package standard library
go1.22.5 Latest Latest

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Published: Jul 2, 2024 License: BSD-3-Clause Imports: 5 Imported by: 79



Package iotest implements Readers and Writers useful mainly for testing.




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var ErrTimeout = errors.New("timeout")

ErrTimeout is a fake timeout error.


func DataErrReader

func DataErrReader(r io.Reader) io.Reader

DataErrReader changes the way errors are handled by a Reader. Normally, a Reader returns an error (typically EOF) from the first Read call after the last piece of data is read. DataErrReader wraps a Reader and changes its behavior so the final error is returned along with the final data, instead of in the first call after the final data.

func ErrReader added in go1.16

func ErrReader(err error) io.Reader

ErrReader returns an io.Reader that returns 0, err from all Read calls.

package main

import (

func main() {
	// A reader that always returns a custom error.
	r := iotest.ErrReader(errors.New("custom error"))
	n, err := r.Read(nil)
	fmt.Printf("n:   %d\nerr: %q\n", n, err)


n:   0
err: "custom error"

func HalfReader

func HalfReader(r io.Reader) io.Reader

HalfReader returns a Reader that implements Read by reading half as many requested bytes from r.

func NewReadLogger

func NewReadLogger(prefix string, r io.Reader) io.Reader

NewReadLogger returns a reader that behaves like r except that it logs (using log.Printf) each read to standard error, printing the prefix and the hexadecimal data read.

func NewWriteLogger

func NewWriteLogger(prefix string, w io.Writer) io.Writer

NewWriteLogger returns a writer that behaves like w except that it logs (using log.Printf) each write to standard error, printing the prefix and the hexadecimal data written.

func OneByteReader

func OneByteReader(r io.Reader) io.Reader

OneByteReader returns a Reader that implements each non-empty Read by reading one byte from r.

func TestReader added in go1.16

func TestReader(r io.Reader, content []byte) error

TestReader tests that reading from r returns the expected file content. It does reads of different sizes, until EOF. If r implements io.ReaderAt or io.Seeker, TestReader also checks that those operations behave as they should.

If TestReader finds any misbehaviors, it returns an error reporting them. The error text may span multiple lines.

func TimeoutReader

func TimeoutReader(r io.Reader) io.Reader

TimeoutReader returns ErrTimeout on the second read with no data. Subsequent calls to read succeed.

func TruncateWriter

func TruncateWriter(w io.Writer, n int64) io.Writer

TruncateWriter returns a Writer that writes to w but stops silently after n bytes.


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