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. A more complete, but slow,
55 cgocheck mode can be enabled using GOEXPERIMENT (which
56 requires a rebuild), see https://pkg.go.dev/internal/goexperiment for details.
58 dontfreezetheworld: by default, the start of a fatal panic or throw
59 "freezes the world", stopping all goroutines, which makes it possible
60 to traceback all goroutines (running goroutines cannot be traced), and
61 keeps their state close to the point of panic. Setting
62 dontfreezetheworld=1 disables freeze, allowing goroutines to continue
63 executing during panic processing. This can be useful when debugging
64 the runtime scheduler, as freezetheworld perturbs scheduler state and
65 thus may hide problems.
67 efence: setting efence=1 causes the allocator to run in a mode
68 where each object is allocated on a unique page and addresses are
71 gccheckmark: setting gccheckmark=1 enables verification of the
72 garbage collector's concurrent mark phase by performing a
73 second mark pass while the world is stopped. If the second
74 pass finds a reachable object that was not found by concurrent
75 mark, the garbage collector will panic.
77 gcpacertrace: setting gcpacertrace=1 causes the garbage collector to
78 print information about the internal state of the concurrent pacer.
80 gcshrinkstackoff: setting gcshrinkstackoff=1 disables moving goroutines
81 onto smaller stacks. In this mode, a goroutine's stack can only grow.
83 gcstoptheworld: setting gcstoptheworld=1 disables concurrent garbage collection,
84 making every garbage collection a stop-the-world event. Setting gcstoptheworld=2
85 also disables concurrent sweeping after the garbage collection finishes.
87 gctrace: setting gctrace=1 causes the garbage collector to emit a single line to standard
88 error at each collection, summarizing the amount of memory collected and the
89 length of the pause. The format of this line is subject to change.
91 gc # @#s #%: #+#+# ms clock, #+#/#/#+# ms cpu, #->#-># MB, # MB goal, # MB stacks, #MB globals, # P
92 where the fields are as follows:
93 gc # the GC number, incremented at each GC
94 @#s time in seconds since program start
95 #% percentage of time spent in GC since program start
96 #+...+# wall-clock/CPU times for the phases of the GC
97 #->#-># MB heap size at GC start, at GC end, and live heap
98 # MB goal goal heap size
99 # MB stacks estimated scannable stack size
100 # MB globals scannable global size
101 # P number of processors used
102 The phases are stop-the-world (STW) sweep termination, concurrent
103 mark and scan, and STW mark termination. The CPU times
104 for mark/scan are broken down in to assist time (GC performed in
105 line with allocation), background GC time, and idle GC time.
106 If the line ends with "(forced)", this GC was forced by a
109 harddecommit: setting harddecommit=1 causes memory that is returned to the OS to
110 also have protections removed on it. This is the only mode of operation on Windows,
111 but is helpful in debugging scavenger-related issues on other platforms. Currently,
112 only supported on Linux.
114 inittrace: setting inittrace=1 causes the runtime to emit a single line to standard
115 error for each package with init work, summarizing the execution time and memory
116 allocation. No information is printed for inits executed as part of plugin loading
117 and for packages without both user defined and compiler generated init work.
118 The format of this line is subject to change. Currently, it is:
119 init # @#ms, # ms clock, # bytes, # allocs
120 where the fields are as follows:
121 init # the package name
122 @# ms time in milliseconds when the init started since program start
123 # clock wall-clock time for package initialization work
124 # bytes memory allocated on the heap
125 # allocs number of heap allocations
127 madvdontneed: setting madvdontneed=0 will use MADV_FREE
128 instead of MADV_DONTNEED on Linux when returning memory to the
129 kernel. This is more efficient, but means RSS numbers will
130 drop only when the OS is under memory pressure. On the BSDs and
131 Illumos/Solaris, setting madvdontneed=1 will use MADV_DONTNEED instead
132 of MADV_FREE. This is less efficient, but causes RSS numbers to drop
135 memprofilerate: setting memprofilerate=X will update the value of runtime.MemProfileRate.
