Source: print.go in package fmt


package fmt

import (
	"internal/fmtsort"
	"io"
	"os"
	"reflect"
	"sync"
	"unicode/utf8"
)

Strings for use with buffer.WriteString. This is less overhead than using buffer.Write with byte arrays.

const (
	commaSpaceString  = ", "
	nilAngleString    = "<nil>"
	nilParenString    = "(nil)"
	nilString         = "nil"
	mapString         = "map["
	percentBangString = "%!"
	missingString     = "(MISSING)"
	badIndexString    = "(BADINDEX)"
	panicString       = "(PANIC="
	extraString       = "%!(EXTRA "
	badWidthString    = "%!(BADWIDTH)"
	badPrecString     = "%!(BADPREC)"
	noVerbString      = "%!(NOVERB)"
	invReflectString  = "<invalid reflect.Value>"
)

State represents the printer state passed to custom formatters. It provides access to the io.Writer interface plus information about the flags and options for the operand's format specifier.

Write is the function to call to emit formatted output to be printed.

Width returns the value of the width option and whether it has been set.

Precision returns the value of the precision option and whether it has been set.

	Precision() (prec int, ok bool)

Flag reports whether the flag c, a character, has been set.

	Flag(c int) bool
}

Formatter is implemented by any value that has a Format method. The implementation controls how State and rune are interpreted, and may call Sprint(f) or Fprint(f) etc. to generate its output.

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

Stringer is implemented by any value that has a String method, which defines the ``native'' format for that value. The String method is used to print values passed as an operand to any format that accepts a string or to an unformatted printer such as Print.

type Stringer interface {
	String() string
}

GoStringer is implemented by any value that has a GoString method, which defines the Go syntax for that value. The GoString method is used to print values passed as an operand to a %#v format.

type GoStringer interface {
	GoString() string
}

Use simple []byte instead of bytes.Buffer to avoid large dependency.

type buffer []byte

func (b *buffer) write(p []byte) {
	*b = append(*b, p...)
}

func (b *buffer) writeString(s string) {
	*b = append(*b, s...)
}

func (b *buffer) writeByte(c byte) {
	*b = append(*b, c)
}

func (bp *buffer) writeRune(r rune) {
	if r < utf8.RuneSelf {
		*bp = append(*bp, byte(r))
		return
	}

	b := *bp
	n := len(b)
	for n+utf8.UTFMax > cap(b) {
		b = append(b, 0)
	}
	w := utf8.EncodeRune(b[n:n+utf8.UTFMax], r)
	*bp = b[:n+w]
}

pp is used to store a printer's state and is reused with sync.Pool to avoid allocations.

type pp struct {
	buf buffer

arg holds the current item, as an interface{}.

	arg interface{}

value is used instead of arg for reflect values.

	value reflect.Value

fmt is used to format basic items such as integers or strings.

	fmt fmt

reordered records whether the format string used argument reordering.

goodArgNum records whether the most recent reordering directive was valid.

panicking is set by catchPanic to avoid infinite panic, recover, panic, ... recursion.

erroring is set when printing an error string to guard against calling handleMethods.

wrapErrs is set when the format string may contain a %w verb.

wrappedErr records the target of the %w verb.

	wrappedErr error
}

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

newPrinter allocates a new pp struct or grabs a cached one.

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

free saves used pp structs in ppFree; avoids an allocation per invocation.

Proper usage of a sync.Pool requires each entry to have approximately the same memory cost. To obtain this property when the stored type contains a variably-sized buffer, we add a hard limit on the maximum buffer to place back in the pool. See https://golang.org/issue/23199

	if cap(p.buf) > 64<<10 {
		return
	}

	p.buf = p.buf[:0]
	p.arg = nil
	p.value = reflect.Value{}
	p.wrappedErr = nil
	ppFree.Put(p)
}

func (p *pp) Width() (wid int, ok bool) { return p.fmt.wid, p.fmt.widPresent }

func (p *pp) Precision() (prec int, ok bool) { return p.fmt.prec, p.fmt.precPresent }

func (p *pp) Flag(b int) bool {
	switch b {
	case '-':
		return p.fmt.minus
	case '+':
		return p.fmt.plus || p.fmt.plusV
	case '#':
		return p.fmt.sharp || p.fmt.sharpV
	case ' ':
		return p.fmt.space
	case '0':
		return p.fmt.zero
	}
	return false
}

Implement Write so we can call Fprintf on a pp (through State), for recursive use in custom verbs.

func (p *pp) Write(b []byte) (ret int, err error) {
	p.buf.write(b)
	return len(b), nil
}

Implement WriteString so that we can call io.WriteString on a pp (through state), for efficiency.

func (p *pp) WriteString(s string) (ret int, err error) {
	p.buf.writeString(s)
	return len(s), nil
}

These routines end in 'f' and take a format string.

