Just using "go get" is not working, but "go mod init" then "go get" is working

[sunrise2575]

Environment:

• Ubuntu 18.04
• gcc 7.5.0

Prerequisites:

apt install libsodium-dev libczmq-dev libzmq3-dev -y

---

A. Not-working command:

mkdir zmqtest
go get github.com/zeromq/goczmq

Output:

# github.com/zeromq/goczmq
../go/src/github.com/zeromq/goczmq/sock_option.go:37:3: could not determine kind of name for C.zsock_set_router_notify

Compiling is failed with same message like ... C.zsock_set_router_notify

---

B. Working command:

mkdir zmqtest
go mod init zmqtest
go get github.com/zeromq/goczmq

Output:

go: github.com/zeromq/goczmq upgrade => v4.1.0+incompatible

go mod version successfully compiles the goczmq example code.

But the install manual didn't tell that go mod is mandatory.