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", preempting all threads to stop all running
60 goroutines, which makes it possible to traceback all goroutines, and
61 keeps their state close to the point of panic. Setting
62 dontfreezetheworld=1 disables this preemption, allowing goroutines to
63 continue executing during panic processing. Note that goroutines that
64 naturally enter the scheduler will still stop. This can be useful when
65 debugging the runtime scheduler, as freezetheworld perturbs scheduler
66 state and thus may hide problems.
68 efence: setting efence=1 causes the allocator to run in a mode
69 where each object is allocated on a unique page and addresses are
72 gccheckmark: setting gccheckmark=1 enables verification of the
73 garbage collector's concurrent mark phase by performing a
74 second mark pass while the world is stopped. If the second
75 pass finds a reachable object that was not found by concurrent
76 mark, the garbage collector will panic.
78 gcpacertrace: setting gcpacertrace=1 causes the garbage collector to
79 print information about the internal state of the concurrent pacer.
81 gcshrinkstackoff: setting gcshrinkstackoff=1 disables moving goroutines
82 onto smaller stacks. In this mode, a goroutine's stack can only grow.
84 gcstoptheworld: setting gcstoptheworld=1 disables concurrent garbage collection,
85 making every garbage collection a stop-the-world event. Setting gcstoptheworld=2
86 also disables concurrent sweeping after the garbage collection finishes.
88 gctrace: setting gctrace=1 causes the garbage collector to emit a single line to standard
89 error at each collection, summarizing the amount of memory collected and the
90 length of the pause. The format of this line is subject to change.
92 gc # @#s #%: #+#+# ms clock, #+#/#/#+# ms cpu, #->#-># MB, # MB goal, # MB stacks, #MB globals, # P
93 where the fields are as follows:
94 gc # the GC number, incremented at each GC
95 @#s time in seconds since program start
96 #% percentage of time spent in GC since program start
97 #+...+# wall-clock/CPU times for the phases of the GC
98 #->#-># MB heap size at GC start, at GC end, and live heap
99 # MB goal goal heap size
100 # MB stacks estimated scannable stack size
101 # MB globals scannable global size
102 # P number of processors used
103 The phases are stop-the-world (STW) sweep termination, concurrent
104 mark and scan, and STW mark termination. The CPU times
105 for mark/scan are broken down in to assist time (GC performed in
106 line with allocation), background GC time, and idle GC time.
107 If the line ends with "(forced)", this GC was forced by a
110 harddecommit: setting harddecommit=1 causes memory that is returned to the OS to
111 also have protections removed on it. This is the only mode of operation on Windows,
112 but is helpful in debugging scavenger-related issues on other platforms. Currently,
113 only supported on Linux.
115 inittrace: setting inittrace=1 causes the runtime to emit a single line to standard
116 error for each package with init work, summarizing the execution time and memory
117 allocation. No information is printed for inits executed as part of plugin loading
118 and for packages without both user defined and compiler generated init work.
119 The format of this line is subject to change. Currently, it is:
120 init # @#ms, # ms clock, # bytes, # allocs
121 where the fields are as follows:
122 init # the package name
123 @# ms time in milliseconds when the init started since program start
124 # clock wall-clock time for package initialization work
125 # bytes memory allocated on the heap
126 # allocs number of heap allocations
128 madvdontneed: setting madvdontneed=0 will use MADV_FREE
129 instead of MADV_DONTNEED on Linux when returning memory to the
130 kernel. This is more efficient, but means RSS numbers will
131 drop only when the OS is under memory pressure. On the BSDs and
132 Illumos/Solaris, setting madvdontneed=1 will use MADV_DONTNEED instead
133 of MADV_FREE. This is less efficient, but causes RSS numbers to drop
136 memprofilerate: setting memprofilerate=X will update the value of runtime.MemProfileRate.
137 When set to 0 memory profiling is disabled. Refer to the description of
138 MemProfileRate for the default value.
140 pagetrace: setting pagetrace=/path/to/file will write out a trace of page events
141 that can be viewed, analyzed, and visualized using the x/debug/cmd/pagetrace tool.
142 Build your program with GOEXPERIMENT=pagetrace to enable this functionality. Do not
143 enable this functionality if your program is a setuid binary as it introduces a security
144 risk in that scenario. Currently not supported on Windows, plan9 or js/wasm. Setting this
145 option for some applications can produce large traces, so use with care.