Fprintf formats according to a format specifier and writes to w. It returns the number of bytes written and any write error encountered.

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
}

Printf formats according to a format specifier and writes to standard output. It returns the number of bytes written and any write error encountered.

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

Sprintf formats according to a format specifier and returns the resulting string.

func Sprintf(format string, a ...interface{}) string {
	p := newPrinter()
	p.doPrintf(format, a)
	s := string(p.buf)
	p.free()
	return s
}

These routines do not take a format string

Fprint formats using the default formats for its operands and writes to w. Spaces are added between operands when neither is a string. It returns the number of bytes written and any write error encountered.

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
}

Print formats using the default formats for its operands and writes to standard output. Spaces are added between operands when neither is a string. It returns the number of bytes written and any write error encountered.

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

Sprint formats using the default formats for its operands and returns the resulting string. Spaces are added between operands when neither is a string.

func Sprint(a ...interface{}) string {
	p := newPrinter()
	p.doPrint(a)
	s := string(p.buf)
	p.free()
	return s
}

These routines end in 'ln', do not take a format string, always add spaces between operands, and add a newline after the last operand.

Fprintln formats using the default formats for its operands and writes to w. Spaces are always added between operands and a newline is appended. It returns the number of bytes written and any write error encountered.

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
}

Println formats using the default formats for its operands and writes to standard output. Spaces are always added between operands and a newline is appended. It returns the number of bytes written and any write error encountered.

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

Sprintln formats using the default formats for its operands and returns the resulting string. Spaces are always added between operands and a newline is appended.

func Sprintln(a ...interface{}) string {
	p := newPrinter()
	p.doPrintln(a)
	s := string(p.buf)
	p.free()
	return s
}

getField gets the i'th field of the struct value. If the field is itself is an interface, return a value for the thing inside the interface, not the interface itself.

func getField(v reflect.Value, i int) reflect.Value {
	val := v.Field(i)
	if val.Kind() == reflect.Interface && !val.IsNil() {
		val = val.Elem()
	}
	return val
}

tooLarge reports whether the magnitude of the integer is too large to be used as a formatting width or precision.

func tooLarge(x int) bool {
	const max int = 1e6
	return x > max || x < -max
}

parsenum converts ASCII to integer. num is 0 (and isnum is false) if no number present.

func parsenum(s string, start, end int) (num int, isnum bool, newi int) {
	if start >= end {
		return 0, false, end
	}
	for newi = start; newi < end && '0' <= s[newi] && s[newi] <= '9'; newi++ {
		if tooLarge(num) {
			return 0, false, end // Overflow; crazy long number most likely.
		}
		num = num*10 + int(s[newi]-'0')
		isnum = true
	}
	return
}

func (p *pp) unknownType(v reflect.Value) {
	if !v.IsValid() {
		p.buf.writeString(nilAngleString)
		return
	}
	p.buf.writeByte('?')
	p.buf.writeString(v.Type().String())
	p.buf.writeByte('?')
}

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')
	case p.value.IsValid():
		p.buf.writeString(p.value.Type().String())
		p.buf.writeByte('=')
		p.printValue(p.value, 'v', 0)
	default:
		p.buf.writeString(nilAngleString)
	}
	p.buf.writeByte(')')
	p.erroring = false
}

func (p *pp) fmtBool(v bool, verb rune) {
	switch verb {
	case 't', 'v':
		p.fmt.fmtBoolean(v)
	default:
		p.badVerb(verb)
	}
}

fmt0x64 formats a uint64 in hexadecimal and prefixes it with 0x or not, as requested, by temporarily setting the sharp flag.

func (p *pp) fmt0x64(v uint64, leading0x bool) {
	sharp := p.fmt.sharp
	p.fmt.sharp = leading0x
	p.fmt.fmtInteger(v, 16, unsigned, 'v', ldigits)
	p.fmt.sharp = sharp
}

fmtInteger formats a signed or unsigned integer.

