go-stdlib-part-1-io.slide

The Go Standard Library 
Part 1: I/O

# Go is a general-purpose language that bridges the gap between efficient
# statically typed languages and productive dynamic language. But it’s not just
# the language that makes Go special – Go has broad and consistent standard
# libraries and powerful but simple tools.
# 
# This talk gives an introduction to Go, followed by a tour of some real
# programs that demonstrate the power, scope, and simplicity of the Go
# programming environment.

Fabrizio Milo
Los Angeles Meetup Group -  November 20 2012 
@fabmilo
lagomeetup@gmail.com
http://www.meetup.com/Los-Angeles-Gophers/


* What will be covered:
- io
- os 
- ioutils
- bufio
- bytes
- encoding/binary
- gzip


* Standard library

* io

The `io` package provides fundamental I/O interfaces:

- *Reader* 
    type Reader interface {
        Read(p []byte) (n int, err error) 
    }

.link http://golang.org/pkg/io/#Reader

- *Writer* 
    type Writer interface {
        Write(p []byte) (n int, err error) 
    }

.link http://golang.org/pkg/io/#Writer

* io

- *ReaderWriter* 
    type ReadWriter interface {
       Reader  
       Writer
    }

.link http://golang.org/pkg/io/#ReadWriter

* Examples

- *os.File _(io.ReadWriter)_
- *bytes.Buffer _(io.ReadWriter)_
- *bufio.Writer _(io.Writer)_
- *bufio.Reader _(io.Reader)_
- *string.Reader _(io.Reader)_

.link http://golang.org/pkg/bytes/#Buffer
.link http://golang.org/pkg/bufio/#Writer
.link http://golang.org/pkg/bufio/#Reader
.link http://golang.org/pkg/strings/#Reader


* os

To open a file:

.play go-stdlib-part-1/simple_read_file.go  /START1/,/STOP1/

* os

Read Contents:

.code go-stdlib-part-1/simple_read_file.go  /START2/,/STOP2/


* io/ioutil

to read the whole file: 

   func ReadFile(filename string) ([]byte, error) // (does not return EOF)

.link http://golang.org/pkg/io/ioutil/#ReadFile


   func WriteFile(filename string) ([]byte, error)

.link http://golang.org/pkg/io/ioutil/#WriteFile

* io/ioutil

Handy Variables: 

     util.Discard

.link http://golang.org/pkg/io/ioutil/#Discard

Example:

.play go-stdlib-part-1/copy_trash.go /START/,/STOP/

* os 

other handy utilites:

- os.ExpandEnv( )

.code go-stdlib-part-1/simple_read_file.go  /{path/,/path}/

.link http://golang.org/pkg/os#ExpandEnv

* os

- os.Args
- os.Stdin, os.Stdout, os.Stderr

* os/user

Get the current user info:

.play go-stdlib-part-1/user_info.go


* os 

- ExpandEnv(string)
- Getpagesize()
- Hostname()
- Pipe()
- StartProcess()

    func StartProcess(name string, argv []string, attr *ProcAttr) (*Process, error)

* path/filepath

- Clean(path string)
- Glob(pattern string)
- Walk()

.link http://golang.org/pkg/path/filepath/

* os

How to check if a path exists ?

- No default way

.code go-stdlib-part-1/simple_read_file.go  /<exists/,/exists>/

* bufio

    func NewReader(rd io.Reader) *Reader
   
Example: 

.play go-stdlib-part-1/console1.go /START/,/STOP/


* bufio.Reader

Peek:  

    func (b *Reader) Peek(n int) ([]byte, error)

ReadByte:

    func (b *Reader) ReadByte() (c byte, err error)

UnreadByte:

    func (*Reader) UnreadByte()

.link http://golang.org/pkg/bufio

* bufio.Reader

notice that bufio.Reader 

    //bufio
    type Reader struct {
        // contains filtered or unexported fields
    }

is a *different* type from io.Reader:

    // io    
    type Reader interface {
        Read(p []byte) (n int, err error)
    } 

* bufio.Writer

Pay attention to flush:

    func (b *Writer) Flush() error

Example 1:

.play go-stdlib-part-1/console-progress.go /START/,/STOP/ 

* bufio.Writer

Example 2:

  f := bufio.NewWriter(os.Stdout)
  defer f.Flush() 
  f.Write(b)

    
.link http://golang.org/pkg/bufio#Writer.Flush

* bytes

New Buffer from Bytes:

    func NewBuffer(buf []byte) *Buffer

New Buffer String:
    
    func NewBufferString(s string) *Buffer

* Caveats using the buffers

What's wrong with this code ?:

.play go-stdlib-part-1/double-buffer-caveats.go /START1/,/START2/

* Caveats using the buffers

What's wrong with this code ?:

.play go-stdlib-part-1/double-buffer-caveats-2.go /START1/,/START2/

More Caveats:

    func ReadFull(r Reader, buf []byte) (n int, err error)


* gzip
    "compress/gzip"
    reader, err = gzip.NewReader(response.Body)
    defer reader.Close()

* gzip

Example: Reading from a string and then compressing it

.code go-stdlib-part-1/gzip-example.go /WRITE1/,/WRITE2/  

* gzip

Reverse Example: uncompressing a string

.play go-stdlib-part-1/gzip-example.go /READ1/,/READ2/  

* encoding/binary
    
    func Read(r io.Reader, order ByteOrder, data interface{}) error

.link http://golang.org/pkg/encoding/binary/#Read

* encoding/binary

    var f float32
    err = binary.Read(fd, binary.BigEndian, &f)
    fmt.Println(err, f)

* encoding/binary
 
 Note the *interface{}* of the last type
 
    func Read(r io.Reader, order ByteOrder, data interface{}) error
    
* encoding/binary
 
You can Read entire structures from a stream:
 
.play go-stdlib-part-1/binary_struct.go /START/,/END/
 
No worries about padding & byte alignment, is all taken care of
Good post on byte endianess: 

.link http://commandcenter.blogspot.com/2012/04/byte-order-fallacy.html