147 invalidptr: invalidptr=1 (the default) causes the garbage collector and stack
148 copier to crash the program if an invalid pointer value (for example, 1)
149 is found in a pointer-typed location. Setting invalidptr=0 disables this check.
150 This should only be used as a temporary workaround to diagnose buggy code.
151 The real fix is to not store integers in pointer-typed locations.
153 sbrk: setting sbrk=1 replaces the memory allocator and garbage collector
154 with a trivial allocator that obtains memory from the operating system and
155 never reclaims any memory.
157 scavtrace: setting scavtrace=1 causes the runtime to emit a single line to standard
158 error, roughly once per GC cycle, summarizing the amount of work done by the
159 scavenger as well as the total amount of memory returned to the operating system
160 and an estimate of physical memory utilization. The format of this line is subject
161 to change, but currently it is:
162 scav # KiB work (bg), # KiB work (eager), # KiB total, #% util
163 where the fields are as follows:
164 # KiB work (bg) the amount of memory returned to the OS in the background since
166 # KiB work (eager) the amount of memory returned to the OS eagerly since the last line
167 # KiB now the amount of address space currently returned to the OS
168 #% util the fraction of all unscavenged heap memory which is in-use
169 If the line ends with "(forced)", then scavenging was forced by a
170 debug.FreeOSMemory() call.
172 scheddetail: setting schedtrace=X and scheddetail=1 causes the scheduler to emit
173 detailed multiline info every X milliseconds, describing state of the scheduler,
174 processors, threads and goroutines.
176 schedtrace: setting schedtrace=X causes the scheduler to emit a single line to standard
177 error every X milliseconds, summarizing the scheduler state.
179 tracebackancestors: setting tracebackancestors=N extends tracebacks with the stacks at
180 which goroutines were created, where N limits the number of ancestor goroutines to
181 report. This also extends the information returned by runtime.Stack. Ancestor's goroutine
182 IDs will refer to the ID of the goroutine at the time of creation; it's possible for this
183 ID to be reused for another goroutine. Setting N to 0 will report no ancestry information.
185 tracefpunwindoff: setting tracefpunwindoff=1 forces the execution tracer to
186 use the runtime's default stack unwinder instead of frame pointer unwinding.
187 This increases tracer overhead, but could be helpful as a workaround or for
188 debugging unexpected regressions caused by frame pointer unwinding.
190 asyncpreemptoff: asyncpreemptoff=1 disables signal-based
191 asynchronous goroutine preemption. This makes some loops
192 non-preemptible for long periods, which may delay GC and
193 goroutine scheduling. This is useful for debugging GC issues
194 because it also disables the conservative stack scanning used
195 for asynchronously preempted goroutines.
197 The net and net/http packages also refer to debugging variables in GODEBUG.
198 See the documentation for those packages for details.
200 The GOMAXPROCS variable limits the number of operating system threads that
201 can execute user-level Go code simultaneously. There is no limit to the number of threads
202 that can be blocked in system calls on behalf of Go code; those do not count against
203 the GOMAXPROCS limit. This package's GOMAXPROCS function queries and changes
206 The GORACE variable configures the race detector, for programs built using -race.
207 See https://golang.org/doc/articles/race_detector.html for details.
209 The GOTRACEBACK variable controls the amount of output generated when a Go
210 program fails due to an unrecovered panic or an unexpected runtime condition.
211 By default, a failure prints a stack trace for the current goroutine,
212 eliding functions internal to the run-time system, and then exits with exit code 2.
213 The failure prints stack traces for all goroutines if there is no current goroutine
214 or the failure is internal to the run-time.
215 GOTRACEBACK=none omits the goroutine stack traces entirely.
216 GOTRACEBACK=single (the default) behaves as described above.
217 GOTRACEBACK=all adds stack traces for all user-created goroutines.
218 GOTRACEBACK=system is like “all” but adds stack frames for run-time functions
219 and shows goroutines created internally by the run-time.
220 GOTRACEBACK=crash is like “system” but crashes in an operating system-specific
221 manner instead of exiting. For example, on Unix systems, the crash raises
222 SIGABRT to trigger a core dump.
223 GOTRACEBACK=wer is like “crash” but doesn't disable Windows Error Reporting (WER).
224 For historical reasons, the GOTRACEBACK settings 0, 1, and 2 are synonyms for
225 none, all, and system, respectively.