func (p *pp) fmtInteger(v uint64, isSigned bool, verb rune) {
	switch verb {
	case 'v':
		if p.fmt.sharpV && !isSigned {
			p.fmt0x64(v, true)
		} else {
			p.fmt.fmtInteger(v, 10, isSigned, verb, ldigits)
		}
	case 'd':
		p.fmt.fmtInteger(v, 10, isSigned, verb, ldigits)
	case 'b':
		p.fmt.fmtInteger(v, 2, isSigned, verb, ldigits)
	case 'o', 'O':
		p.fmt.fmtInteger(v, 8, isSigned, verb, ldigits)
	case 'x':
		p.fmt.fmtInteger(v, 16, isSigned, verb, ldigits)
	case 'X':
		p.fmt.fmtInteger(v, 16, isSigned, verb, udigits)
	case 'c':
		p.fmt.fmtC(v)
	case 'q':
		p.fmt.fmtQc(v)
	case 'U':
		p.fmt.fmtUnicode(v)
	default:
		p.badVerb(verb)
	}
}

fmtFloat formats a float. The default precision for each verb is specified as last argument in the call to fmt_float.

func (p *pp) fmtFloat(v float64, size int, verb rune) {
	switch verb {
	case 'v':
		p.fmt.fmtFloat(v, size, 'g', -1)
	case 'b', 'g', 'G', 'x', 'X':
		p.fmt.fmtFloat(v, size, verb, -1)
	case 'f', 'e', 'E':
		p.fmt.fmtFloat(v, size, verb, 6)
	case 'F':
		p.fmt.fmtFloat(v, size, 'f', 6)
	default:
		p.badVerb(verb)
	}
}

fmtComplex formats a complex number v with r = real(v) and j = imag(v) as (r+ji) using fmtFloat for r and j formatting.

Make sure any unsupported verbs are found before the calls to fmtFloat to not generate an incorrect error string.

	switch verb {
	case 'v', 'b', 'g', 'G', 'x', 'X', 'f', 'F', 'e', 'E':
		oldPlus := p.fmt.plus
		p.buf.writeByte('(')

Imaginary part always has a sign.

		p.fmt.plus = true
		p.fmtFloat(imag(v), size/2, verb)
		p.buf.writeString("i)")
		p.fmt.plus = oldPlus
	default:
		p.badVerb(verb)
	}
}

func (p *pp) fmtString(v string, verb rune) {
	switch verb {
	case 'v':
		if p.fmt.sharpV {
			p.fmt.fmtQ(v)
		} else {
			p.fmt.fmtS(v)
		}
	case 's':
		p.fmt.fmtS(v)
	case 'x':
		p.fmt.fmtSx(v, ldigits)
	case 'X':
		p.fmt.fmtSx(v, udigits)
	case 'q':
		p.fmt.fmtQ(v)
	default:
		p.badVerb(verb)
	}
}

func (p *pp) fmtBytes(v []byte, verb rune, typeString string) {
	switch verb {
	case 'v', 'd':
		if p.fmt.sharpV {
			p.buf.writeString(typeString)
			if v == nil {
				p.buf.writeString(nilParenString)
				return
			}
			p.buf.writeByte('{')
			for i, c := range v {
				if i > 0 {
					p.buf.writeString(commaSpaceString)
				}
				p.fmt0x64(uint64(c), true)
			}
			p.buf.writeByte('}')
		} else {
			p.buf.writeByte('[')
			for i, c := range v {
				if i > 0 {
					p.buf.writeByte(' ')
				}
				p.fmt.fmtInteger(uint64(c), 10, unsigned, verb, ldigits)
			}
			p.buf.writeByte(']')
		}
	case 's':
		p.fmt.fmtBs(v)
	case 'x':
		p.fmt.fmtBx(v, ldigits)
	case 'X':
		p.fmt.fmtBx(v, udigits)
	case 'q':
		p.fmt.fmtQ(string(v))
	default:
		p.printValue(reflect.ValueOf(v), verb, 0)
	}
}

