Snippet

Preventing Memory Leaks with Sub-Slice Copying

When you slice a slice, the new slice shares the same underlying array as the original. If a small sub-slice of a huge slice remains in memory, the garbage collector cannot reclaim the large underlying array. Copying the needed data into a new, smaller slice permits the large slice to be garbage collected.

snippet.go

package main
 
import "fmt"
 
func getImportantData(largeData []byte) []byte {
	result := make([]byte, 2)
	copy(result, largeData[:2])
	return result
}
 
func main() {
	bigSlice := make([]byte, 1000000)
	bigSlice[0], bigSlice[1] = 42, 99
 
	smallSlice := getImportantData(bigSlice)
	fmt.Printf("Length: %d, Capacity: %d\n", len(smallSlice), cap(smallSlice))
}

Breakdown

result := make([]byte, 2)

Creates a new slice with length and capacity of 2, allocating a separate small underlying array.

copy(result, largeData[:2])

Copies only the required bytes from the large slice into the newly allocated slice.

return result

Returns the new small slice, allowing the caller to drop reference to the large slice.

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Preventing Memory Leaks with Sub-Slice Copying

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