Source file src/runtime/extern.go
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. 4 5 /* 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. 10 11 # Environment Variables 12 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. 16 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. 22 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 33 time. 34 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: 37 38 clobberfree: setting clobberfree=1 causes the garbage collector to 39 clobber the memory content of an object with bad content when it frees 40 the object. 41 42 cpu.*: cpu.all=off disables the use of all optional instruction set extensions. 43 cpu.extension=off disables use of instructions from the specified instruction set extension. 44 extension is the lower case name for the instruction set extension such as sse41 or avx 45 as listed in internal/cpu package. As an example cpu.avx=off disables runtime detection 46 and thereby use of AVX instructions. 47 48 cgocheck: setting cgocheck=0 disables all checks for packages 49 using cgo to incorrectly pass Go pointers to non-Go code. 50 Setting cgocheck=1 (the default) enables relatively cheap 51 checks that may miss some errors. A more complete, but slow, 52 cgocheck mode can be enabled using GOEXPERIMENT (which 53 requires a rebuild), see https://pkg.go.dev/internal/goexperiment for details. 54 55 disablethp: setting disablethp=1 on Linux disables transparent huge pages for the heap. 56 It has no effect on other platforms. disablethp is meant for compatibility with versions 57 of Go before 1.21, which stopped working around a Linux kernel default that can result 58 in significant memory overuse. See https://go.dev/issue/64332. This setting will be 59 removed in a future release, so operators should tweak their Linux configuration to suit 60 their needs before then. See https://go.dev/doc/gc-guide#Linux_transparent_huge_pages. 61 62 dontfreezetheworld: by default, the start of a fatal panic or throw 63 "freezes the world", preempting all threads to stop all running 64 goroutines, which makes it possible to traceback all goroutines, and 65 keeps their state close to the point of panic. Setting 66 dontfreezetheworld=1 disables this preemption, allowing goroutines to 67 continue executing during panic processing. Note that goroutines that 68 naturally enter the scheduler will still stop. This can be useful when 69 debugging the runtime scheduler, as freezetheworld perturbs scheduler 70 state and thus may hide problems. 71 72 efence: setting efence=1 causes the allocator to run in a mode 73 where each object is allocated on a unique page and addresses are 74 never recycled. 75 76 gccheckmark: setting gccheckmark=1 enables verification of the 77 garbage collector's concurrent mark phase by performing a 78 second mark pass while the world is stopped. If the second 79 pass finds a reachable object that was not found by concurrent 80 mark, the garbage collector will panic. 81 82 gcpacertrace: setting gcpacertrace=1 causes the garbage collector to 83 print information about the internal state of the concurrent pacer. 84 85 gcshrinkstackoff: setting gcshrinkstackoff=1 disables moving goroutines 86 onto smaller stacks. In this mode, a goroutine's stack can only grow. 87 88 gcstoptheworld: setting gcstoptheworld=1 disables concurrent garbage collection, 89 making every garbage collection a stop-the-world event. Setting gcstoptheworld=2 90 also disables concurrent sweeping after the garbage collection finishes. 91 92 gctrace: setting gctrace=1 causes the garbage collector to emit a single line to standard 93 error at each collection, summarizing the amount of memory collected and the 94 length of the pause. The format of this line is subject to change. Included in 95 the explanation below is also the relevant runtime/metrics metric for each field. 96 Currently, it is: 97 gc # @#s #%: #+#+# ms clock, #+#/#/#+# ms cpu, #->#-># MB, # MB goal, # MB stacks, #MB globals, # P 98 where the fields are as follows: 99 gc # the GC number, incremented at each GC 100 @#s time in seconds since program start 101 #% percentage of time spent in GC since program start 102 #+...+# wall-clock/CPU times for the phases of the GC 103 #->#-># MB heap size at GC start, at GC end, and live heap, or /gc/scan/heap:bytes 104 # MB goal goal heap size, or /gc/heap/goal:bytes 105 # MB stacks estimated scannable stack size, or /gc/scan/stack:bytes 106 # MB globals scannable global size, or /gc/scan/globals:bytes 107 # P number of processors used, or /sched/gomaxprocs:threads 108 The phases are stop-the-world (STW) sweep termination, concurrent 109 mark and scan, and STW mark termination. The CPU times 110 for mark/scan are broken down in to assist time (GC performed in 111 line with allocation), background GC time, and idle GC time. 112 If the line ends with "(forced)", this GC was forced by a 113 runtime.GC() call. 114 115 harddecommit: setting harddecommit=1 causes memory that is returned to the OS to 116 also have protections removed on it. This is the only mode of operation on Windows, 117 but is helpful in debugging scavenger-related issues on other platforms. Currently, 118 only supported on Linux. 119 120 inittrace: setting inittrace=1 causes the runtime to emit a single line to standard 121 error for each package with init work, summarizing the execution time and memory 122 allocation. No information is printed for inits executed as part of plugin loading 123 and for packages without both user defined and compiler generated init work. 124 The format of this line is subject to change. Currently, it is: 125 init # @#ms, # ms clock, # bytes, # allocs 126 where the fields are as follows: 127 init # the package name 128 @# ms time in milliseconds when the init started since program start 129 # clock wall-clock time for package initialization work 130 # bytes memory allocated on the heap 131 # allocs number of heap allocations 132 133 madvdontneed: setting madvdontneed=0 will use MADV_FREE 134 instead of MADV_DONTNEED on Linux when returning memory to the 135 kernel. This is more efficient, but means RSS numbers will 136 drop only when the OS is under memory pressure. On the BSDs and 137 Illumos/Solaris, setting madvdontneed=1 will use MADV_DONTNEED instead 138 of MADV_FREE. This is less efficient, but causes RSS numbers to drop 139 more quickly. 