func (p *pp) fmtPointer(value reflect.Value, verb rune) {
	var u uintptr
	switch value.Kind() {
	case reflect.Chan, reflect.Func, reflect.Map, reflect.Ptr, reflect.Slice, reflect.UnsafePointer:
		u = value.Pointer()
	default:
		p.badVerb(verb)
		return
	}

	switch verb {
	case 'v':
		if p.fmt.sharpV {
			p.buf.writeByte('(')
			p.buf.writeString(value.Type().String())
			p.buf.writeString(")(")
			if u == 0 {
				p.buf.writeString(nilString)
			} else {
				p.fmt0x64(uint64(u), true)
			}
			p.buf.writeByte(')')
		} else {
			if u == 0 {
				p.fmt.padString(nilAngleString)
			} else {
				p.fmt0x64(uint64(u), !p.fmt.sharp)
			}
		}
	case 'p':
		p.fmt0x64(uint64(u), !p.fmt.sharp)
	case 'b', 'o', 'd', 'x', 'X':
		p.fmtInteger(uint64(u), unsigned, verb)
	default:
		p.badVerb(verb)
	}
}

func (p *pp) catchPanic(arg interface{}, verb rune, method string) {

If it's a nil pointer, just say "<nil>". The likeliest causes are a Stringer that fails to guard against nil or a nil pointer for a value receiver, and in either case, "<nil>" is a nice result.

		if v := reflect.ValueOf(arg); v.Kind() == reflect.Ptr && v.IsNil() {
			p.buf.writeString(nilAngleString)
			return

Otherwise print a concise panic message. Most of the time the panic value will print itself nicely.

Nested panics; the recursion in printArg cannot succeed.

			panic(err)
		}

For this output we want default behavior.

		p.fmt.clearflags()

		p.buf.writeString(percentBangString)
		p.buf.writeRune(verb)
		p.buf.writeString(panicString)
		p.buf.writeString(method)
		p.buf.writeString(" method: ")
		p.panicking = true
		p.printArg(err, 'v')
		p.panicking = false
		p.buf.writeByte(')')

		p.fmt.fmtFlags = oldFlags
	}
}

func (p *pp) handleMethods(verb rune) (handled bool) {
	if p.erroring {
		return
	}

It is invalid to use %w other than with Errorf, more than once, or with a non-error arg.

		err, ok := p.arg.(error)
		if !ok || !p.wrapErrs || p.wrappedErr != nil {
			p.wrappedErr = nil
			p.wrapErrs = false
			p.badVerb(verb)
			return true
		}

If the arg is a Formatter, pass 'v' as the verb to it.

		verb = 'v'
	}

Is it a Formatter?

	if formatter, ok := p.arg.(Formatter); ok {
		handled = true
		defer p.catchPanic(p.arg, verb, "Format")
		formatter.Format(p, verb)
		return
	}

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

	if p.fmt.sharpV {
		if stringer, ok := p.arg.(GoStringer); ok {
			handled = true

Print the result of GoString unadorned.

			p.fmt.fmtS(stringer.GoString())
			return
		}

If a string is acceptable according to the format, see if the value satisfies one of the string-valued interfaces. Println etc. set verb to %v, which is "stringable".

		switch verb {

Is it an error or Stringer? The duplication in the bodies is necessary: setting handled and deferring catchPanic must happen before calling the method.

			switch v := p.arg.(type) {
			case error:
				handled = true
				defer p.catchPanic(p.arg, verb, "Error")
				p.fmtString(v.Error(), verb)
				return

			case Stringer:
				handled = true
				defer p.catchPanic(p.arg, verb, "String")
				p.fmtString(v.String(), verb)
				return
			}
		}
	}
	return false
}

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
	}

Special processing considerations. %T (the value's type) and %p (its address) are special; we always do them first.

	switch verb {
	case 'T':
		p.fmt.fmtS(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 float64:
		p.fmtFloat(f, 64, verb)
	case complex64:
		p.fmtComplex(complex128(f), 64, verb)
	case complex128:
		p.fmtComplex(f, 128, verb)
	case int:
		p.fmtInteger(uint64(f), signed, verb)
	case int8:
		p.fmtInteger(uint64(f), signed, verb)
	case int16:
		p.fmtInteger(uint64(f), signed, verb)
	case int32:
		p.fmtInteger(uint64(f), signed, verb)
	case int64:
		p.fmtInteger(uint64(f), signed, verb)
	case uint:
		p.fmtInteger(uint64(f), unsigned, verb)
	case uint8:
		p.fmtInteger(uint64(f), unsigned, verb)
	case uint16:
		p.fmtInteger(uint64(f), unsigned, verb)
	case uint32:
		p.fmtInteger(uint64(f), unsigned, verb)
	case uint64:
		p.fmtInteger(f, unsigned, verb)
	case uintptr:
		p.fmtInteger(uint64(f), unsigned, verb)
	case string:
		p.fmtString(f, verb)
	case []byte:
		p.fmtBytes(f, verb, "[]byte")

