CODEWITHSUNDEEP
pointersgo
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Reputation: 761

Pointers golang from string

My application accepts a pointer from os.Args.

For example

pointer := os.Args[1] //"0x7ffc47e43200"

How can I use that pointer and get the value that is stored on that location?

Upvotes: 2

Views: 2027

Answers (1)

joshlf
joshlf

Reputation: 23557

Disclaimer: As you are probably aware, this is dangerous and if you're going to do this in a production application, you'd better have a really good reason. That being said...

You need to do a few things. Here's the code, and then we'll walk through it.

package main

import (
    "fmt"
    "os"
    "strconv"
    "unsafe"
)

func main() {
    str := "7ffc47e43200" // strconv.ParseUint doesn't like a "0x" prefix
    u, err := strconv.ParseUint(str, 16, 64)
    if err != nil {
        fmt.Fprintln(os.Stderr, "could not parse pointer:", err)
        os.Exit(1)
    }

    ptr := unsafe.Pointer(uintptr(u)) // generic pointer (like void* in C)
    intptr := (*int)(ptr)             // typed pointer to int
    fmt.Println(*intptr)
}

You can run this on the Go Playground.

First, we need to parse the string as a numerical value. In your example, you gave a hexadecimal number, so we'll parse in base 16 (that's the "16" argument to strconv.ParseUint). Note that strconv.ParseUint doesn't like the "0x" prefix, so I removed it.

Then, we need to convert the number into a pointer type. For this, we will use the unsafe.Pointer type, which is special to the Go compiler. Normally, the compiler won't let you convert between pointer types. The exception is that, according to the unsafe.Pointer documentation:

  • A pointer value of any type can be converted to a Pointer.
  • A Pointer can be converted to a pointer value of any type.
  • A uintptr can be converted to a Pointer.
  • A Pointer can be converted to a uintptr.

Thus, in order to convert to a pointer, we'll need to first convert to a uintptr and then to an unsafe.Pointer. From here, we can convert to any pointer type we want. In this example, we will convert to an int pointer, but we could choose any other pointer type as well. We then dereference the pointer (which panics in this case).

Upvotes: 8

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