原文链接： http://www.cloudnoter.com/?p=131
flag包是用来处理命令参数的。总得来说，其通过将命令行标志与某一具体变量进行绑定，开发人员通过使用该变量进行业务逻辑处理。
一、FlagSet是该包的核心类型：type FlagSet struct {
// Usage is the function called when an error occurs while parsing flags.
// The field is a function (not a method) that may be changed to point to
// a custom error handler.
Usage func()
name string
parsed bool
actual map[string]*Flag // 存放命令行中实际输入的标志映射
formal map[string]*Flag // 存放该实例可以处理的命令行标志映射
args []string // arguments after flags 存放非标志的参数列表（即标志后面的参数）
exitOnError bool // does the program exit if there's an error?
errorHandling ErrorHandling
output io.Writer // nil means stderr; use out() accessor
}该类型同时提供了一系列的方法集合【MethodSet】，通过该方法集用于可以实现灵活的命令行标志处理。二、flag包export的变量:CommandLine
// CommandLine is the default set of command-line flags, parsed from os.Args. // The top-level functions such as BoolVar, Arg, and on are wrappers for the // methods of CommandLine. var CommandLine = NewFlagSet(os.Args[0], ExitOnError)该包提供了一个默认变量：CommandLine，其为FlatSet的一个变量（用面向对象的术语叫做：FlagSet的一个实例）
该flag包export的所有函数本质上都是调用FlagSet类型变量（实例）：CommandLine的方法实现。如下：
// Int defines an int flag with specified name, default value, and usage string.
// The return value is the address of an int variable that stores the value of the flag.
func Int(name string, value int, usage string) *int {
return CommandLine.Int(name, value, usage)
}三、flag包支持的标志格式
Command line flag syntax:
-flag // 代表bool值，相当于-flag=true
-flag=x
-flag x // non-boolean flags only 不支持bool值标志
One or two minus signs may be used; they are equivalent. // -flag=value与--flag=value是等效的
The last form is not permitted for boolean flags because the
meaning of the command
cmd -x *
will change if there is a file called 0, false, etc. You must
use the -flag=false form to turn off a boolean flag.
Flag parsing stops just before the first non-flag argument
("-" is a non-flag argument) or after the terminator "--". // 碰到连续两个"-"号且参数长度为2时则终止标志解析四、标志绑定相关方法：以绑定int类型为例。
// IntVar defines an int flag with specified name, default value, and usage string.
// The argument p points to an int variable in which to store the value of the flag.
// FlagSet提供的绑定int类型标志的方法，无返回值。通过传入int类型指针变量进行绑定，当调用该方法后，会将绑定信息存入FlagSet.formal映射中
func (f *FlagSet) IntVar(p *int, name string, value int, usage string) {
f.Var(newIntValue(value, p), name, usage)
}
//与上述方法相对应的flag包export的函数：
// IntVar defines an int flag with specified name, default value, and usage string.
// The argument p points to an int variable in which to store the value of the flag.
func IntVar(p *int, name string, value int, usage string) {
CommandLine.Var(newIntValue(value, p), name, usage)
}
//使用示例
var flagvar int
flag.IntVar(&flagvar, "flagname2", 1234, "help message for flagname2")
// Int defines an int flag with specified name, default value, and usage string.
// The return value is the address of an int variable that stores the value of the flag.
// FlagSet提供的绑定int类型标志的方法，有返回值，返回int类型指针，当调用该方法后，会将绑定信息存入FlagSet.formal映射中
func (f *FlagSet) Int(name string, value int, usage string) *int {
p := new(int)
f.IntVar(p, name, value, usage)
return p
}
//与上述方法相对应的flag包export的函数：
// Int defines an int flag with specified name, default value, and usage string.
// The return value is the address of an int variable that stores the value of the flag.
func Int(name string, value int, usage string) *int {
return CommandLine.Int(name, value, usage)
}
//使用示例
var flagvar = flag.Int("flagname", 1234, "help message for flagname")五、解析标志的相关关键源码
// Parse parses the command-line flags from os.Args[1:]. Must be called
// after all flags are defined and before flags are accessed by the program.
// flag包export的函数，调用时机为：在设置好标志与变量的绑定关系后，调用flag.Parse()。
func Parse() {
// Ignore errors; CommandLine is set for ExitOnError.
CommandLine.Parse(os.Args[1:])
}
// Parse parses flag definitions from the argument list, which should not
// include the command name. Must be called after all flags in the FlagSet
// are defined and before flags are accessed by the program.
// The return value will be ErrHelp if -help was set but not defined.
// FlagSet类型提供的实现方法
func (f *FlagSet) Parse(arguments []string) error {
f.parsed = true
f.args = arguments
for {
seen, err := f.parseOne()
if seen {
continue
}
if err == nil {
break
}
switch f.errorHandling {
case ContinueOnError:
return err
case ExitOnError:
os.Exit(2)
case PanicOnError:
panic(err)
}
}
return nil
}
// parseOne parses one flag. It reports whether a flag was seen.
// 解析每个标志并返回相关结果，若碰到 '-' 或 '--' 时也会直接终止整个标志解析过程，每解析成功一个标志就会将该标志信息放入FlagSet.actual映射中
func (f *FlagSet) parseOne() (bool, error) {
if len(f.args) == 0 {
return false, nil
}
s := f.args[0]
if len(s) == 0 || s[0] != '-' || len(s) == 1 {
return false, nil
}
num_minuses := 1
if s[1] == '-' {
num_minuses++
if len(s) == 2 { // "--" terminates the flags
f.args = f.args[1:]
return false, nil
}
}
name := s[num_minuses:]
if len(name) == 0 || name[0] == '-' || name[0] == '=' {
return false, f.failf("bad flag syntax: %s", s)
}
// it's a flag. does it have an argument?
f.args = f.args[1:]
has_value := false
value := ""
for i := 1; i < len(name); i++ { // equals cannot be first
if name[i] == '=' {
value = name[i+1:]
has_value = true
name = name[0:i]
break
}
}
m := f.formal
flag, alreadythere := m[name] // BUG
if !alreadythere {
if name == "help" || name == "h" { // special case for nice help message.
f.usage()
return false, ErrHelp
}
return false, f.failf("flag provided but not defined: -%s", name)
}
if fv, ok := flag.Value.(boolFlag); ok && fv.IsBoolFlag() { // special case: doesn't need an arg
if has_value {
if err := fv.Set(value); err != nil {
return false, f.failf("invalid boolean value %q for -%s: %v", value, name, err)
}
} else {
fv.Set("true")
}
} else {
// It must have a value, which might be the next argument.
if !has_value && len(f.args) > 0 {
// value is the next arg
has_value = true
value, f.args = f.args[0], f.args[1:]
}
if !has_value {
return false, f.failf("flag needs an argument: -%s", name)
}
if err := flag.Value.Set(value); err != nil {
return false, f.failf("invalid value %q for flag -%s: %v", value, name, err)
}
}
if f.actual == nil {
f.actual = make(map[string]*Flag)
}
f.actual[name] = flag
return true, nil
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