title: when compile protocol buffer, missing input or program is not executable error
body: <p>I'm trying to make golang+grpc server on mac. I installed golang, grpc, protocol buffer and checked that grpc server following this example <a href="https://grpc.io/docs/quickstart/go/" rel="nofollow noreferrer">https://grpc.io/docs/quickstart/go/</a> 
  The problem is when I try to compile .proto file using protocol buffer it said 'missing input' or 'program is not executable'. </p>
  
  <p>install protocol buffer and grpc</p>
  
  <pre><code>brew install grpc protobuf
  </code></pre>
  
  <p>install golang based protoc plugin  </p>
  
  <pre><code>go get github.com/golang/protobuf/protoc-gen-go
  </code></pre>
  
  <p>protoc path</p>
  
  <pre><code>which protoc
  /usr/local/bin/protoc
  </code></pre>
  
  <p>example location</p>
  
  <pre><code>/Users/usrname/go/src/google.golang.org/grpc/examples/helloworld
  </code></pre>
  
  <p>run proto compiler</p>
  
  <pre><code>protoc -I=/Users/username/go/src/google.golang.org/grpc/examples/helloworld/helloworld/ --go_out=/Users/username/go/src/google.golang.org/grpc/examples/helloworld/helloworld/ /Users/username/go/src/google.golang.org/grpc/examples/helloworld/helloworld/helloworld.proto
  </code></pre>
  
  <p>error message</p>
  
  <pre><code>protoc-gen-go: program not found or is not executable
  Please specify a program using absolute path or make sure the program is available in your PATH system variable
  --go_out: protoc-gen-go: Plugin failed with status code 1.
  </code></pre>