What does golang mean by non-reentrant?

In the field of computer science, reentrancy refers to the characteristic that a function can be called multiple times during execution. However, some languages ​​or environments do not support function reentrancy. In Golang, functions are not reentrant, this is due to a design decision in Golang itself.

First, let us understand what a non-reentrant function is. A non-reentrant function is a function that cannot execute multiple instances simultaneously. Due to Golang's memory management system and coroutine scheduling mechanism, if two coroutines call non-reentrant functions at the same time, the data may be illegally overwritten, leading to unpredictable results.

This may sound a bit abstract, so let’s look at an example. Consider the following code snippet, where the foo function is a non-reentrant function because it contains operations of reading and writing data from global variables:

package main

import "fmt"

var global int

func foo() {
    global = global + 1
}

func main() {
    go foo()
    go foo()
    fmt.Println(global)
}

In this example, we The foo function is called in the two coroutines at the same time, and the global variable global is incremented at the same time. If Golang is reentrant, then we expect global to have a value of 2. However, due to the non-reentrant nature of Golang, calling the foo function in two coroutines at the same time will cause data competition, and the result may be uncertain. Sometimes the value of global is 1, sometimes 2.

So, how to avoid this situation?

One solution is to encapsulate important data inside the function and use Golang's channel or mutex lock mechanism to ensure concurrency correctness. This method can ensure that the data is only accessed and modified by one coroutine at the same time, thus avoiding the problem of data competition.

Another solution is to avoid using non-reentrant functions. Correspondingly, in Golang programming practice, we usually try to avoid using global variables. In order to ensure safety between coroutines, we use local variables and parameters. This ensures that each coroutine has its own variables and does not interfere with each other.

In short, Golang’s non-reentrancy requires our special attention in actual programming. It reminds us to always pay attention to details such as thread safety and concurrency issues when writing code to ensure the correctness and stability of the program.

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