What is the init() function and when does it run?

init() is an optional special function that runs automatically after all package-level variable initialisations — before main(). It takes no arguments, returns nothing, and cannot be called directly. A single file may contain multiple init() functions.

package main

import (
    "fmt"
    _ "github.com/lib/pq"  // blank import: run pq's init() for side effects
)                            // registers the postgres driver

var cfg *Config

func init() {
    // Runs BEFORE main(), AFTER package-level vars are initialised
    var err error
    cfg, err = loadConfig("app.yaml")
    if err != nil {
        panic(fmt.Sprintf("config init failed: %v", err))
    }
    fmt.Println("config loaded")
}

func main() {
    fmt.Println("main running")
    // cfg is guaranteed to be non-nil here
}

// INITIALISATION ORDER within a package:
// 1. Package-level variables (in dependency order)
// 2. init() functions (in source file order, multiple per file allowed)
// 3. main() — only in package main

// Across packages: imported packages initialise first
// Go guarantees no circular init dependencies

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