copy

        # IO::Buffer.copy

(from ruby core)
### Implementation from Buffer
---
    copy(source, [offset, [length, [source_offset]]]) -> size

---

Efficiently copy data from a source IO::Buffer into the buffer, at
`offset` using `memcpy`. For copying String instances, see #set_string.

    buffer = IO::Buffer.new(32)
    #  =>
    # #<IO::Buffer 0x0000555f5ca22520+32 INTERNAL>
    # 0x00000000  00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
    # 0x00000010  00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................  *

    buffer.copy(IO::Buffer.for("test"), 8)
    # => 4 -- size of data copied
    buffer
    #  =>
    # #<IO::Buffer 0x0000555f5cf8fe40+32 INTERNAL>
    # 0x00000000  00 00 00 00 00 00 00 00 74 65 73 74 00 00 00 00 ........test....
    # 0x00000010  00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................ *

#copy can be used to put data into strings associated with buffer:

    string= "data:    "
    # => "data:    "
    buffer = IO::Buffer.for(str)
    buffer.copy(IO::Buffer.for("test"), 5)
    # => 4
    string
    # => "data:test"

Attempt to copy into a read-only buffer will fail:

    File.write('test.txt', 'test')
    buffer = IO::Buffer.map(File.open('test.txt'), nil, 0, IO::Buffer::READONLY)
    buffer.copy(IO::Buffer.for("test"), 8)
    # in `copy': Buffer is not writable! (IO::Buffer::AccessError)

See ::map for details of creation of mutable file mappings, this will
work:

    buffer = IO::Buffer.map(File.open('test.txt', 'r+'))
    buffer.copy("boom", 0)
    # => 4
    File.read('test.txt')
    # => "boom"

Attempt to copy the data which will need place outside of buffer's
bounds will fail:

    buffer = IO::Buffer.new(2)
    buffer.copy('test', 0)
    # in `copy': Specified offset+length exceeds source size! (ArgumentError)