Handle extractable values with special methods since printValue does not handle them at depth 0.

		if f.IsValid() && f.CanInterface() {
			p.arg = f.Interface()
			if p.handleMethods(verb) {
				return
			}
		}
		p.printValue(f, verb, 0)

If the type is not simple, it might have methods.

Need to use reflection, since the type had no interface methods that could be used for formatting.

			p.printValue(reflect.ValueOf(f), verb, 0)
		}
	}
}

printValue is similar to printArg but starts with a reflect value, not an interface{} value. It does not handle 'p' and 'T' verbs because these should have been already handled by printArg.

Handle values with special methods if not already handled by printArg (depth == 0).

	if depth > 0 && value.IsValid() && value.CanInterface() {
		p.arg = value.Interface()
		if p.handleMethods(verb) {
			return
		}
	}
	p.arg = nil
	p.value = value

	switch f := value; value.Kind() {
	case reflect.Invalid:
		if depth == 0 {
			p.buf.writeString(invReflectString)
		} else {
			switch verb {
			case 'v':
				p.buf.writeString(nilAngleString)
			default:
				p.badVerb(verb)
			}
		}
	case reflect.Bool:
		p.fmtBool(f.Bool(), verb)
	case reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64:
		p.fmtInteger(uint64(f.Int()), signed, verb)
	case reflect.Uint, reflect.Uint8, reflect.Uint16, reflect.Uint32, reflect.Uint64, reflect.Uintptr:
		p.fmtInteger(f.Uint(), unsigned, verb)
	case reflect.Float32:
		p.fmtFloat(f.Float(), 32, verb)
	case reflect.Float64:
		p.fmtFloat(f.Float(), 64, verb)
	case reflect.Complex64:
		p.fmtComplex(f.Complex(), 64, verb)
	case reflect.Complex128:
		p.fmtComplex(f.Complex(), 128, verb)
	case reflect.String:
		p.fmtString(f.String(), verb)
	case reflect.Map:
		if p.fmt.sharpV {
			p.buf.writeString(f.Type().String())
			if f.IsNil() {
				p.buf.writeString(nilParenString)
				return
			}
			p.buf.writeByte('{')
		} else {
			p.buf.writeString(mapString)
		}
		sorted := fmtsort.Sort(f)
		for i, key := range sorted.Key {
			if i > 0 {
				if p.fmt.sharpV {
					p.buf.writeString(commaSpaceString)
				} else {
					p.buf.writeByte(' ')
				}
			}
			p.printValue(key, verb, depth+1)
			p.buf.writeByte(':')
			p.printValue(sorted.Value[i], verb, depth+1)
		}
		if p.fmt.sharpV {
			p.buf.writeByte('}')
		} else {
			p.buf.writeByte(']')
		}
	case reflect.Struct:
		if p.fmt.sharpV {
			p.buf.writeString(f.Type().String())
		}
		p.buf.writeByte('{')
		for i := 0; i < f.NumField(); i++ {
			if i > 0 {
				if p.fmt.sharpV {
					p.buf.writeString(commaSpaceString)
				} else {
					p.buf.writeByte(' ')
				}
			}
			if p.fmt.plusV || p.fmt.sharpV {
				if name := f.Type().Field(i).Name; name != "" {
					p.buf.writeString(name)
					p.buf.writeByte(':')
				}
			}
			p.printValue(getField(f, i), verb, depth+1)
		}
		p.buf.writeByte('}')
	case reflect.Interface:
		value := f.Elem()
		if !value.IsValid() {
			if p.fmt.sharpV {
				p.buf.writeString(f.Type().String())
				p.buf.writeString(nilParenString)
			} else {
				p.buf.writeString(nilAngleString)
			}
		} else {
			p.printValue(value, verb, depth+1)
		}
	case reflect.Array, reflect.Slice:
		switch verb {

