Golang basic

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Overview

Golang basic 내용 정리

select

The select statement is used to choose from multiple send/receive channel operations. The select statement blocks until one of the send/receive operation is ready.

If one or more of the communications can proceed, a single one that can proceed is chosen via a uniform pseudo-random selection. Otherwise, if there is a default case, that case is chosen. If there is no default case, the "select" statement blocks until at least one of the communications can proceed. <source lang=go> package main

import ( "context" "fmt" "time" )

func main() { fmt.Println("Clear is better than clever.") ctx, canceled := context.WithCancel(context.Background()) canceled()

t := time.NewTicker(1 * time.Second)

out := make(chan int, 1) select { case <-ctx.Done(): fmt.Println("nope: canceled context selected") case out <- work(): fmt.Println("work selected") case <-t.C: fmt.Println("ticker selected") } }

func work() int { time.Sleep(10 * time.Second) fmt.Printf("work work %#v\n", time.Now()) return 1 } </source>

Values

Variables

var 키워드를 이용해서 선언한다. var 키워드 뒤에 변수명을 적고, 그 뒤에 변수타입을 적는다. 다음은 정수(int) 타입의 변수 a 를 선언한 것이다. <source lang=go> var a int </source>

변수 선언문에서 변수 초기값을 할당할 수도 있다. 즉, float32 타입에 초기값을 입력하고자 한다면 다음과 같이 하면 된다. <source lang=go> var b float32 = 12.3 </source>

일단 선언된 변수는 해당 타입의 값을 할당할 수 있다. <source lang=go> a = 10 b = 1.23 </source>

만약 선언된 변수가 Go 프로그램에서 사용되지 않는다면, 에러를 발생시킨다. 따라서 사용되지 않는 변수는 프로그램에서 삭제해야 한다. 

$ go build ./main.go 
# command-line-arguments
./main.go:22:7: k declared and not used

동일한 타입의 변수가 여러 개 있을 경우, 변수들을 나열하고 마지막에 타입을 한번만 지정할 수 있다. <source lang=go> var i, j, k int = 1, 2, 3 </source>

변수를 선언하면서 초기값을 지정하지 않으면, Go 는 zero-value 를 기본적으로 할당한다. 숫자형에는 0, bool 타입에는 false, string 타입에는 ""(빈 문자열)을 할당한다.

Go 에서는 할당되는 값을 보고 그 타입을 추론하는 기능이 자주 사용된다.

아래의 예제에서는 i 에는 int, s 에는 string 타입이 자동으로 할당된다. <source lang=go> var i = 1 var s = "hi" </source>

Debug

GOTRACEBACK

환경변수에 GOTRACEBACK=crash 를 지정해놓으면, 문제 발생시 core dump 를 출력한다. 

$ ./main
panic: duplicate metrics collector registration attempted

goroutine 1 [running]:
panic(0x80b2a0, 0xc00000d040)
	/usr/local/go/src/runtime/panic.go:556 +0x2cb fp=0xc0000dbe38 sp=0xc0000dbda8 pc=0x42c95b
github.com/prometheus/client_golang/prometheus.(*Registry).MustRegister(0xc0000a04b0, 0xc000083050, 0x1, 0x1)
	/home/sungtaekim/go/src/github.com/prometheus/client_golang/prometheus/registry.go:391 +0xad fp=0xc0000dbe80 sp=0xc0000dbe38 pc=0x75cb9d
github.com/prometheus/client_golang/prometheus.MustRegister(0xc000083050, 0x1, 0x1)
	/home/sungtaekim/go/src/github.com/prometheus/client_golang/prometheus/registry.go:176 +0x53 fp=0xc0000dbeb0 sp=0xc0000dbe80 pc=0x75b883
github.com/prometheus/client_golang/prometheus/promauto.NewCounter(0x0, 0x0, 0x0, 0x0, 0xc00008a270, 0x27, 0x861b42, 0x1f, 0x0, 0x8c80e0, ...)
	/home/sungtaekim/go/src/github.com/prometheus/client_golang/prometheus/promauto/auto.go:136 +0xf2 fp=0xc0000dbf30 sp=0xc0000dbeb0 pc=0x7670c2
main.init()
	/home/sungtaekim/worksapce/asterisk_exporter/main.go:106 +0xa55 fp=0xc0000dbf98 sp=0xc0000dbf30 pc=0x77cd35
runtime.main()
	/usr/local/go/src/runtime/proc.go:189 +0x1bd fp=0xc0000dbfe0 sp=0xc0000dbf98 pc=0x42e63d
runtime.goexit()
	/usr/local/go/src/runtime/asm_amd64.s:1333 +0x1 fp=0xc0000dbfe8 sp=0xc0000dbfe0 pc=0x4591d1