136 When set to 0 memory profiling is disabled. Refer to the description of
137 MemProfileRate for the default value.
139 pagetrace: setting pagetrace=/path/to/file will write out a trace of page events
140 that can be viewed, analyzed, and visualized using the x/debug/cmd/pagetrace tool.
141 Build your program with GOEXPERIMENT=pagetrace to enable this functionality. Do not
142 enable this functionality if your program is a setuid binary as it introduces a security
143 risk in that scenario. Currently not supported on Windows, plan9 or js/wasm. Setting this
144 option for some applications can produce large traces, so use with care.
146 invalidptr: invalidptr=1 (the default) causes the garbage collector and stack
147 copier to crash the program if an invalid pointer value (for example, 1)
148 is found in a pointer-typed location. Setting invalidptr=0 disables this check.
149 This should only be used as a temporary workaround to diagnose buggy code.
150 The real fix is to not store integers in pointer-typed locations.
152 sbrk: setting sbrk=1 replaces the memory allocator and garbage collector
153 with a trivial allocator that obtains memory from the operating system and
154 never reclaims any memory.
156 scavtrace: setting scavtrace=1 causes the runtime to emit a single line to standard
157 error, roughly once per GC cycle, summarizing the amount of work done by the
158 scavenger as well as the total amount of memory returned to the operating system
159 and an estimate of physical memory utilization. The format of this line is subject
160 to change, but currently it is:
161 scav # KiB work, # KiB total, #% util
162 where the fields are as follows:
163 # KiB work the amount of memory returned to the OS since the last line
164 # KiB total the total amount of memory returned to the OS
165 #% util the fraction of all unscavenged memory which is in-use
166 If the line ends with "(forced)", then scavenging was forced by a
167 debug.FreeOSMemory() call.
169 scheddetail: setting schedtrace=X and scheddetail=1 causes the scheduler to emit
170 detailed multiline info every X milliseconds, describing state of the scheduler,
171 processors, threads and goroutines.
173 schedtrace: setting schedtrace=X causes the scheduler to emit a single line to standard
174 error every X milliseconds, summarizing the scheduler state.
176 tracebackancestors: setting tracebackancestors=N extends tracebacks with the stacks at
177 which goroutines were created, where N limits the number of ancestor goroutines to
178 report. This also extends the information returned by runtime.Stack. Ancestor's goroutine
179 IDs will refer to the ID of the goroutine at the time of creation; it's possible for this
180 ID to be reused for another goroutine. Setting N to 0 will report no ancestry information.
182 asyncpreemptoff: asyncpreemptoff=1 disables signal-based
183 asynchronous goroutine preemption. This makes some loops
184 non-preemptible for long periods, which may delay GC and
185 goroutine scheduling. This is useful for debugging GC issues
186 because it also disables the conservative stack scanning used
187 for asynchronously preempted goroutines.
189 The net and net/http packages also refer to debugging variables in GODEBUG.
190 See the documentation for those packages for details.
192 The GOMAXPROCS variable limits the number of operating system threads that
193 can execute user-level Go code simultaneously. There is no limit to the number of threads
194 that can be blocked in system calls on behalf of Go code; those do not count against
195 the GOMAXPROCS limit. This package's GOMAXPROCS function queries and changes
198 The GORACE variable configures the race detector, for programs built using -race.
199 See https://golang.org/doc/articles/race_detector.html for details.
201 The GOTRACEBACK variable controls the amount of output generated when a Go
202 program fails due to an unrecovered panic or an unexpected runtime condition.
203 By default, a failure prints a stack trace for the current goroutine,
204 eliding functions internal to the run-time system, and then exits with exit code 2.
205 The failure prints stack traces for all goroutines if there is no current goroutine
206 or the failure is internal to the run-time.
