Why Do I Need to Synchronize Access to String Variables in Go, Even Though Strings Are Immutable?

Immutability of Strings and Thread Synchronization

While strings in Go are immutable, the variables holding their references are not. Therefore, when working with strings in a multithreaded environment, it is necessary to synchronize access to the string variable, not the string itself.

Why Strings Aren't Atomically Typed

An atomic type guarantees that its values are never modified after initialization. However, since string variables can be reassigned, strings are not atomically typed.

Synchronization for String Variables

Synchronization is required whenever multiple threads access a string variable, where at least one access is a write. This is because the string's value can only be changed by reassigning the variable, not by modifying the string itself.

In Practice

If you have a string value "hello", it will remain "hello" indefinitely, assuming you don't assign a new value to the variable. However, if you have a slice value []byte{1, 2, 3}, its elements can be modified concurrently, even if the slice is passed by value.

Consider the following example:

var sig = make(chan int)

func main() {
    s := []byte{1, 2, 3}
    go func() {
        <-sig
        s[0] = 100
        sig <- 0
    }()
    sliceTest(s)
}

func sliceTest(s []byte) {
    fmt.Println("First  s =", s)

    sig <- 0 // send signal to modify now
    <-sig    // Wait for modification to complete

    fmt.Println("Second s =", s)
}

Output:

First  s = [1 2 3]
Second s = [100 2 3]

In this example, sliceTest() receives a slice and prints its initial value. It then waits for another goroutine to modify the slice and then prints its modified value. This demonstrates that the slice value can be changed concurrently. However, if sliceTest() were to receive a string argument, this modification would not occur.

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