Go: Package initialization and program execution order

In a nutshell

  • First the main package is initialized
    • Imported packages are initialized before the package itself.
    • Packages are initialized one at a time:
      • first package-level variables are initialized in declaration order,
      • then the init functions are run.
  • The main function is called.

Program execution

Program execution begins by initializing the main package and then calling the function main. When main returns, the program exits. It does not wait for other goroutines to complete.

Package initialization

  • Package-level variables are initialized in declaration order, but after any of the variables they depend on.

  • Initialization of variables declared in multiple files is done in lexical file name order. Variables declared in the first file are declared before any of the variables declared in the second file.

  • Initialization cycles are not allowed.

  • Dependency analysis is performed per package; only references referring to variables, functions, and methods declared in the current package are considered.

Example: In this example (from the Go language specification)…
var (
	a = c + b
	b = f()
	c = f()
	d = 3
)

func f() int {
	d++
	return d
}
…the initialization order is d, b, c, a.

Init function

Variables may also be initialized using init functions:

func init() { … }

Multiple such functions may be defined. They cannot be called from inside a program.

  • A package with no imports is initialized
    • by assigning initial values to all its package-level variables,
    • followed by calling all init functions in the order they appear in the source.
  • Imported packages are initialized before the package itself.

  • Each package is initialized once, regardless if it’s imported by multiple other packages.

It follows that there can be no cyclic dependencies.

Package initialization happens in a single goroutine, sequentially, one package at a time.

Warning

Lexical ordering according to file names is not part of the formal language specification:

To ensure reproducible initialization behavior, build systems are encouraged to present multiple files belonging to the same package in lexical file name order to a compiler.
The Go Programming Language Specification: Package initialization

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