issue with accessing GET parameters net/http in golang

Question

Following is my go program to extract GET parameters. (URL: /mysql?hostname=example.com)

package main  import (         "net/http"         "fmt"         //"encoding/json"         //"html"         "github.com/kr/pretty" );  func main(){         http.HandleFunc("/", foo)         http.ListenAndServe(":80", nil) }  func foo(w http.ResponseWriter, r * http.Request){         w.Header().Set("Server","A Go WebServer")          w.Header().Set("Content-Type", "text/html")          hostname := r.URL.Query()["hostname"]         //w.Write([]byte(hostname[0]))         fmt.Printf("%# v", pretty.Formatter(hostname[0]))           w.Write([]byte(hostname)) } 

Error shown on console.

"example.com"2014/09/01 02:57:22 http: panic serving 172.17.92.14:49411: runtime error: index out of range /usr/lib/go/src/pkg/net/http/server.go:576 (0x42dc41)     _func_003: buf.Write(debug.Stack()) /home/michael/DPKG/golang/src/pkg/runtime/proc.c:1443 (0x40f8b5) /home/michael/DPKG/golang/src/pkg/runtime/runtime.c:128 (0x410381) /home/michael/DPKG/golang/src/pkg/runtime/runtime.c:85 (0x410228) /root/go/src/api/main.go:34 (0x400d77)     foo: fmt.Printf("%# v", pretty.Formatter(hostname[0])) /usr/lib/go/src/pkg/net/http/server.go:690 (0x421bed)     HandlerFunc.ServeHTTP: f(w, r) /usr/lib/go/src/pkg/net/http/server.go:926 (0x422a6f)     (*ServeMux).ServeHTTP: mux.handler(r).ServeHTTP(w, r) /usr/lib/go/src/pkg/net/http/server.go:656 (0x421a00)     (*conn).serve: handler.ServeHTTP(w, w.req) /home/michael/DPKG/golang/src/pkg/runtime/proc.c:271 (0x40d9bb) 2014/09/01 02:57:22 http: panic serving 172.17.92.14:49412: runtime error: index out of range /usr/lib/go/src/pkg/net/http/server.go:576 (0x42dc41)     _func_003: buf.Write(debug.Stack()) /home/michael/DPKG/golang/src/pkg/runtime/proc.c:1443 (0x40f8b5) /home/michael/DPKG/golang/src/pkg/runtime/runtime.c:128 (0x410381) /home/michael/DPKG/golang/src/pkg/runtime/runtime.c:85 (0x410228) /root/go/src/api/main.go:34 (0x400d77)     foo: fmt.Printf("%# v", pretty.Formatter(hostname[0])) /usr/lib/go/src/pkg/net/http/server.go:690 (0x421bed)     HandlerFunc.ServeHTTP: f(w, r) /usr/lib/go/src/pkg/net/http/server.go:926 (0x422a6f)     (*ServeMux).ServeHTTP: mux.handler(r).ServeHTTP(w, r) /usr/lib/go/src/pkg/net/http/server.go:656 (0x421a00)     (*conn).serve: handler.ServeHTTP(w, w.req) /home/michael/DPKG/golang/src/pkg/runtime/proc.c:271 (0x40d9bb) 2014/09/01 02:57:22 http: panic serving 172.17.92.14:49413: runtime error: index out of range /usr/lib/go/src/pkg/net/http/server.go:576 (0x42dc41)     _func_003: buf.Write(debug.Stack()) /home/michael/DPKG/golang/src/pkg/runtime/proc.c:1443 (0x40f8b5) /home/michael/DPKG/golang/src/pkg/runtime/runtime.c:128 (0x410381) /home/michael/DPKG/golang/src/pkg/runtime/runtime.c:85 (0x410228) /root/go/src/api/main.go:34 (0x400d77)     foo: fmt.Printf("%# v", pretty.Formatter(hostname[0])) /usr/lib/go/src/pkg/net/http/server.go:690 (0x421bed)     HandlerFunc.ServeHTTP: f(w, r) /usr/lib/go/src/pkg/net/http/server.go:926 (0x422a6f)     (*ServeMux).ServeHTTP: mux.handler(r).ServeHTTP(w, r) /usr/lib/go/src/pkg/net/http/server.go:656 (0x421a00)     (*conn).serve: handler.ServeHTTP(w, w.req) /home/michael/DPKG/golang/src/pkg/runtime/proc.c:271 (0x40d9bb) 

I am having trouble with determining the data type of hostname variable. I should get it as a string.