goroutine 2 [force gc (idle)]:
runtime.gopark(0x872f90, 0xb98140, 0x1410, 0x1)
	/usr/local/go/src/runtime/proc.go:302 +0xeb fp=0xc00003ef80 sp=0xc00003ef60 pc=0x42ea7b
runtime.goparkunlock(0xb98140, 0x1410, 0x1)
	/usr/local/go/src/runtime/proc.go:308 +0x53 fp=0xc00003efb0 sp=0xc00003ef80 pc=0x42eb23
runtime.forcegchelper()
	/usr/local/go/src/runtime/proc.go:251 +0xb3 fp=0xc00003efe0 sp=0xc00003efb0 pc=0x42e8f3
runtime.goexit()
	/usr/local/go/src/runtime/asm_amd64.s:1333 +0x1 fp=0xc00003efe8 sp=0xc00003efe0 pc=0x4591d1
created by runtime.init.4
	/usr/local/go/src/runtime/proc.go:240 +0x35

goroutine 3 [GC sweep wait]:
runtime.gopark(0x872f90, 0xb983c0, 0x42140c, 0x1)
	/usr/local/go/src/runtime/proc.go:302 +0xeb fp=0xc00003f780 sp=0xc00003f760 pc=0x42ea7b
runtime.goparkunlock(0xb983c0, 0x8b140c, 0x1)
	/usr/local/go/src/runtime/proc.go:308 +0x53 fp=0xc00003f7b0 sp=0xc00003f780 pc=0x42eb23
runtime.bgsweep(0xc000066000)
	/usr/local/go/src/runtime/mgcsweep.go:52 +0x8f fp=0xc00003f7d8 sp=0xc00003f7b0 pc=0x4213bf
runtime.goexit()
	/usr/local/go/src/runtime/asm_amd64.s:1333 +0x1 fp=0xc00003f7e0 sp=0xc00003f7d8 pc=0x4591d1
created by runtime.gcenable
	/usr/local/go/src/runtime/mgc.go:216 +0x58

goroutine 18 [finalizer wait]:
runtime.gopark(0x872f90, 0xbb6048, 0x20000140f, 0x1)
	/usr/local/go/src/runtime/proc.go:302 +0xeb fp=0xc00003e728 sp=0xc00003e708 pc=0x42ea7b
runtime.goparkunlock(0xbb6048, 0x40140f, 0x1)
	/usr/local/go/src/runtime/proc.go:308 +0x53 fp=0xc00003e758 sp=0xc00003e728 pc=0x42eb23
runtime.runfinq()
	/usr/local/go/src/runtime/mfinal.go:175 +0x99 fp=0xc00003e7e0 sp=0xc00003e758 pc=0x418d99
runtime.goexit()
	/usr/local/go/src/runtime/asm_amd64.s:1333 +0x1 fp=0xc00003e7e8 sp=0xc00003e7e0 pc=0x4591d1
created by runtime.createfing
	/usr/local/go/src/runtime/mfinal.go:156 +0x61
Aborted (core dumped)

Marshal/Unmarshal

<source lang=go> package main

import ( "encoding/json" "fmt" "log" )

const input = ` { "type": "sound", "msg": { "description": "dynamite", "authority": "the Bruce Dickinson" } } `

type Envelope struct { Type string Msg interface{} }

type Sound struct { Description string Authority string }

func main() { var msg json.RawMessage env := Envelope{ Msg: &msg, } if err := json.Unmarshal([]byte(input), &env); err != nil { log.Fatal(err) } switch env.Type { case "sound": var s Sound if err := json.Unmarshal(msg, &s); err != nil { log.Fatal(err) } var desc string = s.Description fmt.Println(desc) default: log.Fatalf("unknown message type: %q", env.Type) } } </sourcE>

Reference

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