I am trying to simualte M\M\1 Queuing Model using OMNET++ on my Mac M1.
The name of the project is "MM1QueueModel" while the same for .ned and .cc are "MM1Queue.ned" and "MM1Queue.cc" respectively.
In the "MM1Queue.ned" file, I am using the package as:
package MM1QueueModel;
but this is giving an error "Declared package does not match expected package in OMNET++"
The solution described in Solution is not working for me. Please help
Take a look at Packages Overview and check whether your directories meet that requirement:
The package name that follows from the directory name and the declared package must match; it is an error if they don't.
Related
I have a directory view_test with two files: main_test.go and test.go.
Both files start with
main_test.go:
package view_test
import (
test.go:
package view_test
import (
Now when I try to test with go test I get:
found packages view (main_test.go) and view_test (test.go) in /foo/internal/resources/view_test
Seems like there are some unknown assumptions where go starts clobbering together package names. Could someone enlighten me what these are?
The _test suffix in test files is special. From go help test:
Test files that declare a package with the suffix "_test" will be compiled as a separate package, and then linked and run with the main test binary.
This feature exists so you can enforce black box tests. Normally tests are defined in the same package as non-test code, so tests have access to unexported identifiers. If that's not desired (i.e. because you want to make sure that tests only use the public API), tests can be declared in a *_test package instead. It's an exception to the rule that all go files in a directory must declare the same package name.
As pointed out in the comments in your case this leads to a package named "view" in the context of tests, and a package named "view_test " in the context of production code. That's the wrong way around and therefore causes multiple defined packages in the same directory.
Don't use the _test suffix for production code to fix the issue.
I created the github.com project qjson/qjson-go that contains the package name qjson that you can see here. I named the github project this way because github.com/qjson/ contains other projects for different languages (e.g. qjson-c).
Unfortunately, I get the following error when I try to import the project as github.com/qjson/qjson-go:
$ go mod tidy
go: finding module for package github.com/qjson/qjson-go
go: downloading github.com/qjson/qjson-go v0.0.0-20210128102242-170c47e2db46
github.com/xxx/xxx imports
github.com/qjson/qjson-go: module github.com/qjson/qjson-go#latest found (v0.0.0-20210128102242-170c47e2db46), but does not contain package github.com/qjson/qjson-go
I’m apparently doing it wrong. I understand that due to the import statement we are then expected to use gjson-go as package identifier.
What must I do so that the git project can be named qjson-go and the package qjson ?
I assume that one solution is to create a sub-directory named qjson inside qjson-go and move all package files in it. The user would then import "github.com/qson/qson-go/qjson". Is that correct ? Is there another solution avoiding the stutter ?
This program works as expected:
package main
import (
"fmt"
"github.com/qjson/qjson-go/qjson"
)
func main() {
fmt.Println(qjson.ErrDivisionByZero)
}
The issue is that you are using this file structure:
qjson/engine.go
qjson/errors.go
When you should just be putting them at the top level, like this:
engine.go
errors.go
So you can either fix the directory, and tag a new version, or just leave the
files as is, and change your imports to match what I have above.
I'm new to omnet++. How to set the correct package name?
I created a new .ned file in omnet++5.4.1, but there was not a package name.And I type a name,the omnetide show me that declared package "org.car2x.veins.base.modules" does not match expected package "".
How to know which is the correct package name?
Thank you.
I use omnet++5.4.1 and veins 5a1.
The OMNeT++ manual has an explanation for the definition of a package.
Directories in a NED source tree correspond to packages. If NED files are in the /a/b/c directory (where is listed in NED path), then the package name is a.b.c. The package name has to be explicitly declared at the top of the NED files as well, like this:
package a.b.c;
Every Go file starts with package <something>.
As far as I understand - and this is probably where I am missing some information - there are only two possible values for <something>: The name of the directory it is in*, or main. If it is main, all other files in that directory can only have main, too. If it is something else, the project is inconsistent/violating convention.
Now if it is the name of the directory, it's redundant, because the same information is, well, in the name of the directory.
If it is main, it's kind of useless, because as far as I can see there is no way to tell go build to "please build all main packages".
* Because, in other words, one directory is one package.
The name of the package does not have to coincide with the directory name. It is possible to have package foobar in the directory xyz/go-foobar. In this case, xyz/go-foobar becomes an import path, but the package name that you use to quality the identifiers (functions, types etc.) would be foobar.
