Source: print.go in package runtime


package runtime

import (
	"runtime/internal/atomic"
	"runtime/internal/sys"
	"unsafe"
)

The compiler knows that a print of a value of this type should use printhex instead of printuint (decimal).

type hex uint64

func bytes(s string) (ret []byte) {
	rp := (*slice)(unsafe.Pointer(&ret))
	sp := stringStructOf(&s)
	rp.array = sp.str
	rp.len = sp.len
	rp.cap = sp.len
	return
}

printBacklog is a circular buffer of messages written with the builtin print* functions, for use in postmortem analysis of core dumps.

	printBacklog      [512]byte
	printBacklogIndex int
)

recordForPanic maintains a circular buffer of messages written by the runtime leading up to a process crash, allowing the messages to be extracted from a core dump. The text written during a process crash (following "panic" or "fatal error") is not saved, since the goroutine stacks will generally be readable from the runtime datastructures in the core file.

func recordForPanic(b []byte) {
	printlock()

Not actively crashing: maintain circular buffer of print output.

		for i := 0; i < len(b); {
			n := copy(printBacklog[printBacklogIndex:], b[i:])
			i += n
			printBacklogIndex += n
			printBacklogIndex %= len(printBacklog)
		}
	}

	printunlock()
}

var debuglock mutex

The compiler emits calls to printlock and printunlock around the multiple calls that implement a single Go print or println statement. Some of the print helpers (printslice, for example) call print recursively. There is also the problem of a crash happening during the print routines and needing to acquire the print lock to print information about the crash. For both these reasons, let a thread acquire the printlock 'recursively'.


func printlock() {
	mp := getg().m
	mp.locks++ // do not reschedule between printlock++ and lock(&debuglock).
	mp.printlock++
	if mp.printlock == 1 {
		lock(&debuglock)
	}
	mp.locks-- // now we know debuglock is held and holding up mp.locks for us.
}

func printunlock() {
	mp := getg().m
	mp.printlock--
	if mp.printlock == 0 {
		unlock(&debuglock)
	}
}

write to goroutine-local buffer if diverting output, or else standard error.

func gwrite(b []byte) {
	if len(b) == 0 {
		return
	}
	recordForPanic(b)

Don't use the writebuf if gp.m is dying. We want anything written through gwrite to appear in the terminal rather than be written to in some buffer, if we're in a panicking state. Note that we can't just clear writebuf in the gp.m.dying case because a panic isn't allowed to have any write barriers.

	if gp == nil || gp.writebuf == nil || gp.m.dying > 0 {
		writeErr(b)
		return
	}

	n := copy(gp.writebuf[len(gp.writebuf):cap(gp.writebuf)], b)
	gp.writebuf = gp.writebuf[:len(gp.writebuf)+n]
}

func printsp() {
	printstring(" ")
}

func printnl() {
	printstring("\n")
}

func printbool(v bool) {
	if v {
		printstring("true")
	} else {
		printstring("false")
	}
}

func printfloat(v float64) {
	switch {
	case v != v:
		printstring("NaN")
		return
	case v+v == v && v > 0:
		printstring("+Inf")
		return
	case v+v == v && v < 0:
		printstring("-Inf")
		return
	}

	const n = 7 // digits printed
	var buf [n + 7]byte
	buf[0] = '+'
	e := 0 // exp
	if v == 0 {
		if 1/v < 0 {
			buf[0] = '-'
		}
	} else {
		if v < 0 {
			v = -v
			buf[0] = '-'
		}

normalize

		for v >= 10 {
			e++
			v /= 10
		}
		for v < 1 {
			e--
			v *= 10
		}

round

		h := 5.0
		for i := 0; i < n; i++ {
			h /= 10
		}
		v += h
		if v >= 10 {
			e++
			v /= 10
		}
	}

format +d.dddd+edd

	for i := 0; i < n; i++ {
		s := int(v)
		buf[i+2] = byte(s + '0')
		v -= float64(s)
		v *= 10
	}
	buf[1] = buf[2]
	buf[2] = '.'

	buf[n+2] = 'e'
	buf[n+3] = '+'
	if e < 0 {
		e = -e
		buf[n+3] = '-'
	}

	buf[n+4] = byte(e/100) + '0'
	buf[n+5] = byte(e/10)%10 + '0'
	buf[n+6] = byte(e%10) + '0'
	gwrite(buf[:])
}

func printcomplex(c complex128) {
	print("(", real(c), imag(c), "i)")
}

func printuint(v uint64) {
	var buf [100]byte
	i := len(buf)
	for i--; i > 0; i-- {
		buf[i] = byte(v%10 + '0')
		if v < 10 {
			break
		}
		v /= 10
	}
	gwrite(buf[i:])
}

func printint(v int64) {
	if v < 0 {
		printstring("-")
		v = -v
	}
	printuint(uint64(v))
}

func printhex(v uint64) {
	const dig = "0123456789abcdef"
	var buf [100]byte
	i := len(buf)
	for i--; i > 0; i-- {
		buf[i] = dig[v%16]
		if v < 16 {
			break
		}
		v /= 16
	}
	i--
	buf[i] = 'x'
	i--
	buf[i] = '0'
	gwrite(buf[i:])
}

func printpointer(p unsafe.Pointer) {
	printhex(uint64(uintptr(p)))
}
func printuintptr(p uintptr) {
	printhex(uint64(p))
}

func printstring(s string) {
	gwrite(bytes(s))
}

func printslice(s []byte) {
	sp := (*slice)(unsafe.Pointer(&s))
	print("[", len(s), "/", cap(s), "]")
	printpointer(sp.array)
}

func printeface(e eface) {
	print("(", e._type, ",", e.data, ")")
}

func printiface(i iface) {
	print("(", i.tab, ",", i.data, ")")
}

hexdumpWords prints a word-oriented hex dump of [p, end). If mark != nil, it will be called with each printed word's address and should return a character mark to appear just before that word's value. It can return 0 to indicate no mark.

func hexdumpWords(p, end uintptr, mark func(uintptr) byte) {
	p1 := func(x uintptr) {
		var buf [2 * sys.PtrSize]byte
		for i := len(buf) - 1; i >= 0; i-- {
			if x&0xF < 10 {
				buf[i] = byte(x&0xF) + '0'
			} else {
				buf[i] = byte(x&0xF) - 10 + 'a'
			}
			x >>= 4
		}
		gwrite(buf[:])
	}

	printlock()
	var markbuf [1]byte
	markbuf[0] = ' '
	for i := uintptr(0); p+i < end; i += sys.PtrSize {
		if i%16 == 0 {
			if i != 0 {
				println()
			}
			p1(p + i)
			print(": ")
		}

		if mark != nil {
			markbuf[0] = mark(p + i)
			if markbuf[0] == 0 {
				markbuf[0] = ' '
			}
		}
		gwrite(markbuf[:])
		val := *(*uintptr)(unsafe.Pointer(p + i))
		p1(val)
		print(" ")

Can we symbolize val?

		fn := findfunc(val)
		if fn.valid() {
			print("<", funcname(fn), "+", val-fn.entry, "> ")
		}
	}
	println()
	printunlock()