Listing 5.3: Setting FlagSet's output

What's changed?

--- a/hit/cmd/hit/config.go
+++ b/hit/cmd/hit/config.go
@@ -3,7 +3,8 @@
 import (
 	"errors"
 	"flag"
 	"fmt"
+	"io"
 	"net/url"
 	"strconv"
 )
@@ -18,24 +19,26 @@
-func parseArgs(c *config, args []string) error {
+func parseArgs(c *config, args []string, stderr io.Writer) error {
 	fs := flag.NewFlagSet("hit", flag.ContinueOnError)
+	fs.SetOutput(stderr)
+
 	fs.Usage = func() {
 		fmt.Fprintf(fs.Output(), "usage: %s [options] url\n", fs.Name())
 		fs.PrintDefaults()
 	}
 
 	fs.Var(asPositiveIntValue(&c.n), "n", "Number of requests")
 	fs.Var(asPositiveIntValue(&c.c), "c", "Concurrency level")
 	fs.Var(asPositiveIntValue(&c.rps), "rps", "Requests per second")
 
 	if err := fs.Parse(args); err != nil {
 		return err
 	}
 	c.url = fs.Arg(0)
 
 	if err := validateArgs(c); err != nil {
 		fmt.Fprintln(fs.Output(), err)
 		fs.Usage()
 		return err
 	}
 
 	return nil
 }

Code in the file

hit / cmd / hit / config.go

package main

import (
	"errors"
	"flag"
	"fmt"
	"io"
	"net/url"
	"strconv"
)

type config struct {
	url string
	n   int
	c   int
	rps int
}

func parseArgs(c *config, args []string, stderr io.Writer) error {
	fs := flag.NewFlagSet("hit", flag.ContinueOnError)
	fs.SetOutput(stderr)

	fs.Usage = func() {
		fmt.Fprintf(fs.Output(), "usage: %s [options] url\n", fs.Name())
		fs.PrintDefaults()
	}

	fs.Var(asPositiveIntValue(&c.n), "n", "Number of requests")
	fs.Var(asPositiveIntValue(&c.c), "c", "Concurrency level")
	fs.Var(asPositiveIntValue(&c.rps), "rps", "Requests per second")

	if err := fs.Parse(args); err != nil {
		return err
	}
	c.url = fs.Arg(0)

	if err := validateArgs(c); err != nil {
		fmt.Fprintln(fs.Output(), err)
		fs.Usage()
		return err
	}

	return nil
}

func validateArgs(c *config) error {
	u, err := url.Parse(c.url)
	if err != nil {
		return fmt.Errorf("invalid value %q for url: %w", c.url, err)
	}
	if c.url == "" || u.Host == "" || u.Scheme == "" {
		return fmt.Errorf("invalid value %q for url: requires a valid url", c.url)
	}
	if c.n < c.c {
		return fmt.Errorf("invalid value %d for flag -n: should be greater than flag -c: %d", c.n, c.c)
	}
	return nil
}

type positiveIntValue int

func asPositiveIntValue(p *int) *positiveIntValue {
	return (*positiveIntValue)(p)
}

func (n *positiveIntValue) String() string {
	return strconv.Itoa(int(*n))
}

func (n *positiveIntValue) Set(s string) error {
	v, err := strconv.ParseInt(s, 0, strconv.IntSize)
	if err != nil {
		return err
	}
	if v <= 0 {
		return errors.New("should be greater than zero")
	}
	*n = positiveIntValue(v)

	return nil
}