// Copyright 2009 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

/*
Package runtime contains operations that interact with Go's runtime system,
such as functions to control goroutines. It also includes the low-level type information
used by the reflect package; see reflect's documentation for the programmable
interface to the run-time type system.

Environment Variables

The following environment variables ($name or %name%, depending on the host
operating system) control the run-time behavior of Go programs. The meanings
and use may change from release to release.

The GOGC variable sets the initial garbage collection target percentage.
A collection is triggered when the ratio of freshly allocated data to live data
remaining after the previous collection reaches this percentage. The default
is GOGC=100. Setting GOGC=off disables the garbage collector entirely.
The runtime/debug package's SetGCPercent function allows changing this
percentage at run time. See https://golang.org/pkg/runtime/debug/#SetGCPercent.

The GODEBUG variable controls debugging variables within the runtime.
It is a comma-separated list of name=val pairs setting these named variables:

	allocfreetrace: setting allocfreetrace=1 causes every allocation to be
	profiled and a stack trace printed on each object's allocation and free.

	cgocheck: setting cgocheck=0 disables all checks for packages
	using cgo to incorrectly pass Go pointers to non-Go code.
	Setting cgocheck=1 (the default) enables relatively cheap
	checks that may miss some errors.  Setting cgocheck=2 enables
	expensive checks that should not miss any errors, but will
	cause your program to run slower.

	efence: setting efence=1 causes the allocator to run in a mode
	where each object is allocated on a unique page and addresses are
	never recycled.

	gccheckmark: setting gccheckmark=1 enables verification of the
	garbage collector's concurrent mark phase by performing a
	second mark pass while the world is stopped.  If the second
	pass finds a reachable object that was not found by concurrent
	mark, the garbage collector will panic.

	gcpacertrace: setting gcpacertrace=1 causes the garbage collector to
	print information about the internal state of the concurrent pacer.

	gcshrinkstackoff: setting gcshrinkstackoff=1 disables moving goroutines
	onto smaller stacks. In this mode, a goroutine's stack can only grow.

	gcstackbarrieroff: setting gcstackbarrieroff=1 disables the use of stack barriers
	that allow the garbage collector to avoid repeating a stack scan during the
	mark termination phase.

	gcstackbarrierall: setting gcstackbarrierall=1 installs stack barriers
	in every stack frame, rather than in exponentially-spaced frames.

	gcrescanstacks: setting gcrescanstacks=1 enables stack
	re-scanning during the STW mark termination phase. This is
	helpful for debugging if objects are being prematurely
	garbage collected.

	gcstoptheworld: setting gcstoptheworld=1 disables concurrent garbage collection,
	making every garbage collection a stop-the-world event. Setting gcstoptheworld=2
	also disables concurrent sweeping after the garbage collection finishes.

	gctrace: setting gctrace=1 causes the garbage collector to emit a single line to standard
	error at each collection, summarizing the amount of memory collected and the
	length of the pause. Setting gctrace=2 emits the same summary but also
	repeats each collection. The format of this line is subject to change.
	Currently, it is:
		gc # @#s #%: #+#+# ms clock, #+#/#/#+# ms cpu, #->#-># MB, # MB goal, # P
	where the fields are as follows:
		gc #        the GC number, incremented at each GC
		@#s         time in seconds since program start
		#%          percentage of time spent in GC since program start
		#+...+#     wall-clock/CPU times for the phases of the GC
		#->#-># MB  heap size at GC start, at GC end, and live heap
		# MB goal   goal heap size
		# P         number of processors used
	The phases are stop-the-world (STW) sweep termination, concurrent
	mark and scan, and STW mark termination. The CPU times
	for mark/scan are broken down in to assist time (GC performed in
	line with allocation), background GC time, and idle GC time.
	If the line ends with "(forced)", this GC was forced by a
	runtime.GC() call and all phases are STW.

