Golang 源码剖析：fmt 标准库 --- Print* 是怎么样输出的？

package main

import (
	"fmt"
)

func main() {
	fmt.Println("Hello World!")
}

func Print(a ...interface{}) (n int, err error) {
	return Fprint(os.Stdout, a...)
}

func Println(a ...interface{}) (n int, err error) {
	return Fprintln(os.Stdout, a...)
}

func Printf(format string, a ...interface{}) (n int, err error) {
	return Fprintf(os.Stdout, format, a...)
}

func Print(a ...interface{}) (n int, err error) {
	return Fprint(os.Stdout, a...)
}

func Fprint(w io.Writer, a ...interface{}) (n int, err error) {
	p := newPrinter()
	p.doPrint(a)
	n, err = w.Write(p.buf)
	p.free()
	return
}

var ppFree = sync.Pool{
	New: func() interface{} { return new(pp) },
}

func newPrinter() *pp {
	p := ppFree.Get().(*pp)
	p.panicking = false
	p.erroring = false
	p.fmt.init(&p.buf)
	return p
}

func (p *pp) doPrint(a []interface{}) {
	prevString := false
	for argNum, arg := range a {
	    true && false
		isString := arg != nil && reflect.TypeOf(arg).Kind() == reflect.String
		// Add a space between two non-string arguments.
		if argNum > 0 && !isString && !prevString {
			p.buf.WriteByte(' ')
		}
		p.printArg(arg, 'v')
		prevString = isString
	}
}

p.printArg(arg, 'v')

%v	the value in a default format
	when printing structs, the plus flag (%+v) adds field names
%#v	a Go-syntax representation of the value
%T	a Go-syntax representation of the type of the value
%%	a literal percent sign; consumes no value
%t	the word true or false

+	always print a sign for numeric values;
	guarantee ASCII-only output for %q (%+q)
-	pad with spaces on the right rather than the left (left-justify the field)
#	alternate format: add leading 0 for octal (%#o), 0x for hex (%#x);
	0X for hex (%#X); suppress 0x for %p (%#p);
	for %q, print a raw (backquoted) string if strconv.CanBackquote
	returns true;
	always print a decimal point for %e, %E, %f, %F, %g and %G;
	do not remove trailing zeros for %g and %G;
	write e.g. U+0078 'x' if the character is printable for %U (%#U).
' '	(space) leave a space for elided sign in numbers (% d);
	put spaces between bytes printing strings or slices in hex (% x, % X)
0	pad with leading zeros rather than spaces;
	for numbers, this moves the padding after the sign

func Fprintf(w io.Writer, format string, a ...interface{}) (n int, err error) {
	p := newPrinter()
	p.doPrintf(format, a)
	n, err = w.Write(p.buf)
	p.free()
	return
}

func (p *pp) doPrintf(format string, a []interface{}) {
	end := len(format)
	argNum := 0         // we process one argument per non-trivial format
	afterIndex := false // previous item in format was an index like [3].
	p.reordered = false
formatLoop:
	for i := 0; i < end; {
		p.goodArgNum = true
		lasti := i
		for i < end && format[i] != '%' {
			i++
		}
		if i > lasti {
			p.buf.WriteString(format[lasti:i])
		}
		if i >= end {
			// done processing format string
			break
		}

		// Process one verb
		i++

		// Do we have flags?
		p.fmt.clearflags()
	simpleFormat:
		for ; i < end; i++ {
			c := format[i]
			switch c {
			case '#':   //'#'、'0'、'+'、'-'、' '
				...
			default:
				if 'a' <= c && c <= 'z' && argNum < len(a) {
					...
					p.printArg(a[argNum], rune(c))
					argNum++
					i++
					continue formatLoop
				}
				
				break simpleFormat
			}
		}

		// Do we have an explicit argument index?
		argNum, i, afterIndex = p.argNumber(argNum, format, i, len(a))