Here's an example to make it more concrete: I created a test package http://godoc.org/github.com/dmitris/go-foobar (source in https://github.com/dmitris/go-foobar) - you can see from the documentation page, that the import path is "github.com/dmitris/go-foobar" but the package name is foobar, so you would call the function it provides as foobar.Demo() (not go-foobar.Demo()).
A similar real-life example - the import path for the NSQ Messaging platform is "github.com/nsqio/go-nsq" while the package name is "nsq": http://godoc.org/github.com/nsqio/go-nsq. However, for the sake of user-friendliness and simplicity, the standard and recommended practice is to keep the last portions of the import path and the package name being the same whenever possible.
package main is not useless - it tells the Go compiler to create an executable as opposed to a .a library file (with go install or go get; go build discards the compilation result). The executable is named after the directory name in which the package main file or files are placed. Again a concrete example - I made a test program https://github.com/dmitris/go-foobar-client, you install it with go get github.com/dmitris/go-foobar-client and you should get a go-foobar-client executable placed in your $GOPATH/bin directory. It is from the the directory name where the package main file is placed that the Go compiler takes the name of the executable from. The filename of the .go file that contains the main() function is not important - in the example above, we can rename main.go to client.go or something else, but as long as the enclosing directory is called go-foobar-client, that's how the resulting executable will be named.
For an additional accessible and practically oriented reading about Go packages, I recommend Dave Cheney's article "Five suggestions for setting up a Go project" http://dave.cheney.net/2014/12/01/five-suggestions-for-setting-up-a-go-project.
The missing information you "have" is that the package name does not need to be the same as the directory name.
It is perfectly fine to use a package name other than the folder name. If you do so, you still have to import the package based on the directory structure, but after the import you have to refer to it by the name you used in the package clause.
For example, if you have a folder $GOPATH/src/mypck, and in it you have a file a.go:
package apple
const Pi = 3.14
Using this package:
package main
import (
"mypck"
"fmt"
)
func main() {
fmt.Println(apple.Pi)
}
Just like you are allowed to use relative imports but is not advisable, you may use package names other that their containing folder, but this is not advisable also to avoid further misunderstanding.
Note that the specification doesn't even require all files belonging to the same package to be in the same folder (but it may be an implementation requirement). Spec: Package clause:
A set of files sharing the same PackageName form the implementation of a package. An implementation may require that all source files for a package inhabit the same directory.
What's the use of this?
Simple. A package name is a Go identifier:
identifier = letter { letter | unicode_digit } .
Which allows unicode letters to be used in identifiers, e.g. αβ is a valid identifier in Go. Folder and file names are not handled by Go but by the Operating System, and different file systems have different restrictions. There are actually many file systems which would not allow all valid Go identifiers as folder names, so you would not be able to name your packages what otherwise the language spec would allow.
So in one hand not all valid Go identifiers may be valid folder names. And on the other hand, not all valid folder names are valid Go identifiers, for example go-math is a valid folder name in most (all?) file systems, but it's not a valid Go identifier (as identifiers cannot contain the dash - character).
Having the option to use package names different than their containing folders, you have the option to really name your packages what the language spec allows, regardless of the underlying operating and file system, and put it in a folder named anything that the underlying OS and file system allows - regardless of the package name.
This repo has 3 go files all begin with "package lumber".
To use this package, I'm supposed to put this in my GOROOT and simply
import lumber
in my program. How do variables and types in this package connect with each other across multiple files? How does the go compiler know which file to begin reading first?
In case I want to read the package, where should I begin reading to understand the package? What exactly is the flow of things here?
To elaborate on jnml's answer:
When you use import "foo/bar" in your code, you are not referring to the source files (which will be located in $GOPATH/src/foo/bar/).
Instead, you are referring to a compiled package file at $GOPATH/pkg/$GOOS_$GOARCH/foo/bar.a. When you build your own code, and the compiler finds that the foo/bar package has not yet been compiled (or is out of date), it will do this for you automatically.
It does this by collating* all the relevant source files in the $GOPATH/src/foo/bar directory and building them into a single bar.a file, which it installs in the pkg directory. Compilation then resumes with your own program.
This process is repeated for all imported packages, and packages imported by those as well, all the way down the dependency chain.
*) How the files are collated, depends on how the file itself is named and what kind of build tags are present inside it.
For a deeper understanding of how this works, refer to the build docs.
No, you're not "supposed to put this in my GOROOT". You're supposed to execute
$ go get github.com/jcelliott/lumber
which will clone the repository into $GOPATH/src/github.com/jcelliott/lumber. Then you can use the package by importing it in your code as
import "github.com/jcelliott/lumber"
About the scoping rules: Declarations and scope