	Setting gctrace to any value > 0 also causes the garbage collector
	to emit a summary when memory is released back to the system.
	This process of returning memory to the system is called scavenging.
	The format of this summary is subject to change.
	Currently it is:
		scvg#: # MB released  printed only if non-zero
		scvg#: inuse: # idle: # sys: # released: # consumed: # (MB)
	where the fields are as follows:
		scvg#        the scavenge cycle number, incremented at each scavenge
		inuse: #     MB used or partially used spans
		idle: #      MB spans pending scavenging
		sys: #       MB mapped from the system
		released: #  MB released to the system
		consumed: #  MB allocated from the system

	memprofilerate: setting memprofilerate=X will update the value of runtime.MemProfileRate.
	When set to 0 memory profiling is disabled.  Refer to the description of
	MemProfileRate for the default value.

	memprofilerate:  setting memprofilerate=X changes the setting for
	runtime.MemProfileRate.  Refer to the description of this variable for how
	it is used and its default value.

	sbrk: setting sbrk=1 replaces the memory allocator and garbage collector
	with a trivial allocator that obtains memory from the operating system and
	never reclaims any memory.

	scavenge: scavenge=1 enables debugging mode of heap scavenger.

	scheddetail: setting schedtrace=X and scheddetail=1 causes the scheduler to emit
	detailed multiline info every X milliseconds, describing state of the scheduler,
	processors, threads and goroutines.

	schedtrace: setting schedtrace=X causes the scheduler to emit a single line to standard
	error every X milliseconds, summarizing the scheduler state.

The net and net/http packages also refer to debugging variables in GODEBUG.
See the documentation for those packages for details.

The GOMAXPROCS variable limits the number of operating system threads that
can execute user-level Go code simultaneously. There is no limit to the number of threads
that can be blocked in system calls on behalf of Go code; those do not count against
the GOMAXPROCS limit. This package's GOMAXPROCS function queries and changes
the limit.

The GOTRACEBACK variable controls the amount of output generated when a Go
program fails due to an unrecovered panic or an unexpected runtime condition.
By default, a failure prints a stack trace for the current goroutine,
eliding functions internal to the run-time system, and then exits with exit code 2.
The failure prints stack traces for all goroutines if there is no current goroutine
or the failure is internal to the run-time.
GOTRACEBACK=none omits the goroutine stack traces entirely.
GOTRACEBACK=single (the default) behaves as described above.
GOTRACEBACK=all adds stack traces for all user-created goroutines.
GOTRACEBACK=system is like ``all'' but adds stack frames for run-time functions
and shows goroutines created internally by the run-time.
GOTRACEBACK=crash is like ``system'' but crashes in an operating system-specific
manner instead of exiting. For example, on Unix systems, the crash raises
SIGABRT to trigger a core dump.
For historical reasons, the GOTRACEBACK settings 0, 1, and 2 are synonyms for
none, all, and system, respectively.
The runtime/debug package's SetTraceback function allows increasing the
amount of output at run time, but it cannot reduce the amount below that
specified by the environment variable.
See https://golang.org/pkg/runtime/debug/#SetTraceback.

The GOARCH, GOOS, GOPATH, and GOROOT environment variables complete
the set of Go environment variables. They influence the building of Go programs
(see https://golang.org/cmd/go and https://golang.org/pkg/go/build).
GOARCH, GOOS, and GOROOT are recorded at compile time and made available by
constants or functions in this package, but they do not influence the execution
of the run-time system.
*/
package runtime

import "runtime/internal/sys"

// Gosched yields the processor, allowing other goroutines to run.  It does not
// suspend the current goroutine, so execution resumes automatically.
func Gosched()

// Caller reports file and line number information about function invocations on
// the calling goroutine's stack. The argument skip is the number of stack frames
// to ascend, with 0 identifying the caller of Caller.  (For historical reasons the
// meaning of skip differs between Caller and Callers.) The return values report the
// program counter, file name, and line number within the file of the corresponding
// call.  The boolean ok is false if it was not possible to recover the information.
func Caller(skip int) (pc uintptr, file string, line int, ok bool)

// Callers fills the slice pc with the return program counters of function invocations
// on the calling goroutine's stack. The argument skip is the number of stack frames
// to skip before recording in pc, with 0 identifying the frame for Callers itself and
// 1 identifying the caller of Callers.
// It returns the number of entries written to pc.
func Callers(skip int, pc []uintptr) int

