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// Copyright 2022 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
package slog
import (
const nAttrsInline = 5
// A Record holds information about a log event.
// Copies of a Record share state.
// Do not modify a Record after handing out a copy to it.
// Use [Record.Clone] to create a copy with no shared state.
type Record struct {
// The time at which the output method (Log, Info, etc.) was called.
Time time.Time
// The log message.
Message string
// The level of the event.
Level Level
// The program counter at the time the record was constructed, as determined
// by runtime.Callers. If zero, no program counter is available.
// The only valid use for this value is as an argument to
// [runtime.CallersFrames]. In particular, it must not be passed to
// [runtime.FuncForPC].
PC uintptr
// Allocation optimization: an inline array sized to hold
// the majority of log calls (based on examination of open-source
// code). It holds the start of the list of Attrs.
front [nAttrsInline]Attr
// The number of Attrs in front.
nFront int
// The list of Attrs except for those in front.
// Invariants:
// - len(back) > 0 iff nFront == len(front)
// - Unused array elements are zero. Used to detect mistakes.
back []Attr
// NewRecord creates a Record from the given arguments.
// Use [Record.AddAttrs] to add attributes to the Record.
// NewRecord is intended for logging APIs that want to support a [Handler] as
// a backend.
func NewRecord(t time.Time, level Level, msg string, pc uintptr) Record {
return Record{
Time: t,
Message: msg,
Level: level,
PC: pc,
// Clone returns a copy of the record with no shared state.
// The original record and the clone can both be modified
// without interfering with each other.
func (r Record) Clone() Record {
r.back = slices.Clip(r.back) // prevent append from mutating shared array
return r
// NumAttrs returns the number of attributes in the Record.
func (r Record) NumAttrs() int {
return r.nFront + len(r.back)
// Attrs calls f on each Attr in the Record.
// Iteration stops if f returns false.
func (r Record) Attrs(f func(Attr) bool) {
for i := 0; i < r.nFront; i++ {
if !f(r.front[i]) {
for _, a := range r.back {
if !f(a) {
// AddAttrs appends the given Attrs to the Record's list of Attrs.
func (r *Record) AddAttrs(attrs ...Attr) {
n := copy(r.front[r.nFront:], attrs)
r.nFront += n
// Check if a copy was modified by slicing past the end
// and seeing if the Attr there is non-zero.
if cap(r.back) > len(r.back) {
end := r.back[:len(r.back)+1][len(r.back)]
if !end.isEmpty() {
panic("copies of a slog.Record were both modified")
r.back = append(r.back, attrs[n:]...)
// Add converts the args to Attrs as described in [Logger.Log],
// then appends the Attrs to the Record's list of Attrs.
func (r *Record) Add(args ...any) {
var a Attr
for len(args) > 0 {
a, args = argsToAttr(args)
if r.nFront < len(r.front) {
r.front[r.nFront] = a
} else {
if r.back == nil {
r.back = make([]Attr, 0, countAttrs(args))
r.back = append(r.back, a)
// countAttrs returns the number of Attrs that would be created from args.
func countAttrs(args []any) int {
n := 0
for i := 0; i < len(args); i++ {
if _, ok := args[i].(string); ok {
return n
const badKey = "!BADKEY"
// argsToAttr turns a prefix of the nonempty args slice into an Attr
// and returns the unconsumed portion of the slice.
// If args[0] is an Attr, it returns it.
// If args[0] is a string, it treats the first two elements as
// a key-value pair.
// Otherwise, it treats args[0] as a value with a missing key.
func argsToAttr(args []any) (Attr, []any) {
switch x := args[0].(type) {
case string:
if len(args) == 1 {
return String(badKey, x), nil
return Any(x, args[1]), args[2:]
case Attr:
return x, args[1:]
return Any(badKey, x), args[1:]
// Source describes the location of a line of source code.
type Source struct {
// Function is the package path-qualified function name containing the
// source line. If non-empty, this string uniquely identifies a single
// function in the program. This may be the empty string if not known.
Function string `json:"function"`
// File and Line are the file name and line number (1-based) of the source
// line. These may be the empty string and zero, respectively, if not known.
File string `json:"file"`
Line int `json:"line"`
// attrs returns the non-zero fields of s as a slice of attrs.
// It is similar to a LogValue method, but we don't want Source
// to implement LogValuer because it would be resolved before
// the ReplaceAttr function was called.
func (s *Source) group() Value {
var as []Attr
if s.Function != "" {
as = append(as, String("function", s.Function))
if s.File != "" {
as = append(as, String("file", s.File))
if s.Line != 0 {
as = append(as, Int("line", s.Line))
return GroupValue(as...)
// source returns a Source for the log event.
// If the Record was created without the necessary information,
// or if the location is unavailable, it returns a non-nil *Source
// with zero fields.
func (r Record) source() *Source {
fs := runtime.CallersFrames([]uintptr{r.PC})
f, _ := fs.Next()
return &Source{
Function: f.Function,
File: f.File,
Line: f.Line,