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// 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
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.
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.
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.
The GOARCH, GOOS, GOPATH, and GOROOT environment variables complete
the set of Go environment variables. They influence the building of Go programs
(see and
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) {
// Ask for two PCs: the one we were asked for
// and what it called, so that we can see if it
// "called" sigpanic.
var rpc [2]uintptr
if callers(1+skip-1, rpc[:]) < 2 {
f := findfunc(rpc[1])
if f == nil {
// TODO(rsc): Probably a bug?
// The C version said "have retpc at least"
// but actually returned pc=0.
ok = true
pc = rpc[1]
xpc := pc
g := findfunc(rpc[0])
// All architectures turn faults into apparent calls to sigpanic.
// If we see a call to sigpanic, we do not back up the PC to find
// the line number of the call instruction, because there is no call.
if xpc > f.entry && (g == nil || g.entry != funcPC(sigpanic)) {
file, line32 := funcline(f, xpc)
line = int(line32)
ok = true
// 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.
// Note that since each slice entry pc[i] is a return program counter,
// looking up the file and line for pc[i] (for example, using (*Func).FileLine)
// will normally return the file and line number of the instruction immediately
// following the call.
// To easily look up file/line information for the call sequence, use Frames.
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:
// 386, amd64, arm, or s390x.
const GOARCH string = sys.GOARCH