207 GOTRACEBACK=none omits the goroutine stack traces entirely.
208 GOTRACEBACK=single (the default) behaves as described above.
209 GOTRACEBACK=all adds stack traces for all user-created goroutines.
210 GOTRACEBACK=system is like “all” but adds stack frames for run-time functions
211 and shows goroutines created internally by the run-time.
212 GOTRACEBACK=crash is like “system” but crashes in an operating system-specific
213 manner instead of exiting. For example, on Unix systems, the crash raises
214 SIGABRT to trigger a core dump.
215 For historical reasons, the GOTRACEBACK settings 0, 1, and 2 are synonyms for
216 none, all, and system, respectively.
217 The runtime/debug package's SetTraceback function allows increasing the
218 amount of output at run time, but it cannot reduce the amount below that
219 specified by the environment variable.
220 See https://golang.org/pkg/runtime/debug/#SetTraceback.
222 The GOARCH, GOOS, GOPATH, and GOROOT environment variables complete
223 the set of Go environment variables. They influence the building of Go programs
224 (see https://golang.org/cmd/go and https://golang.org/pkg/go/build).
225 GOARCH, GOOS, and GOROOT are recorded at compile time and made available by
226 constants or functions in this package, but they do not influence the execution
227 of the run-time system.
236 // Caller reports file and line number information about function invocations on
237 // the calling goroutine's stack. The argument skip is the number of stack frames
238 // to ascend, with 0 identifying the caller of Caller. (For historical reasons the
239 // meaning of skip differs between Caller and Callers.) The return values report the
240 // program counter, file name, and line number within the file of the corresponding
241 // call. The boolean ok is false if it was not possible to recover the information.
242 func Caller(skip int) (pc uintptr, file string, line int, ok bool) {
243 rpc := make([]uintptr, 1)
244 n := callers(skip+1, rpc[:])
248 frame, _ := CallersFrames(rpc).Next()
249 return frame.PC, frame.File, frame.Line, frame.PC != 0
252 // Callers fills the slice pc with the return program counters of function invocations
253 // on the calling goroutine's stack. The argument skip is the number of stack frames
254 // to skip before recording in pc, with 0 identifying the frame for Callers itself and
255 // 1 identifying the caller of Callers.
256 // It returns the number of entries written to pc.
258 // To translate these PCs into symbolic information such as function
259 // names and line numbers, use CallersFrames. CallersFrames accounts
260 // for inlined functions and adjusts the return program counters into
261 // call program counters. Iterating over the returned slice of PCs
262 // directly is discouraged, as is using FuncForPC on any of the
263 // returned PCs, since these cannot account for inlining or return
264 // program counter adjustment.
265 func Callers(skip int, pc []uintptr) int {
266 // runtime.callers uses pc.array==nil as a signal
267 // to print a stack trace. Pick off 0-length pc here
268 // so that we don't let a nil pc slice get to it.
272 return callers(skip, pc)
275 var defaultGOROOT string // set by cmd/link
277 // GOROOT returns the root of the Go tree. It uses the
278 // GOROOT environment variable, if set at process start,
279 // or else the root used during the Go build.
280 func GOROOT() string {
281 s := gogetenv("GOROOT")
288 // buildVersion is the Go tree's version string at build time.
290 // If any GOEXPERIMENTs are set to non-default values, it will include
291 // "X:<GOEXPERIMENT>".
293 // This is set by the linker.
295 // This is accessed by "go version <binary>".
296 var buildVersion string
298 // Version returns the Go tree's version string.
299 // It is either the commit hash and date at the time of the build or,
300 // when possible, a release tag like "go1.3".
301 func Version() string {
305 // GOOS is the running program's operating system target:
306 // one of darwin, freebsd, linux, and so on.
307 // To view possible combinations of GOOS and GOARCH, run "go tool dist list".
308 const GOOS string = goos.GOOS
310 // GOARCH is the running program's architecture target:
311 // one of 386, amd64, arm, s390x, and so on.
312 const GOARCH string = goarch.GOARCH