Module initialization in Go - Step-by-Step Execution

Concept Flow - Module initialization

Start Program

↓

Import Packages

↓

Run init() functions in imported packages

↓

Run init() in main package

↓

Execute main() function

↓

Program Ends

The program starts by importing packages, then runs all init() functions in order, and finally runs main().

Execution Sample

Go

package main
import "fmt"
func init() {
    fmt.Println("Init called")
}
func main() {
    fmt.Println("Main called")
}

This Go program prints messages from init() first, then from main().

Execution Table

Step	Action	Output	Next Step
1	Start program		Import packages
2	Import packages		Run init() functions in imported packages
3	Run init() in main package	Init called	Execute main() function
4	Execute main() function	Main called	Program ends
5	Program ends		No further steps

💡 Program ends after main() completes

Variable Tracker

Variable	Start	After init()	After main()	Final
No variables	-	-	-	-

Key Moments - 2 Insights

Why does init() run before main() even though main() is the entry point?

init() functions run automatically after package imports and before main(), as shown in execution_table step 3 before step 4.

Can there be multiple init() functions?

Yes, each package can have multiple init() functions, all run before main(), as indicated in step 2 and 3 of execution_table.

Visual Quiz - 3 Questions

Look at the execution_table, what is printed at step 3?

A Program starts

B Main called

C Init called

D No output

💡 Hint

Check the Output column at step 3 in execution_table

At which step does the main() function run?

A Step 2

B Step 4

C Step 3

D Step 5

💡 Hint

Look at the Action column to find when main() executes

If there were multiple imported packages each with init(), when would they run?

A Before main(), during package import

B Only if called explicitly

C After main()

D They do not run automatically

💡 Hint

Refer to execution_table step 2 and 3 about init() timing

Concept Snapshot

Go module initialization:
- Program starts by importing packages
- All init() functions run automatically after imports
- init() runs before main()
- main() is the program entry point
- Multiple init() functions can exist and run in order

Full Transcript

In Go, when a program starts, it first imports all required packages. After importing, Go runs all init() functions found in those packages automatically. Then, it runs the init() function in the main package. Only after all init() functions finish does the main() function run. This means init() functions prepare the program before main() starts. Multiple init() functions can exist and they all run before main(). The program ends after main() completes.