1 // Copyright 2009 The Go Authors. All rights reserved.
2 // Use of this source code is governed by a BSD-style
3 // license that can be found in the LICENSE file.
6 Package runtime contains operations that interact with Go's runtime system,
7 such as functions to control goroutines. It also includes the low-level type information
8 used by the reflect package; see reflect's documentation for the programmable
9 interface to the run-time type system.
11 # Environment Variables
13 The following environment variables ($name or %name%, depending on the host
14 operating system) control the run-time behavior of Go programs. The meanings
15 and use may change from release to release.
17 The GOGC variable sets the initial garbage collection target percentage.
18 A collection is triggered when the ratio of freshly allocated data to live data
19 remaining after the previous collection reaches this percentage. The default
20 is GOGC=100. Setting GOGC=off disables the garbage collector entirely.
21 [runtime/debug.SetGCPercent] allows changing this percentage at run time.
23 The GOMEMLIMIT variable sets a soft memory limit for the runtime. This memory limit
24 includes the Go heap and all other memory managed by the runtime, and excludes
25 external memory sources such as mappings of the binary itself, memory managed in
26 other languages, and memory held by the operating system on behalf of the Go
27 program. GOMEMLIMIT is a numeric value in bytes with an optional unit suffix.
28 The supported suffixes include B, KiB, MiB, GiB, and TiB. These suffixes
29 represent quantities of bytes as defined by the IEC 80000-13 standard. That is,
30 they are based on powers of two: KiB means 2^10 bytes, MiB means 2^20 bytes,
31 and so on. The default setting is math.MaxInt64, which effectively disables the
32 memory limit. [runtime/debug.SetMemoryLimit] allows changing this limit at run
35 The GODEBUG variable controls debugging variables within the runtime.
36 It is a comma-separated list of name=val pairs setting these named variables:
38 allocfreetrace: setting allocfreetrace=1 causes every allocation to be
39 profiled and a stack trace printed on each object's allocation and free.
41 clobberfree: setting clobberfree=1 causes the garbage collector to
42 clobber the memory content of an object with bad content when it frees
45 cpu.*: cpu.all=off disables the use of all optional instruction set extensions.
46 cpu.extension=off disables use of instructions from the specified instruction set extension.
47 extension is the lower case name for the instruction set extension such as sse41 or avx
48 as listed in internal/cpu package. As an example cpu.avx=off disables runtime detection
49 and thereby use of AVX instructions.
51 cgocheck: setting cgocheck=0 disables all checks for packages
52 using cgo to incorrectly pass Go pointers to non-Go code.
53 Setting cgocheck=1 (the default) enables relatively cheap
54 checks that may miss some errors. Setting cgocheck=2 enables
55 expensive checks that should not miss any errors, but will
56 cause your program to run slower.
58 efence: setting efence=1 causes the allocator to run in a mode
59 where each object is allocated on a unique page and addresses are
62 gccheckmark: setting gccheckmark=1 enables verification of the
63 garbage collector's concurrent mark phase by performing a
64 second mark pass while the world is stopped. If the second
65 pass finds a reachable object that was not found by concurrent
66 mark, the garbage collector will panic.
68 gcpacertrace: setting gcpacertrace=1 causes the garbage collector to
69 print information about the internal state of the concurrent pacer.
71 gcshrinkstackoff: setting gcshrinkstackoff=1 disables moving goroutines
72 onto smaller stacks. In this mode, a goroutine's stack can only grow.
74 gcstoptheworld: setting gcstoptheworld=1 disables concurrent garbage collection,
75 making every garbage collection a stop-the-world event. Setting gcstoptheworld=2
76 also disables concurrent sweeping after the garbage collection finishes.
78 gctrace: setting gctrace=1 causes the garbage collector to emit a single line to standard
79 error at each collection, summarizing the amount of memory collected and the
80 length of the pause. The format of this line is subject to change.
82 gc # @#s #%: #+#+# ms clock, #+#/#/#+# ms cpu, #->#-># MB, # MB goal, # MB stacks, #MB globals, # P
83 where the fields are as follows:
84 gc # the GC number, incremented at each GC
85 @#s time in seconds since program start
86 #% percentage of time spent in GC since program start
87 #+...+# wall-clock/CPU times for the phases of the GC
88 #->#-># MB heap size at GC start, at GC end, and live heap
89 # MB goal goal heap size
90 # MB stacks estimated scannable stack size
91 # MB globals scannable global size
92 # P number of processors used
93 The phases are stop-the-world (STW) sweep termination, concurrent
94 mark and scan, and STW mark termination. The CPU times
95 for mark/scan are broken down in to assist time (GC performed in
96 line with allocation), background GC time, and idle GC time.
