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Source file src/runtime/extern.go

Documentation: runtime

  // 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.
  	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.
  	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.
  	invalidptr: defaults to invalidptr=1, causing the garbage collector and stack
  	copier to crash the program if an invalid pointer value (for example, 1)
  	is found in a pointer-typed location. Setting invalidptr=0 disables this check.
  	This should only be used as a temporary workaround to diagnose buggy code.
  	The real fix is to not store integers in pointer-typed locations.
  	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"
  // 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) {
  	// Make room for three PCs: the one we were asked for,
  	// what it called, so that CallersFrames can see if it "called"
  	// sigpanic, and possibly a PC for skipPleaseUseCallersFrames.
  	var rpc [3]uintptr
  	if callers(1+skip-1, rpc[:]) < 2 {
  	var stackExpander stackExpander
  	callers := stackExpander.init(rpc[:])
  	// We asked for one extra, so skip that one. If this is sigpanic,
  	// stepping over this frame will set up state in Frames so the
  	// next frame is correct.
  	callers, _, ok = stackExpander.next(callers)
  	if !ok {
  	_, frame, _ := stackExpander.next(callers)
  	pc = frame.PC
  	file = frame.File
  	line = frame.Line
  // 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.
  // To translate these PCs into symbolic information such as function
  // names and line numbers, use CallersFrames. CallersFrames accounts
  // for inlined functions and adjusts the return program counters into
  // call program counters. Iterating over the returned slice of PCs
  // directly is discouraged, as is using FuncForPC on any of the
  // returned PCs, since these cannot account for inlining or return
  // program counter adjustment.
  func Callers(skip int, pc []uintptr) int {
  	// runtime.callers uses pc.array==nil as a signal
  	// to print a stack trace. Pick off 0-length pc here
  	// so that we don't let a nil pc slice get to it.
  	if len(pc) == 0 {
  		return 0
  	return callers(skip, pc)
  // 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:
  // one of 386, amd64, arm, s390x, and so on.
  const GOARCH string = sys.GOARCH

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