printf - args num ok? Umeda.go #3 2018 - 02 - 24 @decafe09 

fmt.Printf fmt.Printf("%s", "moji") 

fmt.Printf fmt.Printf("%s", "moji", "yobun")  


fmt.Printf $ go build # ここでは気づけない $ go run main.go # ここで気づける moji%!(EXTRA string=yobun) 

fmt.Printf $ go build # ここでは気づけない $ go run main.go # ここで気づける moji%!(EXTRA string=yobun) func main() { http.HandleFunc( "/xxx", func(w http.ResponseWriter, r *http.Request) { fmt.Printf("%s", "moji", "yobun") return }, ) http.ListenAndServe(":8080", nil) } 

fmt.Printf $ go build # ここでは気づけない $ go run main.go # ここでは気づけない $ curl localhost:8080/xxx # ここで気づける moji%!(EXTRA string=yobun) 

go vet $ go vet ./main.go:6: Printf call needs 1 arg but has 2 args go vet !
curl    " go vet CI  go vet 

go vet –printfuncs $ go vet –printfuncs=Infof ./main.go:6: Infof call needs 1 arg but has 2 args func Infof(format string, a ...interface{}) (n int, err error) { return fmt.Printf(format, a...) } 

go vet @ Go 1.10 $ go test ./main.go:6: Printf call needs 1 arg but has 2 args 
go1.9.3 main.go:6: wrong number of args for format in Printf call: 1 needed but 2 args go1.10 main.go:6: Printf call needs 1 arg but has 2 args go test  go vet  

go test -printfuncs 


Humm... format := "%s" fmt.Printf(format, "moji", ”yobun") 

Humm... $ go build # ここでは気づけない $ go vet # ここでは気づけない $ go run main.go # ここで気づける moji%!(EXTRA string=yobun) http ! % #  &$"
%   

Background API6=3.;?>/2,BW!
:20/)O  APIVK" C%!8>?4+?5JSAPIRFP XXNTL@
YYNTLIEH  XXYYEH  XX, YY O(&'!
format!O(& %D 71? +<:20/GQ&72-/M%A
format#xx,yy*9=-.;?U$  

Thinking... "0(' ! 0(',1#%$ /0( $   ! +.)*format !format  !&-... 

Result func check(format string) error 1. format %! 2. formatmessage 3. format %! 4. %!    
func main() { format := "%s" err := check(format) if err != nil { panic(err) } fmt.Println(message(format)) } func message(format string) string { return fmt.Sprintf(format, "moji", "yobun") } func check(format string) error { pre := strings.Count(format, "%!") msg := message(format) post := strings.Count(msg, "%!") if post > pre { return errors.New("format error: " + msg) } return nil } 

OK All errors begin with the string "%!" followed sometimes by a single character (the verb) and end with a parenthesized description. https://golang.org/pkg/fmt/ 

Check $ go run main.go panic: format error: moji%!(EXTRA string=yobun) 

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