Using Unsafe to Obtain a Byte Slice from a String without Copying

Go strings are immutable, meaning that converting them to byte slices involves a memory copy. This can potentially impact performance when working with large datasets. This article explores how to use unsafe to avoid this copy operation while emphasizing the critical aspects and limitations.

Background

The standard library function []byte(s) creates a copy of the string s. If memory consumption is a concern, it is desirable to obtain the byte slice without incurring this overhead.

Unsafe Conversion

Utilizing the unsafe package provides a way to achieve this goal. By casting the string value to a pointer to an array of bytes, we can access the underlying byte slice without creating a copy.

func unsafeGetBytes(s string) []byte {
    return (*[0x7fff0000]byte)(unsafe.Pointer(
        (*reflect.StringHeader)(unsafe.Pointer(&s)).Data),
    )[:len(s):len(s)]
}

Cautions

It is crucial to note that this approach carries inherent risks. Strings in Go are immutable, so modifying the byte slice obtained through unsafeGetBytes could result in unexpected behavior or even data corruption. Therefore, this technique should be used only in controlled internal environments where memory performance is paramount.

Handling Empty Strings

Note that the empty string ("") has no bytes, so its data field is indeterminate. If your code may encounter empty strings, explicitly checking for them is essential.

func unsafeGetBytes(s string) []byte {
    if s == "" {
        return nil // or []byte{}
    }
    return (*[0x7fff0000]byte)(unsafe.Pointer(
        (*reflect.StringHeader)(unsafe.Pointer(&s)).Data),
    )[:len(s):len(s)]
}

Performance Considerations

While this conversion avoids the overhead of copying, it is essential to keep in mind that compression operations, such as the one you mentioned using gzipWriter, are computationally intensive. The potential performance gain from avoiding the memory copy may be negligible compared to the computation required for compression.

Alternative Approaches

Alternatively, the io.WriteString function can be leveraged to write strings to an io.Writer without invoking the copy operation. The function checks for the existence of the WriteString method on the io.Writer and invokes it if available.

Related Questions and Resources

For further exploration, consider the following resources:

• [Go GitHub Issue 25484](https://github.com/golang/go/issues/25484)
• [unsafe.String](https://pkg.go.dev/unsafe#String)
• [unsafe.StringData](https://pkg.go.dev/unsafe#StringData)
• [[]byte(string) vs []byte(*string)](https://stackoverflow.com/questions/23369632/)
• [What are the possible consequences of using unsafe conversion from []byte to string in go?](https://stackoverflow.com/questions/67306718/)