初始化RWMutex在Golang中崩溃？

Question

The following code crashes with the following error, why??

package main

import (
    "sync"

)

var foo *sync.RWMutex

func main() {
    foo.Lock()
}

outputs:

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0xffffffff addr=0x0 pc=0x8125c]

goroutine 1 [running]:
sync.(*RWMutex).Lock(0x0, 0x104000f0)
    /usr/local/go/src/sync/rwmutex.go:86 +0x1c
main.main()
    /tmp/sandbox093456788/main.go:11 +0x20

Program exited.

it will not happen, when you use one of the following lines

var foo = new(sync.RWMutex)
var foo sync.RWMutex

for the initialization of the foo variable.

Answer 1

The mutexes are generally designed to work without any type of initialization. That is, a zero valued mutex is all you need to use them. To fix your program, declare foo as a value. Not a pointer to a mutex:

var foo sync.RWMutex

func main() {
    foo.Lock()
}

When you declare foo as a pointer:

var foo *sync.RWMutex

foo is a zero valued pointer (to type sync.RWMutex), i.e. it does not point to a valid mutex value. foo is nil. That's why you get the nil pointer exception when calling a method.

However, note that to share the mutex between goroutines/functions, you're going to have to pass them as pointers, after they have been initialized (like above), or they'll not be shared.

From the docs:

An RWMutex must not be copied after first use.

Docs: https://golang.org/pkg/sync/#RWMutex