Io.Copy

As explained in Go in Action, Second Edition

io.Copy transfers data from a Reader to a Writer until the input reaches end of file or an error occurs.

io.Copy transfers data from any io.Reader to any io.Writer until the input reaches end of file or an error occurs. It provides a general stream-copying operation without tying the implementation to files or another specific source and destination. section 10.3

The function fits Go’s composable I/O design: callers can connect different sources and sinks through the small io.Reader and io.Writer interfaces. Use io.CopyN instead when the operation must stop after a specific byte count. section 10.3

Files and streaming data

Use io.Copy for copying files or streaming data between sources and sinks, while choosing io.CopyN for copying only part of a stream. section 10.3

Figure 10.1 io.Reader and io.Writer interface composition with multiple package implementations

The interface-composition figure helps map io.Copy’s generic operation across multiple Reader and Writer implementations. section 10.3

How Go in Action, Second Edition covers it

How deep it goes

Defines efficient Reader-to-Writer transfer and its EOF-or-error boundary, but defers implementation internals.

Examples

Uses file copying, streaming between sources and sinks, and partial-stream copying as practical cases.

What it assumes

Assumes familiarity with the io.Reader and io.Writer interfaces and their composable stream model.

What you can do after

Apply it via io.Copy to move data between any compatible Reader and Writer.

How it connects

Connects io.Reader, io.Writer, io.CopyN, and stream composition.

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Also explained in

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  Go by Exampleio.Copy is a Go standard-library function that copies bytes from an io.Reader to an io.Writer and returns the number of bytes copied and any error.

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Related concepts

• io.Readeruses · io.Reader is a Go standard-library interface whose Read method reads bytes into a supplied buffer and reports the number read and any error.
• io.Writeruses · io.Writer is a Go interface whose Write method accepts a byte sequence and returns the number of bytes written and any error.
• bufferinguses · Buffering is the temporary storage of data or work between production and consumption to accommodate differences in processing rates, absorb interruptions, or improve transfer efficiency.
• payloaduses · A payload is the data, code, or other material carried by or attached to a message, token, system, or operation.

This explainer was generated by Manning's knowledge graph from the text of Go in Action, Second Edition. The section links point to the parts of the book it draws on.