Source: env_unix.go in package syscall

+build aix darwin dragonfly freebsd js,wasm linux netbsd openbsd solaris plan9

Unix environment variables.


package syscall

import (
	"runtime"
	"sync"
)

envOnce guards initialization by copyenv, which populates env.

	envOnce sync.Once

envLock guards env and envs.

	envLock sync.RWMutex

env maps from an environment variable to its first occurrence in envs.

	env map[string]int

envs is provided by the runtime. elements are expected to be of the form "key=value". An empty string means deleted (or a duplicate to be ignored).

	envs []string = runtime_envs()
)

func runtime_envs() []string // in package runtime

setenv_c and unsetenv_c are provided by the runtime but are no-ops if cgo isn't loaded.

func setenv_c(k, v string)
func unsetenv_c(k string)

func copyenv() {
	env = make(map[string]int)
	for i, s := range envs {
		for j := 0; j < len(s); j++ {
			if s[j] == '=' {
				key := s[:j]
				if _, ok := env[key]; !ok {
					env[key] = i // first mention of key

Clear duplicate keys. This permits Unsetenv to safely delete only the first item without worrying about unshadowing a later one, which might be a security problem.

					envs[i] = ""
				}
				break
			}
		}
	}
}

func Unsetenv(key string) error {
	envOnce.Do(copyenv)

	envLock.Lock()
	defer envLock.Unlock()

	if i, ok := env[key]; ok {
		envs[i] = ""
		delete(env, key)
	}
	unsetenv_c(key)
	return nil
}

func Getenv(key string) (value string, found bool) {
	envOnce.Do(copyenv)
	if len(key) == 0 {
		return "", false
	}

	envLock.RLock()
	defer envLock.RUnlock()

	i, ok := env[key]
	if !ok {
		return "", false
	}
	s := envs[i]
	for i := 0; i < len(s); i++ {
		if s[i] == '=' {
			return s[i+1:], true
		}
	}
	return "", false
}

func Setenv(key, value string) error {
	envOnce.Do(copyenv)
	if len(key) == 0 {
		return EINVAL
	}
	for i := 0; i < len(key); i++ {
		if key[i] == '=' || key[i] == 0 {
			return EINVAL
		}

On Plan 9, null is used as a separator, eg in $path.

	if runtime.GOOS != "plan9" {
		for i := 0; i < len(value); i++ {
			if value[i] == 0 {
				return EINVAL
			}
		}
	}

	envLock.Lock()
	defer envLock.Unlock()

	i, ok := env[key]
	kv := key + "=" + value
	if ok {
		envs[i] = kv
	} else {
		i = len(envs)
		envs = append(envs, kv)
	}
	env[key] = i
	setenv_c(key, value)
	return nil
}

func Clearenv() {
	envOnce.Do(copyenv) // prevent copyenv in Getenv/Setenv

	envLock.Lock()
	defer envLock.Unlock()

	for k := range env {
		unsetenv_c(k)
	}
	env = make(map[string]int)
	envs = []string{}
}

func Environ() []string {
	envOnce.Do(copyenv)
	envLock.RLock()
	defer envLock.RUnlock()
	a := make([]string, 0, len(envs))
	for _, env := range envs {
		if env != "" {
			a = append(a, env)
		}
	}
	return a