226 The runtime/debug package's SetTraceback function allows increasing the
227 amount of output at run time, but it cannot reduce the amount below that
228 specified by the environment variable.
229 See https://golang.org/pkg/runtime/debug/#SetTraceback.
231 The GOARCH, GOOS, GOPATH, and GOROOT environment variables complete
232 the set of Go environment variables. They influence the building of Go programs
233 (see https://golang.org/cmd/go and https://golang.org/pkg/go/build).
234 GOARCH, GOOS, and GOROOT are recorded at compile time and made available by
235 constants or functions in this package, but they do not influence the execution
236 of the run-time system.
240 On Unix platforms, Go's runtime system behaves slightly differently when a
241 binary is setuid/setgid or executed with setuid/setgid-like properties, in order
242 to prevent dangerous behaviors. On Linux this is determined by checking for the
243 AT_SECURE flag in the auxiliary vector, on the BSDs and Solaris/Illumos it is
244 determined by checking the issetugid syscall, and on AIX it is determined by
245 checking if the uid/gid match the effective uid/gid.
247 When the runtime determines the binary is setuid/setgid-like, it does three main
249 - The standard input/output file descriptors (0, 1, 2) are checked to be open.
250 If any of them are closed, they are opened pointing at /dev/null.
251 - The value of the GOTRACEBACK environment variable is set to 'none'.
252 - When a signal is received that terminates the program, or the program
253 encounters an unrecoverable panic that would otherwise override the value
254 of GOTRACEBACK, the goroutine stack, registers, and other memory related
255 information are omitted.
264 // Caller reports file and line number information about function invocations on
265 // the calling goroutine's stack. The argument skip is the number of stack frames
266 // to ascend, with 0 identifying the caller of Caller. (For historical reasons the
267 // meaning of skip differs between Caller and Callers.) The return values report the
268 // program counter, file name, and line number within the file of the corresponding
269 // call. The boolean ok is false if it was not possible to recover the information.
270 func Caller(skip int) (pc uintptr, file string, line int, ok bool) {
271 rpc := make([]uintptr, 1)
272 n := callers(skip+1, rpc[:])
276 frame, _ := CallersFrames(rpc).Next()
277 return frame.PC, frame.File, frame.Line, frame.PC != 0
280 // Callers fills the slice pc with the return program counters of function invocations
281 // on the calling goroutine's stack. The argument skip is the number of stack frames
282 // to skip before recording in pc, with 0 identifying the frame for Callers itself and
283 // 1 identifying the caller of Callers.
284 // It returns the number of entries written to pc.
286 // To translate these PCs into symbolic information such as function
287 // names and line numbers, use CallersFrames. CallersFrames accounts
288 // for inlined functions and adjusts the return program counters into
289 // call program counters. Iterating over the returned slice of PCs
290 // directly is discouraged, as is using FuncForPC on any of the
291 // returned PCs, since these cannot account for inlining or return
292 // program counter adjustment.
293 func Callers(skip int, pc []uintptr) int {
294 // runtime.callers uses pc.array==nil as a signal
295 // to print a stack trace. Pick off 0-length pc here
296 // so that we don't let a nil pc slice get to it.
300 return callers(skip, pc)
303 var defaultGOROOT string // set by cmd/link
305 // GOROOT returns the root of the Go tree. It uses the
306 // GOROOT environment variable, if set at process start,
307 // or else the root used during the Go build.
308 func GOROOT() string {
309 s := gogetenv("GOROOT")
316 // buildVersion is the Go tree's version string at build time.
318 // If any GOEXPERIMENTs are set to non-default values, it will include
319 // "X:<GOEXPERIMENT>".
321 // This is set by the linker.
323 // This is accessed by "go version <binary>".
324 var buildVersion string
326 // Version returns the Go tree's version string.
327 // It is either the commit hash and date at the time of the build or,
328 // when possible, a release tag like "go1.3".
329 func Version() string {
333 // GOOS is the running program's operating system target:
334 // one of darwin, freebsd, linux, and so on.
335 // To view possible combinations of GOOS and GOARCH, run "go tool dist list".
336 const GOOS string = goos.GOOS
338 // GOARCH is the running program's architecture target:
339 // one of 386, amd64, arm, s390x, and so on.
340 const GOARCH string = goarch.GOARCH