File: wports.go
   1 /*
   2 The MIT License (MIT)
   3 
   4 Copyright (c) 2026 pacman64
   5 
   6 Permission is hereby granted, free of charge, to any person obtaining a copy of
   7 this software and associated documentation files (the "Software"), to deal
   8 in the Software without restriction, including without limitation the rights to
   9 use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies
  10 of the Software, and to permit persons to whom the Software is furnished to do
  11 so, subject to the following conditions:
  12 
  13 The above copyright notice and this permission notice shall be included in all
  14 copies or substantial portions of the Software.
  15 
  16 THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
  17 IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
  18 FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
  19 AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
  20 LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
  21 OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
  22 SOFTWARE.
  23 */
  24 
  25 /*
  26 To compile a smaller-sized command-line app, you can use the `go` command as
  27 follows:
  28 
  29 go build -ldflags "-s -w" -trimpath wports.go
  30 */
  31 
  32 package main
  33 
  34 import (
  35     "net"
  36     "os"
  37     "runtime"
  38     "strconv"
  39     "sync"
  40     "time"
  41 )
  42 
  43 const info = `
  44 wports [options...]
  45 
  46 Which PORTS finds all localhost TCP ports currently in use.
  47 
  48 The only option available is to show this help message, using any of
  49 "-h", "--h", "-help", or "--help", without the quotes.
  50 `
  51 
  52 const (
  53     // timeout gives plenty of waiting-time to check localhost ports
  54     timeout = 500 * time.Millisecond
  55 
  56     // lastPort = int(^uint16(0))
  57 
  58     lastPort = 1<<16 - 1
  59 )
  60 
  61 // result values report which ports are currently being used
  62 type result struct {
  63     // When is the date/time the port was checked
  64     When time.Time
  65 
  66     // Port is the network-port number checked
  67     Port int
  68 }
  69 
  70 func main() {
  71     args := os.Args[1:]
  72 
  73     if len(args) > 0 {
  74         switch args[0] {
  75         case `-h`, `--h`, `-help`, `--help`:
  76             os.Stdout.WriteString(info[1:])
  77             return
  78 
  79         case `--`:
  80             args = args[1:]
  81         }
  82     }
  83 
  84     _, err := os.Stdout.WriteString("when\tport\n")
  85     if err != nil {
  86         return
  87     }
  88 
  89     results := make(chan result)
  90     go run(results)
  91 
  92     // buf is a buffer big enough for any output line
  93     var buf [32]byte
  94 
  95     for r := range results {
  96         line := buf[:0]
  97         line = r.When.AppendFormat(line, `2006-01-02 15:04:05`)
  98         line = append(line, '\t')
  99         line = strconv.AppendInt(line, int64(r.Port), 10)
 100         line = append(line, '\n')
 101 
 102         _, err := os.Stdout.Write(line)
 103         if err != nil {
 104             return
 105         }
 106     }
 107 }
 108 
 109 // run asynchronously dispatches tasks to check all ports
 110 func run(results chan<- result) {
 111     defer close(results) // allow the main app to end
 112 
 113     var tasks sync.WaitGroup
 114     // the number of tasks is always known in advance
 115     tasks.Add(lastPort)
 116 
 117     // permissions is buffered to limit concurrency to the core-count
 118     permissions := make(chan struct{}, runtime.NumCPU())
 119     defer close(permissions)
 120 
 121     // avoid checking port 0, as it's the `anything-available` port
 122     for port := 1; port <= lastPort; port++ {
 123         // wait until some concurrency-room is available, before proceeding
 124         permissions <- struct{}{}
 125         go check(port, &tasks, permissions, results)
 126     }
 127 
 128     // wait for all checks to finish, before closing the `results` channel,
 129     // which in turn would quit the whole app right away
 130     tasks.Wait()
 131 }
 132 
 133 // check figures out if the TCP port given is being used
 134 func check(port int, tasks *sync.WaitGroup, turn <-chan struct{}, res chan<- result) {
 135     when := time.Now()
 136     defer tasks.Done()
 137 
 138     var buf [24]byte
 139     addr := strconv.AppendInt(append(buf[:0], ':'), int64(port), 10)
 140     conn, err := net.DialTimeout(`tcp`, string(addr), timeout)
 141     if err != nil {
 142         <-turn
 143         return
 144     }
 145     conn.Close()
 146 
 147     <-turn
 148     // only report ports being used
 149     res <- result{When: when, Port: port}
 150 }