97 If the line ends with "(forced)", this GC was forced by a
100 harddecommit: setting harddecommit=1 causes memory that is returned to the OS to
101 also have protections removed on it. This is the only mode of operation on Windows,
102 but is helpful in debugging scavenger-related issues on other platforms. Currently,
103 only supported on Linux.
105 inittrace: setting inittrace=1 causes the runtime to emit a single line to standard
106 error for each package with init work, summarizing the execution time and memory
107 allocation. No information is printed for inits executed as part of plugin loading
108 and for packages without both user defined and compiler generated init work.
109 The format of this line is subject to change. Currently, it is:
110 init # @#ms, # ms clock, # bytes, # allocs
111 where the fields are as follows:
112 init # the package name
113 @# ms time in milliseconds when the init started since program start
114 # clock wall-clock time for package initialization work
115 # bytes memory allocated on the heap
116 # allocs number of heap allocations
118 madvdontneed: setting madvdontneed=0 will use MADV_FREE
119 instead of MADV_DONTNEED on Linux when returning memory to the
120 kernel. This is more efficient, but means RSS numbers will
121 drop only when the OS is under memory pressure. On the BSDs and
122 Illumos/Solaris, setting madvdontneed=1 will use MADV_DONTNEED instead
123 of MADV_FREE. This is less efficient, but causes RSS numbers to drop
126 memprofilerate: setting memprofilerate=X will update the value of runtime.MemProfileRate.
127 When set to 0 memory profiling is disabled. Refer to the description of
128 MemProfileRate for the default value.
130 pagetrace: setting pagetrace=/path/to/file will write out a trace of page events
131 that can be viewed, analyzed, and visualized using the x/debug/cmd/pagetrace tool.
132 Build your program with GOEXPERIMENT=pagetrace to enable this functionality. Do not
133 enable this functionality if your program is a setuid binary as it introduces a security
134 risk in that scenario. Currently not supported on Windows, plan9 or js/wasm. Setting this
135 option for some applications can produce large traces, so use with care.
137 invalidptr: invalidptr=1 (the default) causes the garbage collector and stack
138 copier to crash the program if an invalid pointer value (for example, 1)
139 is found in a pointer-typed location. Setting invalidptr=0 disables this check.
140 This should only be used as a temporary workaround to diagnose buggy code.
141 The real fix is to not store integers in pointer-typed locations.
143 sbrk: setting sbrk=1 replaces the memory allocator and garbage collector
144 with a trivial allocator that obtains memory from the operating system and
145 never reclaims any memory.
147 scavtrace: setting scavtrace=1 causes the runtime to emit a single line to standard
148 error, roughly once per GC cycle, summarizing the amount of work done by the
149 scavenger as well as the total amount of memory returned to the operating system
150 and an estimate of physical memory utilization. The format of this line is subject
151 to change, but currently it is:
152 scav # KiB work, # KiB total, #% util
153 where the fields are as follows:
154 # KiB work the amount of memory returned to the OS since the last line
155 # KiB total the total amount of memory returned to the OS
156 #% util the fraction of all unscavenged memory which is in-use
157 If the line ends with "(forced)", then scavenging was forced by a
158 debug.FreeOSMemory() call.
160 scheddetail: setting schedtrace=X and scheddetail=1 causes the scheduler to emit
161 detailed multiline info every X milliseconds, describing state of the scheduler,
162 processors, threads and goroutines.
164 schedtrace: setting schedtrace=X causes the scheduler to emit a single line to standard
165 error every X milliseconds, summarizing the scheduler state.
167 tracebackancestors: setting tracebackancestors=N extends tracebacks with the stacks at
168 which goroutines were created, where N limits the number of ancestor goroutines to
169 report. This also extends the information returned by runtime.Stack. Ancestor's goroutine
170 IDs will refer to the ID of the goroutine at the time of creation; it's possible for this
171 ID to be reused for another goroutine. Setting N to 0 will report no ancestry information.
173 asyncpreemptoff: asyncpreemptoff=1 disables signal-based
174 asynchronous goroutine preemption. This makes some loops
175 non-preemptible for long periods, which may delay GC and
176 goroutine scheduling. This is useful for debugging GC issues
177 because it also disables the conservative stack scanning used
178 for asynchronously preempted goroutines.