Handle byte and uint8 slices and arrays special for the above verbs.

			t := f.Type()
			if t.Elem().Kind() == reflect.Uint8 {
				var bytes []byte
				if f.Kind() == reflect.Slice {
					bytes = f.Bytes()
				} else if f.CanAddr() {
					bytes = f.Slice(0, f.Len()).Bytes()

We have an array, but we cannot Slice() a non-addressable array, so we build a slice by hand. This is a rare case but it would be nice if reflection could help a little more.

					bytes = make([]byte, f.Len())
					for i := range bytes {
						bytes[i] = byte(f.Index(i).Uint())
					}
				}
				p.fmtBytes(bytes, verb, t.String())
				return
			}
		}
		if p.fmt.sharpV {
			p.buf.writeString(f.Type().String())
			if f.Kind() == reflect.Slice && f.IsNil() {
				p.buf.writeString(nilParenString)
				return
			}
			p.buf.writeByte('{')
			for i := 0; i < f.Len(); i++ {
				if i > 0 {
					p.buf.writeString(commaSpaceString)
				}
				p.printValue(f.Index(i), verb, depth+1)
			}
			p.buf.writeByte('}')
		} else {
			p.buf.writeByte('[')
			for i := 0; i < f.Len(); i++ {
				if i > 0 {
					p.buf.writeByte(' ')
				}
				p.printValue(f.Index(i), verb, depth+1)
			}
			p.buf.writeByte(']')
		}

pointer to array or slice or struct? ok at top level but not embedded (avoid loops)

		if depth == 0 && f.Pointer() != 0 {
			switch a := f.Elem(); a.Kind() {
			case reflect.Array, reflect.Slice, reflect.Struct, reflect.Map:
				p.buf.writeByte('&')
				p.printValue(a, verb, depth+1)
				return
			}
		}
		fallthrough
	case reflect.Chan, reflect.Func, reflect.UnsafePointer:
		p.fmtPointer(f, verb)
	default:
		p.unknownType(f)
	}
}

intFromArg gets the argNumth element of a. On return, isInt reports whether the argument has integer type.

func intFromArg(a []interface{}, argNum int) (num int, isInt bool, newArgNum int) {
	newArgNum = argNum
	if argNum < len(a) {
		num, isInt = a[argNum].(int) // Almost always OK.

Work harder.

			switch v := reflect.ValueOf(a[argNum]); v.Kind() {
			case reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64:
				n := v.Int()
				if int64(int(n)) == n {
					num = int(n)
					isInt = true
				}
			case reflect.Uint, reflect.Uint8, reflect.Uint16, reflect.Uint32, reflect.Uint64, reflect.Uintptr:
				n := v.Uint()
				if int64(n) >= 0 && uint64(int(n)) == n {
					num = int(n)
					isInt = true
				}

Already 0, false.

			}
		}
		newArgNum = argNum + 1
		if tooLarge(num) {
			num = 0
			isInt = false
		}
	}
	return
}

parseArgNumber returns the value of the bracketed number, minus 1 (explicit argument numbers are one-indexed but we want zero-indexed). The opening bracket is known to be present at format[0]. The returned values are the index, the number of bytes to consume up to the closing paren, if present, and whether the number parsed ok. The bytes to consume will be 1 if no closing paren is present.

There must be at least 3 bytes: [n].

	if len(format) < 3 {
		return 0, 1, false
	}

Find closing bracket.

	for i := 1; i < len(format); i++ {
		if format[i] == ']' {
			width, ok, newi := parsenum(format, 1, i)
			if !ok || newi != i {
				return 0, i + 1, false
			}
			return width - 1, i + 1, true // arg numbers are one-indexed and skip paren.
		}
	}
	return 0, 1, false
}

argNumber returns the next argument to evaluate, which is either the value of the passed-in argNum or the value of the bracketed integer that begins format[i:]. It also returns the new value of i, that is, the index of the next byte of the format to process.

