// sample-unix.go: a file with a build constraint.
// Free sample from 99helpers.com/file-types/go
//
// The //go:build line below must come before the package clause, with only
// blank lines and comments above it. This file is compiled only for Unix-like
// targets (Linux, macOS, the BSDs and others), because it uses
// syscall.Getrlimit, which Windows doesn't have.
//
// Build constraints apply to packages, so try it inside a module:
//
//	mkdir unixdemo && cd unixdemo       (and put this file in it)
//	go mod init example.com/unixdemo
//	go run .                            on Linux or macOS: prints the limits
//	GOOS=windows go build .             "build constraints exclude all Go files"
//
// Naming the file instead (go run sample-unix.go) ignores build constraints,
// so on Windows that fails with "undefined: syscall.Rlimit" instead.
//
// A file name can carry a constraint too: a file named sample_linux.go is
// only compiled for Linux, sample_windows_amd64.go only for 64-bit Windows.
// The "unix" constraint has no file-name form; it only works in //go:build.

//go:build unix

package main

import (
	"fmt"
	"runtime"
	"syscall"
)

func main() {
	var limit syscall.Rlimit
	if err := syscall.Getrlimit(syscall.RLIMIT_NOFILE, &limit); err != nil {
		fmt.Println("getrlimit:", err)
		return
	}
	fmt.Printf("Built for %s/%s\n", runtime.GOOS, runtime.GOARCH)
	fmt.Printf("Open files allowed: %d (soft limit), %d (hard limit)\n", limit.Cur, limit.Max)
}