golang unsafe implementation

Golang is a strongly typed, static language with a rich standard library and efficient garbage collection mechanism. The design goal of Golang is to improve program running efficiency and development efficiency. However, in the actual development process, sometimes it is necessary to use some less safe mechanisms, such as pointer operations, underlying memory access, etc., to implement some high-performance code. At this time, you need to use Golang's unsafe package to achieve direct operations on the underlying memory.

This article will introduce the usage and precautions of Golang unsafe package.

unsafe package overview

Golang’s unsafe package is a special package that provides some unsafe operations, such as pointer operations, type conversions, etc. The functions in the unsafe package do not perform type checking and bounds checking during the compilation process, so improper use can cause serious problems and even cause the program to crash. Be very careful when you have to use it.

Using the unsafe package can achieve some high-performance code, but improper use will bring great risks, so it is recommended to use it with caution. Below we will introduce several scenarios for using the unsafe package.

1. Modify unmodifiable values

In Golang, some variables are not allowed to be modified, such as const and string type variables. However, sometimes we need to modify these variables. At this time, you can use unsafe.Pointer to convert the pointers of these variables into the unsafe.Pointer type, and then use the pointer to modify the value of the variable.

Sample code:

package main

import (
    "fmt"
    "unsafe"
)

func main() {
    const a string = "hello"
    var p *string = &a
    var q unsafe.Pointer = unsafe.Pointer(p)
    fmt.Println(*p) // 输出 hello
    *(*string)(q) = "world"
    fmt.Println(*p) // 输出 world
}

In the above code, we converted the pointer p of the string type variable a into the unsafe.Pointer type, assigned it to q, and then passed q a has been modified. Note that this approach is unreliable and can cause exceptions during compiler or runtime.

2. Manipulate the internal structure of Go

In Golang, many internal structures are inaccessible. For example, in the runtime library in the standard library, we cannot directly access its internal structure. However, through the unsafe package, we can gain access to these internal structures, such as access to goroutine, stack and other structures.

Sample code:

package main

import (
    "fmt"
    "unsafe"
)

func main() {
    type slice struct {
        pointer unsafe.Pointer
        len     int
        cap     int
    }
    a := []int{1, 2, 3, 4, 5}
    s := (*slice)(unsafe.Pointer(&a))
    fmt.Println(s.pointer) // 输出 &a[0]
    fmt.Println(s.len)     // 输出 5
    fmt.Println(s.cap)     // 输出 5
}

In the above code, we define a slice type. By converting the pointer of slice a into a slice pointer, we can directly access the underlying array pointer of the slice. Length, capacity and other information.

3. Bypassing the type system to achieve high-performance code

In Golang’s type system, some types cannot be converted directly, such as int type and float32 type. However, sometimes we need to convert between these types. For example, in numerical calculations, we need to convert the int type to the float32 type for calculation. At this point, we can use the Convert function of the unsafe package to complete the conversion.

Sample code:

package main

import (
    "fmt"
    "unsafe"
)

func main() {
    a := 10
    ptr := unsafe.Pointer(&a)
    v1 := *(*float32)(ptr)
    fmt.Println(v1) // 输出 4.591524e-41
    v2 := *(*int)(unsafe.Pointer(&v1))
    fmt.Println(v2) // 输出 1091567616
}

In the above code, we first convert the pointer of an int type variable a into an unsafe.Pointer type pointer ptr, and then convert the ptr into A pointer of type float32 is dereferenced to obtain v1. Finally, convert the pointer of v1 into an int type pointer and dereference it to get v2. This approach can bypass the type system to achieve efficient conversion.

Notes

When using the unsafe package, you need to pay attention to the following points:

1. Do not use the unsafe.Pointer type "cross-domain" pointers, that is, do not cast a pointer of one type to a pointer of another type. This is prone to problems because the converted pointer is in an unknown memory area and may affect other variables.
2. Don't use unsafe.Pointer pointers for some variables that have no allocated memory, because they point to unknown memory areas. May cause unpredictable consequences.
3. Do not dereference pointers at unpredictable times. Because Golang's garbage collection mechanism may clean up memory at any time, and you don't know when garbage collection will be triggered.
4. If you have to use the unsafe package, you should try to reduce the scope of the unsafe operation as much as possible. After using the code block of the unsafe package, you should return to type-safe code as soon as possible.

Summary

Golang’s unsafe package provides some unsafe operations that may lead to serious consequences if used improperly. Therefore, be careful when using unsafe packages. If possible, you should avoid using unsafe packages and choose safer and more reliable ways to implement high-performance code.

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