func (p *pp) argNumber(argNum int, format string, i int, numArgs int) (newArgNum, newi int, found bool) {
	if len(format) <= i || format[i] != '[' {
		return argNum, i, false
	}
	p.reordered = true
	index, wid, ok := parseArgNumber(format[i:])
	if ok && 0 <= index && index < numArgs {
		return index, i + wid, true
	}
	p.goodArgNum = false
	return argNum, i + wid, ok
}

func (p *pp) badArgNum(verb rune) {
	p.buf.writeString(percentBangString)
	p.buf.writeRune(verb)
	p.buf.writeString(badIndexString)
}

func (p *pp) missingArg(verb rune) {
	p.buf.writeString(percentBangString)
	p.buf.writeRune(verb)
	p.buf.writeString(missingString)
}

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])
		}

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 '#':
				p.fmt.sharp = true
			case '0':
				p.fmt.zero = !p.fmt.minus // Only allow zero padding to the left.
			case '+':
				p.fmt.plus = true
			case '-':
				p.fmt.minus = true
				p.fmt.zero = false // Do not pad with zeros to the right.
			case ' ':
				p.fmt.space = true

Fast path for common case of ascii lower case simple verbs without precision or width or argument indices.

				if 'a' <= c && c <= 'z' && argNum < len(a) {

Go syntax

						p.fmt.sharpV = p.fmt.sharp

Struct-field syntax

						p.fmt.plusV = p.fmt.plus
						p.fmt.plus = false
					}
					p.printArg(a[argNum], rune(c))
					argNum++
					i++
					continue formatLoop

Format is more complex than simple flags and a verb or is malformed.

				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] == '*' {
			i++
			p.fmt.wid, p.fmt.widPresent, argNum = intFromArg(a, argNum)

			if !p.fmt.widPresent {
				p.buf.writeString(badWidthString)
			}

We have a negative width, so take its value and ensure that the minus flag is set

			if p.fmt.wid < 0 {
				p.fmt.wid = -p.fmt.wid
				p.fmt.minus = true
				p.fmt.zero = false // Do not pad with zeros to the right.
			}
			afterIndex = false
		} else {
			p.fmt.wid, p.fmt.widPresent, i = parsenum(format, i, end)
			if afterIndex && p.fmt.widPresent { // "%[3]2d"
				p.goodArgNum = false
			}
		}

Do we have precision?

		if i+1 < end && format[i] == '.' {
			i++
			if afterIndex { // "%[3].2d"
				p.goodArgNum = false
			}
			argNum, i, afterIndex = p.argNumber(argNum, format, i, len(a))
			if i < end && format[i] == '*' {
				i++

Negative precision arguments don't make sense

				if p.fmt.prec < 0 {
					p.fmt.prec = 0
					p.fmt.precPresent = false
				}
				if !p.fmt.precPresent {
					p.buf.writeString(badPrecString)
				}
				afterIndex = false
			} else {
				p.fmt.prec, p.fmt.precPresent, i = parsenum(format, i, end)
				if !p.fmt.precPresent {
					p.fmt.prec = 0
					p.fmt.precPresent = true
				}
			}
		}

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

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

		verb, size := rune(format[i]), 1
		if verb >= utf8.RuneSelf {
			verb, size = utf8.DecodeRuneInString(format[i:])
		}
		i += size

		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)

Go syntax

			p.fmt.sharpV = p.fmt.sharp

Struct-field syntax

			p.fmt.plusV = p.fmt.plus
			p.fmt.plus = false
			fallthrough
		default:
			p.printArg(a[argNum], verb)
			argNum++
		}
	}

Check for extra arguments unless the call accessed the arguments out of order, in which case it's too expensive to detect if they've all been used and arguably OK if they're not.

	if !p.reordered && argNum < len(a) {
		p.fmt.clearflags()
		p.buf.writeString(extraString)
		for i, arg := range a[argNum:] {
			if i > 0 {
				p.buf.writeString(commaSpaceString)
			}
			if arg == nil {
				p.buf.writeString(nilAngleString)
			} else {
				p.buf.writeString(reflect.TypeOf(arg).String())
				p.buf.writeByte('=')
				p.printArg(arg, 'v')
			}
		}
		p.buf.writeByte(')')
	}
}

func (p *pp) doPrint(a []interface{}) {
	prevString := false
	for argNum, arg := range a {

Add a space between two non-string arguments.

		if argNum > 0 && !isString && !prevString {
			p.buf.writeByte(' ')
		}
		p.printArg(arg, 'v')
		prevString = isString
	}
}

doPrintln is like doPrint but always adds a space between arguments and a newline after the last argument.

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')