// SetFinalizer sets the finalizer associated with obj to the provided
// finalizer function. When the garbage collector finds an unreachable block
// with an associated finalizer, it clears the association and runs
// finalizer(obj) in a separate goroutine. This makes obj reachable again,
// but now without an associated finalizer. Assuming that SetFinalizer
// is not called again, the next time the garbage collector sees
// that obj is unreachable, it will free obj.
//
// SetFinalizer(obj, nil) clears any finalizer associated with obj.
//
// The argument obj must be a pointer to an object allocated by
// calling new or by taking the address of a composite literal.
// The argument finalizer must be a function that takes a single argument
// to which obj's type can be assigned, and can have arbitrary ignored return
// values. If either of these is not true, SetFinalizer aborts the
// program.
//
// Finalizers are run in dependency order: if A points at B, both have
// finalizers, and they are otherwise unreachable, only the finalizer
// for A runs; once A is freed, the finalizer for B can run.
// If a cyclic structure includes a block with a finalizer, that
// cycle is not guaranteed to be garbage collected and the finalizer
// is not guaranteed to run, because there is no ordering that
// respects the dependencies.
//
// The finalizer for obj is scheduled to run at some arbitrary time after
// obj becomes unreachable.
// There is no guarantee that finalizers will run before a program exits,
// so typically they are useful only for releasing non-memory resources
// associated with an object during a long-running program.
// For example, an os.File object could use a finalizer to close the
// associated operating system file descriptor when a program discards
// an os.File without calling Close, but it would be a mistake
// to depend on a finalizer to flush an in-memory I/O buffer such as a
// bufio.Writer, because the buffer would not be flushed at program exit.
//
// It is not guaranteed that a finalizer will run if the size of *obj is
// zero bytes.
//
// It is not guaranteed that a finalizer will run for objects allocated
// in initializers for package-level variables. Such objects may be
// linker-allocated, not heap-allocated.
//
// A finalizer may run as soon as an object becomes unreachable.
// In order to use finalizers correctly, the program must ensure that
// the object is reachable until it is no longer required.
// Objects stored in global variables, or that can be found by tracing
// pointers from a global variable, are reachable. For other objects,
// pass the object to a call of the KeepAlive function to mark the
// last point in the function where the object must be reachable.
//
// For example, if p points to a struct that contains a file descriptor d,
// and p has a finalizer that closes that file descriptor, and if the last
// use of p in a function is a call to syscall.Write(p.d, buf, size), then
// p may be unreachable as soon as the program enters syscall.Write. The
// finalizer may run at that moment, closing p.d, causing syscall.Write
// to fail because it is writing to a closed file descriptor (or, worse,
// to an entirely different file descriptor opened by a different goroutine).
// To avoid this problem, call runtime.KeepAlive(p) after the call to
// syscall.Write.
//
// A single goroutine runs all finalizers for a program, sequentially.
// If a finalizer must run for a long time, it should do so by starting
// a new goroutine.
func SetFinalizer(obj interface{}, finalizer interface{})

// KeepAlive marks its argument as currently reachable.
// This ensures that the object is not freed, and its finalizer is not run,
// before the point in the program where KeepAlive is called.
//
// A very simplified example showing where KeepAlive is required:
// 	type File struct { d int }
// 	d, err := syscall.Open("/file/path", syscall.O_RDONLY, 0)
// 	// ... do something if err != nil ...
// 	p := &File{d}
// 	runtime.SetFinalizer(p, func(p *File) { syscall.Close(p.d) })
// 	var buf [10]byte
// 	n, err := syscall.Read(p.d, buf[:])
// 	// Ensure p is not finalized until Read returns.
// 	runtime.KeepAlive(p)
// 	// No more uses of p after this point.
//
// Without the KeepAlive call, the finalizer could run at the start of
// syscall.Read, closing the file descriptor before syscall.Read makes
// the actual system call.
func KeepAlive(interface{})

// GOROOT returns the root of the Go tree.
// It uses the GOROOT environment variable, if set,
// or else the root used during the Go build.
func GOROOT() string {
	s := gogetenv("GOROOT")
	if s != "" {
		return s
	}
	return sys.DefaultGoroot
}

// Version returns the Go tree's version string.
// It is either the commit hash and date at the time of the build or,
// when possible, a release tag like "go1.3".
func Version() string {
	return sys.TheVersion
}

// GOOS is the running program's operating system target:
// one of darwin, freebsd, linux, and so on.
const GOOS string = sys.GOOS

// GOARCH is the running program's architecture target:
// 386, amd64, arm, or s390x.
const GOARCH string = sys.GOARCH

// GCCGOTOOLDIR is the Tool Dir for the gccgo build
const GCCGOTOOLDIR string = sys.GccgoToolDir