When I try to write to http response writer

w.Write([]byte(hostname)) 

following build error is shown

go/src/api/main.go:38: cannot convert hostname (type []string) to type []byte 

If I treat it as a string array build is successful but there is run time error

        newhost := hostname[0];         w.Write([]byte(newhost)) 

Error:

    2014/09/01 04:42:40 http: panic serving 172.17.92.14:50404: runtime error: index out of range /usr/lib/go/src/pkg/net/http/server.go:576 (0x42db27)     _func_003: buf.Write(debug.Stack()) /home/michael/DPKG/golang/src/pkg/runtime/proc.c:1443 (0x40f79b) /home/michael/DPKG/golang/src/pkg/runtime/runtime.c:128 (0x410267) /home/michael/DPKG/golang/src/pkg/runtime/runtime.c:85 (0x41010e) /root/go/src/api/main.go:38 (0x400d63)     foo: newhost := hostname[0]; /usr/lib/go/src/pkg/net/http/server.go:690 (0x421ad3)     HandlerFunc.ServeHTTP: f(w, r) /usr/lib/go/src/pkg/net/http/server.go:926 (0x422955)     (*ServeMux).ServeHTTP: mux.handler(r).ServeHTTP(w, r) /usr/lib/go/src/pkg/net/http/server.go:656 (0x4218e6)     (*conn).serve: handler.ServeHTTP(w, w.req) /home/michael/DPKG/golang/src/pkg/runtime/proc.c:271 (0x40d8a1) 2014/09/01 04:42:40 http: panic serving 172.17.92.14:50405: runtime error: index out of range /usr/lib/go/src/pkg/net/http/server.go:576 (0x42db27)     _func_003: buf.Write(debug.Stack()) /home/michael/DPKG/golang/src/pkg/runtime/proc.c:1443 (0x40f79b) /home/michael/DPKG/golang/src/pkg/runtime/runtime.c:128 (0x410267) /home/michael/DPKG/golang/src/pkg/runtime/runtime.c:85 (0x41010e) /root/go/src/api/main.go:38 (0x400d63)     foo: newhost := hostname[0]; /usr/lib/go/src/pkg/net/http/server.go:690 (0x421ad3)     HandlerFunc.ServeHTTP: f(w, r) /usr/lib/go/src/pkg/net/http/server.go:926 (0x422955)     (*ServeMux).ServeHTTP: mux.handler(r).ServeHTTP(w, r) /usr/lib/go/src/pkg/net/http/server.go:656 (0x4218e6)     (*conn).serve: handler.ServeHTTP(w, w.req) /home/michael/DPKG/golang/src/pkg/runtime/proc.c:271 (0x40d8a1) 2014/09/01 04:42:40 http: panic serving 172.17.92.14:50406: runtime error: index out of range /usr/lib/go/src/pkg/net/http/server.go:576 (0x42db27)     _func_003: buf.Write(debug.Stack()) /home/michael/DPKG/golang/src/pkg/runtime/proc.c:1443 (0x40f79b) /home/michael/DPKG/golang/src/pkg/runtime/runtime.c:128 (0x410267) /home/michael/DPKG/golang/src/pkg/runtime/runtime.c:85 (0x41010e) /root/go/src/api/main.go:38 (0x400d63)     foo: newhost := hostname[0]; /usr/lib/go/src/pkg/net/http/server.go:690 (0x421ad3)     HandlerFunc.ServeHTTP: f(w, r) /usr/lib/go/src/pkg/net/http/server.go:926 (0x422955)     (*ServeMux).ServeHTTP: mux.handler(r).ServeHTTP(w, r) /usr/lib/go/src/pkg/net/http/server.go:656 (0x4218e6)     (*conn).serve: handler.ServeHTTP(w, w.req) /home/michael/DPKG/golang/src/pkg/runtime/proc.c:271 (0x40d8a1) 

I also checked length of newhost as @VonC suggested and it gave expected results.

        fmt.Print("\n")         fmt.Println((len(newhost))) 

Output on console

"example.com" 11 2014/09/01 05:05:15 http: panic serving 172.17.92.14:50779: runtime error: index out of range /usr/lib/go/src/pkg/net/http/server.go:576 (0x42dd33) 

Answer 1

You need to use url/Values.Get, it will return the empty string if the value isn't set:

hostname := r.URL.Query().Get("hostname") if len(hostname) != 0 {     io.WriteString(w, hostname)  // or     w.Write([]byte(hostname)) } 

// edit

Let's take it a step by step:

go/src/api/main.go:38: cannot convert hostname (type []string) to type []byte:

• req.URL.Query["hostname"], returns a string slice ([]string) which you can't directly convert to []byte, only string can be converted like that.

The first runtime error: index out of range:

• You're trying to use hostname[0] without checking how many elements in the slice, so in that case there weren't any elements.

That last runtime error: index out of range is weird though, I have no idea why you got it, must be something else you changed in the code.