180 The net and net/http packages also refer to debugging variables in GODEBUG.
181 See the documentation for those packages for details.
183 The GOMAXPROCS variable limits the number of operating system threads that
184 can execute user-level Go code simultaneously. There is no limit to the number of threads
185 that can be blocked in system calls on behalf of Go code; those do not count against
186 the GOMAXPROCS limit. This package's GOMAXPROCS function queries and changes
189 The GORACE variable configures the race detector, for programs built using -race.
190 See https://golang.org/doc/articles/race_detector.html for details.
192 The GOTRACEBACK variable controls the amount of output generated when a Go
193 program fails due to an unrecovered panic or an unexpected runtime condition.
194 By default, a failure prints a stack trace for the current goroutine,
195 eliding functions internal to the run-time system, and then exits with exit code 2.
196 The failure prints stack traces for all goroutines if there is no current goroutine
197 or the failure is internal to the run-time.
198 GOTRACEBACK=none omits the goroutine stack traces entirely.
199 GOTRACEBACK=single (the default) behaves as described above.
200 GOTRACEBACK=all adds stack traces for all user-created goroutines.
201 GOTRACEBACK=system is like “all” but adds stack frames for run-time functions
202 and shows goroutines created internally by the run-time.
203 GOTRACEBACK=crash is like “system” but crashes in an operating system-specific
204 manner instead of exiting. For example, on Unix systems, the crash raises
205 SIGABRT to trigger a core dump.
206 For historical reasons, the GOTRACEBACK settings 0, 1, and 2 are synonyms for
207 none, all, and system, respectively.
208 The runtime/debug package's SetTraceback function allows increasing the
209 amount of output at run time, but it cannot reduce the amount below that
210 specified by the environment variable.
211 See https://golang.org/pkg/runtime/debug/#SetTraceback.
213 The GOARCH, GOOS, GOPATH, and GOROOT environment variables complete
214 the set of Go environment variables. They influence the building of Go programs
215 (see https://golang.org/cmd/go and https://golang.org/pkg/go/build).
216 GOARCH, GOOS, and GOROOT are recorded at compile time and made available by
217 constants or functions in this package, but they do not influence the execution
218 of the run-time system.
227 // Caller reports file and line number information about function invocations on
228 // the calling goroutine's stack. The argument skip is the number of stack frames
229 // to ascend, with 0 identifying the caller of Caller. (For historical reasons the
230 // meaning of skip differs between Caller and Callers.) The return values report the
231 // program counter, file name, and line number within the file of the corresponding
232 // call. The boolean ok is false if it was not possible to recover the information.
233 func Caller(skip int) (pc uintptr, file string, line int, ok bool) {
234 rpc := make([]uintptr, 1)
235 n := callers(skip+1, rpc[:])
239 frame, _ := CallersFrames(rpc).Next()
240 return frame.PC, frame.File, frame.Line, frame.PC != 0
243 // Callers fills the slice pc with the return program counters of function invocations
244 // on the calling goroutine's stack. The argument skip is the number of stack frames
245 // to skip before recording in pc, with 0 identifying the frame for Callers itself and
246 // 1 identifying the caller of Callers.
247 // It returns the number of entries written to pc.
249 // To translate these PCs into symbolic information such as function
250 // names and line numbers, use CallersFrames. CallersFrames accounts
251 // for inlined functions and adjusts the return program counters into
252 // call program counters. Iterating over the returned slice of PCs
253 // directly is discouraged, as is using FuncForPC on any of the
254 // returned PCs, since these cannot account for inlining or return
255 // program counter adjustment.
256 func Callers(skip int, pc []uintptr) int {
257 // runtime.callers uses pc.array==nil as a signal
258 // to print a stack trace. Pick off 0-length pc here
259 // so that we don't let a nil pc slice get to it.
263 return callers(skip, pc)
266 var defaultGOROOT string // set by cmd/link
268 // GOROOT returns the root of the Go tree. It uses the
269 // GOROOT environment variable, if set at process start,
270 // or else the root used during the Go build.
271 func GOROOT() string {
272 s := gogetenv("GOROOT")
279 // buildVersion is the Go tree's version string at build time.
281 // If any GOEXPERIMENTs are set to non-default values, it will include
282 // "X:<GOEXPERIMENT>".
284 // This is set by the linker.
286 // This is accessed by "go version <binary>".
287 var buildVersion string
289 // Version returns the Go tree's version string.
290 // It is either the commit hash and date at the time of the build or,
291 // when possible, a release tag like "go1.3".
292 func Version() string {
296 // GOOS is the running program's operating system target:
297 // one of darwin, freebsd, linux, and so on.
298 // To view possible combinations of GOOS and GOARCH, run "go tool dist list".
299 const GOOS string = goos.GOOS
301 // GOARCH is the running program's architecture target:
302 // one of 386, amd64, arm, s390x, and so on.
303 const GOARCH string = goarch.GOARCH