}

Fix Go Error: runtime.main_main·f: function main is undeclared [Solution]

3 min read

The Error

When you execute go build or go run, you encounter the following error:

runtime.main_main·f: function main is undeclared in the main package

This is a common Go compilation error that frustrates many developers, especially those new to Go. In this comprehensive guide, we'll explain exactly what causes this error and provide multiple solutions to fix it.

What Causes This Error?

The runtime.main_main·f: function main is undeclared in the main package error occurs when Go cannot find a valid entry point for your program. In Go, every executable program must have:

  1. A file with package main declaration
  2. A func main() function in that package

The error typically happens in these scenarios:

Cause 1: Missing main Function

You have a file with package main but forgot to define the func main():

// main.go - WRONG: Missing main function
package main

import "fmt"

func sayHello() {
    fmt.Println("Hello!")
}

Cause 2: Wrong Package Name

Your main file uses a different package name:

// main.go - WRONG: Should be "package main"
package myapp

func main() {
    // This won't work as an entry point
}

Cause 3: Multiple Files with package main

When you have multiple .go files with package main, Go expects exactly ONE func main() across all of them. Having zero or multiple main functions causes issues.

Cause 4: Building a Library Instead of Executable

You're trying to go build on a package that's meant to be a library, not an executable.

Solution 1: Add the main Function

Make sure your main file has both the correct package declaration and the main function:

// main.go - CORRECT
package main

import "fmt"

func main() {
    fmt.Println("Hello, World!")
    sayHello()
}

func sayHello() {
    fmt.Println("Hello from sayHello!")
}

Solution 2: Check Package Declaration

Verify that your entry point file uses package main:

// BEFORE (incorrect)
package myapplication

// AFTER (correct)
package main

Solution 3: Review All Files in the Directory

When you have multiple .go files, check each one:

# List all Go files in your directory
ls *.go

# Check the package declaration in each file
head -1 *.go

Make sure: - All files in the same directory use the same package name - Only ONE file has func main() if using package main - Files intended as libraries use a different package name

Solution 4: Correct Project Structure

For a proper Go project structure:

myproject/
├── main.go          # package main, contains func main()
├── handlers.go      # package main, helper functions
└── utils/
    └── helpers.go   # package utils, library code

Example main.go:

package main

import (
    "fmt"
    "myproject/utils"
)

func main() {
    fmt.Println("Starting application...")
    utils.DoSomething()
}

Example handlers.go (same directory):

package main

import "fmt"

func handleRequest() {
    fmt.Println("Handling request...")
}

Solution 5: Using go mod

If you're using Go modules (recommended for Go 1.11+), ensure your project is properly initialized:

# Initialize a new module
go mod init myproject

# Verify your go.mod file exists
cat go.mod

Your go.mod should look like:

module myproject

go 1.21

Verification Steps

After fixing the error, verify your code works:

# Build the project
go build

# Or run directly
go run main.go

# For multiple files
go run .

Common Mistakes to Avoid

  1. Typo in main function name: It must be exactly func main(), not func Main() or func MAIN()

  2. main function with parameters: The main function cannot take arguments: ```go // WRONG func main(args []string) {}

// CORRECT func main() {} ```

  1. main function with return value: The main function cannot return values: ```go // WRONG func main() int {}

// CORRECT func main() {} ```

  1. Using init() instead of main(): The init() function runs before main, but you still need main: ```go package main

func init() { // Runs first }

func main() { // Entry point - REQUIRED } ```

Summary

The runtime.main_main·f: function main is undeclared error is straightforward to fix once you understand Go's requirements for executable programs:

  • Use package main for your entry point file
  • Define exactly one func main() with no parameters and no return value
  • Keep all files in the same directory using the same package name

Following these guidelines ensures your Go programs compile and run correctly.

GO