Inner struct {
Value int
}
func (c Inner) Run(value int) {
c.Value = value
for {
fmt.Println(c.Value)
time.Sleep(time.Second * 2)
}
}
type Outer struct {
In Inner
}
func (c Outer) Run() {
go c.In.Run(42)
for {
time.Sleep(time.Second)
fmt.Println(c.In)
}
}
func main() {
o := new(Outer)
o.Run()
}
Program printing:
from inner: {42}
from outer: {0}
from outer: {0}
from inner: {42}
from outer: {0}
from inner: {42}
from outer: {0}
from outer: {0}
Maybe it's pointer problem, but I don't know how resolve it.
The most obvious error in your code is that Inner.Run() has a value-receiver, which means it gets a copy of the Inner type. When you modify this, you modify the copy, and the caller won't see any change on the Inner value.
So first modify it to have a pointer-receiver:
func (c *Inner) Run(value int) {
// ...
}
If a method has a pointer-receiver, the address (pointer) of the value the method is called on will be passed to the method. And inside the method you will modify the pointed value, not the pointer. The pointer points to the same value that is present at the caller, so the same value is modified (and not a copy).
This change alone may make your code work. However, the output of your program is non-deterministic because you modify a variable (field) from one goroutine, and you read this variable from another goroutine too, so you must synchronize access to this field in some way.
One way to synchronize access is using sync.RWMutex:
type Inner struct {
m *sync.RWMutex
Value int
}
When you create your Outer value, initialize this mutex:
o := new(Outer)
o.In.m = &sync.RWMutex{}
Or in one line:
o := &Outer{In: Inner{m: &sync.RWMutex{}}}
And in Inner.Run() lock when you access the Inner.Value field:
func (c *Inner) Run(value int) {
c.m.Lock()
c.Value = value
c.m.Unlock()
for {
c.m.RLock()
fmt.Println(c.Value)
c.m.RUnlock()
time.Sleep(time.Second * 2)
}
}
And you also have to use the lock when you access the field in Outer.Run():
func (c Outer) Run() {
go c.In.Run(42)
for {
time.Sleep(time.Second)
c.In.m.RLock()
fmt.Println(c.In)
c.In.m.RUnlock()
}
}
Note:
Your example only changes Inner.Value once, in the beginning of Inner.Run. So the above code does a lot of unnecessary locks/unlocks which could be removed if the loop in Outer.Run() would wait until the value is set, and afterwards both goroutines could read the variable without locking. In general if the variable can be changed at later times too, the above presented locking/unlocking is required at each read/write.
The simplest way to resolve your issue is to use a pointer receiver in your Run function:
func (c *Inner) Run(value int) {
out = make(chan int)
c.Value = value
for {
fmt.Println(c.Value)
time.Sleep(time.Second * 2)
}
}
But another solution would be to use an out channel to which you can send the Inner struct value:
func (c Inner) Run(value int) {
out = make(chan int)
c.Value = value
for {
fmt.Println(c.Value)
time.Sleep(time.Second * 2)
out <- c.Value
}
}
Then in a separate goroutine to receive back the sent value:
for{
go func() {
c.In.Run(42)
<-out
fmt.Println(out)
}()
time.Sleep(time.Second)
}
Here is the full code:
package main
import (
"fmt"
"time"
)
type Inner struct {
Value int
}
var out chan int
func (c Inner) Run(value int) {
out = make(chan int)
c.Value = value
for {
fmt.Println(c.Value)
time.Sleep(time.Second * 2)
out <- c.Value
}
}
type Outer struct {
In Inner
}
func (c Outer) Run() {
for{
go func() {
c.In.Run(42)
<-out
fmt.Println(out)
}()
time.Sleep(time.Second)
}
}
func main() {
o := new(Outer)
o.Run()
}
https://play.golang.org/p/Zt_NAsM98_

Discover the return type of the current function in go

I have a function that is being generated using reflection and reflect.MakeFunc, so I don't actually have the return type until runtime.
Inside the template function that MakeFunc is using, is there a way to determine the return type of the concrete function being templated?
Essentially, is there a way to determine the return type iof the currently executing function at runtime?
I know about the Out method:
fn.Type().Out(0)
And I can find the return type of a function easily enough?
But is there a way to find the return type of the currently executing function (as opposed to an explicit passed function reference).
You should check fn.Type().Out(0).Kind(), for example:
func main() {
fnTmpl := func(in []reflect.Value) []reflect.Value {
return []reflect.Value{in[0]}
}
makeFn := func(fptr interface{}) {
fn := reflect.ValueOf(fptr).Elem()
fn.Set(reflect.MakeFunc(fn.Type(), fnTmpl))
}
var nFn func(int) int
makeFn(&nFn)
kind := reflect.TypeOf(nFn).Out(0).Kind()
switch kind {
case reflect.Int:
fmt.Println("int")
}
}
In the case you are talking about, the return type of the currently executing function is always []reflect.Type (because that is what a function passed to reflect.MakeFunc must return). What you really want is the return type of the reflect.makeFuncStub function that called your function.
There is no way to get that (except perhaps some strange inspection of the call stack), but you can make an enhanced version of MakeFunc that provides the information:
package main
import (
"fmt"
"reflect"
)
// MakeFunc is like reflect.MakeFunc, but fn has an extra argument, retType, which
// is passed the desired return type.
func MakeFunc(typ reflect.Type, fn func(args []reflect.Value, retType reflect.Type) (results []reflect.Value)) reflect.Value {
if n := typ.NumOut(); n != 1 {
panic("wrong number of return values")
}
rt := typ.Out(0)
return reflect.MakeFunc(typ, func(args []reflect.Value) (results []reflect.Value) {
return fn(args, rt)
})
}
func makeReturnOne(fptr interface{}) {
fn := reflect.ValueOf(fptr).Elem()
fn.Set(MakeFunc(fn.Type(), returnOne))
}
func returnOne(args []reflect.Value, retType reflect.Type) []reflect.Value {
ret := reflect.New(retType).Elem()
switch retType.Kind() {
case reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64:
ret.SetInt(1)
case reflect.Float32, reflect.Float64:
ret.SetFloat(1.0)
default:
panic("returnOne only supports int and float types")
}
r := ret.Interface()
fmt.Printf("returning %v as %T\n", r, r)
return []reflect.Value{ret}
}
func main() {
var r1f func() float64
var r1i func() int
makeReturnOne(&r1f)
makeReturnOne(&r1i)
fmt.Println(r1f())
fmt.Println(r1i())
}
I might have misinterpreted what you are trying to achieve, but why not just take the kind of the value you are returning? Modifying OneOfOne's example as follows:
fnTmpl := func(in []reflect.Value) (res []reflect.Value) {
res = []reflect.Value{in[0]}
fmt.Println("Returned:", res[0].Kind())
return res
}
Playground: http://play.golang.org/p/EujmxyGRrI

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