		// Do we have width?
		if i < end && format[i] == '*' {
			...
		}

		// Do we have precision?
		if i+1 < end && format[i] == '.' {
			...
		}

		if !afterIndex {
			argNum, i, afterIndex = p.argNumber(argNum, format, i, len(a))
		}

		if i >= end {
			p.buf.WriteString(noVerbString)
			break
		}

		...

		switch {
		case verb == '%': // Percent does not absorb operands and ignores f.wid and f.prec.
			p.buf.WriteByte('%')
		case !p.goodArgNum:
			p.badArgNum(verb)
		case argNum >= len(a): // No argument left over to print for the current verb.
			p.missingArg(verb)
		case verb == 'v':
			...
			fallthrough
		default:
			p.printArg(a[argNum], verb)
			argNum++
		}
	}

	if !p.reordered && argNum < len(a) {
		...
	}
}

fmt.Printf("%d", 1, 2, 3)
// 1%!(EXTRA int=2, int=3)

func Fprintln(w io.Writer, a ...interface{}) (n int, err error) {
	p := newPrinter()
	p.doPrintln(a)
	n, err = w.Write(p.buf)
	p.free()
	return
}

func (p *pp) doPrintln(a []interface{}) {
	for argNum, arg := range a {
		if argNum > 0 {
			p.buf.WriteByte(' ')
		}
		p.printArg(arg, 'v')
	}
	p.buf.WriteByte('\n')
}

func (p *pp) printArg(arg interface{}, verb rune) {
	p.arg = arg
	p.value = reflect.Value{}

	if arg == nil {
		switch verb {
		case 'T', 'v':
			p.fmt.padString(nilAngleString)
		default:
			p.badVerb(verb)
		}
		return
	}

	switch verb {
	case 'T':
		p.fmt.fmt_s(reflect.TypeOf(arg).String())
		return
	case 'p':
		p.fmtPointer(reflect.ValueOf(arg), 'p')
		return
	}

	// Some types can be done without reflection.
	switch f := arg.(type) {
	case bool:
		p.fmtBool(f, verb)
	case float32:
		p.fmtFloat(float64(f), 32, verb)
	...
	case reflect.Value:
		if f.IsValid() && f.CanInterface() {
			p.arg = f.Interface()
			if p.handleMethods(verb) {
				return
			}
		}
		p.printValue(f, verb, 0)
	default:
		if !p.handleMethods(verb) {
			p.printValue(reflect.ValueOf(f), verb, 0)
		}
	}
}

func (p *pp) badVerb(verb rune) {
	p.erroring = true
	p.buf.WriteString(percentBangString)
	p.buf.WriteRune(verb)
	p.buf.WriteByte('(')
	switch {
	case p.arg != nil:
		p.buf.WriteString(reflect.TypeOf(p.arg).String())
		p.buf.WriteByte('=')
		p.printArg(p.arg, 'v')
	...
	default:
		p.buf.WriteString(nilAngleString)
	}
	p.buf.WriteByte(')')
	p.erroring = false
}

fmt.Printf("%s", []int64{1, 2, 3})
// [%!s(int64=1) %!s(int64=2) %!s(int64=3)]%

func (p *pp) handleMethods(verb rune) (handled bool) {
	if p.erroring {
		return
	}
	// Is it a Formatter?
	if formatter, ok := p.arg.(Formatter); ok {
		handled = true
		defer p.catchPanic(p.arg, verb)
		formatter.Format(p, verb)
		return
	}

	// If we're doing Go syntax and the argument knows how to supply it, take care of it now.
	...
	
	return false
}

type State interface {
	Write(b []byte) (n int, err error)

	Width() (wid int, ok bool)

	Precision() (prec int, ok bool)

	Flag(c int) bool
}

type Formatter interface {
	Format(f State, c rune)
}

type Stringer interface {
	String() string
}

type GoStringer interface {
	GoString() string
}