140 141 memprofilerate: setting memprofilerate=X will update the value of runtime.MemProfileRate. 142 When set to 0 memory profiling is disabled. Refer to the description of 143 MemProfileRate for the default value. 144 145 pagetrace: setting pagetrace=/path/to/file will write out a trace of page events 146 that can be viewed, analyzed, and visualized using the x/debug/cmd/pagetrace tool. 147 Build your program with GOEXPERIMENT=pagetrace to enable this functionality. Do not 148 enable this functionality if your program is a setuid binary as it introduces a security 149 risk in that scenario. Currently not supported on Windows, plan9 or js/wasm. Setting this 150 option for some applications can produce large traces, so use with care. 151 152 panicnil: setting panicnil=1 disables the runtime error when calling panic with nil 153 interface value or an untyped nil. 154 155 runtimecontentionstacks: setting runtimecontentionstacks=1 enables inclusion of call stacks 156 related to contention on runtime-internal locks in the "mutex" profile, subject to the 157 MutexProfileFraction setting. When runtimecontentionstacks=0, contention on 158 runtime-internal locks will report as "runtime._LostContendedRuntimeLock". When 159 runtimecontentionstacks=1, the call stacks will correspond to the unlock call that released 160 the lock. But instead of the value corresponding to the amount of contention that call 161 stack caused, it corresponds to the amount of time the caller of unlock had to wait in its 162 original call to lock. A future release is expected to align those and remove this setting. 163 164 invalidptr: invalidptr=1 (the default) causes the garbage collector and stack 165 copier to crash the program if an invalid pointer value (for example, 1) 166 is found in a pointer-typed location. Setting invalidptr=0 disables this check. 167 This should only be used as a temporary workaround to diagnose buggy code. 168 The real fix is to not store integers in pointer-typed locations. 169 170 sbrk: setting sbrk=1 replaces the memory allocator and garbage collector 171 with a trivial allocator that obtains memory from the operating system and 172 never reclaims any memory. 173 174 scavtrace: setting scavtrace=1 causes the runtime to emit a single line to standard 175 error, roughly once per GC cycle, summarizing the amount of work done by the 176 scavenger as well as the total amount of memory returned to the operating system 177 and an estimate of physical memory utilization. The format of this line is subject 178 to change, but currently it is: 179 scav # KiB work (bg), # KiB work (eager), # KiB total, #% util 180 where the fields are as follows: 181 # KiB work (bg) the amount of memory returned to the OS in the background since 182 the last line 183 # KiB work (eager) the amount of memory returned to the OS eagerly since the last line 184 # KiB now the amount of address space currently returned to the OS 185 #% util the fraction of all unscavenged heap memory which is in-use 186 If the line ends with "(forced)", then scavenging was forced by a 187 debug.FreeOSMemory() call. 188 189 scheddetail: setting schedtrace=X and scheddetail=1 causes the scheduler to emit 190 detailed multiline info every X milliseconds, describing state of the scheduler, 191 processors, threads and goroutines. 192 193 schedtrace: setting schedtrace=X causes the scheduler to emit a single line to standard 194 error every X milliseconds, summarizing the scheduler state. 195 196 tracebackancestors: setting tracebackancestors=N extends tracebacks with the stacks at 197 which goroutines were created, where N limits the number of ancestor goroutines to 198 report. This also extends the information returned by runtime.Stack. 199 Setting N to 0 will report no ancestry information. 200 201 tracefpunwindoff: setting tracefpunwindoff=1 forces the execution tracer to 202 use the runtime's default stack unwinder instead of frame pointer unwinding. 203 This increases tracer overhead, but could be helpful as a workaround or for 204 debugging unexpected regressions caused by frame pointer unwinding. 205 206 traceadvanceperiod: the approximate period in nanoseconds between trace generations. Only 207 applies if a program is built with GOEXPERIMENT=exectracer2. Used primarily for testing 208 and debugging the execution tracer. 209 210 tracecheckstackownership: setting tracecheckstackownership=1 enables a debug check in the 211 execution tracer to double-check stack ownership before taking a stack trace. 212 213 asyncpreemptoff: asyncpreemptoff=1 disables signal-based 214 asynchronous goroutine preemption. This makes some loops 215 non-preemptible for long periods, which may delay GC and 216 goroutine scheduling. This is useful for debugging GC issues 217 because it also disables the conservative stack scanning used 218 for asynchronously preempted goroutines. 219 220 The [net] and [net/http] packages also refer to debugging variables in GODEBUG. 221 See the documentation for those packages for details. 222 223 The GOMAXPROCS variable limits the number of operating system threads that 224 can execute user-level Go code simultaneously. There is no limit to the number of threads 225 that can be blocked in system calls on behalf of Go code; those do not count against 226 the GOMAXPROCS limit. This package's [GOMAXPROCS] function queries and changes 227 the limit. 228 229 The GORACE variable configures the race detector, for programs built using -race. 230 See the [Race Detector article] for details. 231 232 The GOTRACEBACK variable controls the amount of output generated when a Go 233 program fails due to an unrecovered panic or an unexpected runtime condition. 234 By default, a failure prints a stack trace for the current goroutine, 235 eliding functions internal to the run-time system, and then exits with exit code 2. 236 The failure prints stack traces for all goroutines if there is no current goroutine 237 or the failure is internal to the run-time. 238 GOTRACEBACK=none omits the goroutine stack traces entirely. 239 GOTRACEBACK=single (the default) behaves as described above. 240 GOTRACEBACK=all adds stack traces for all user-created goroutines. 241 GOTRACEBACK=system is like “all” but adds stack frames for run-time functions 242 and shows goroutines created internally by the run-time. 243 GOTRACEBACK=crash is like “system” but crashes in an operating system-specific 244 manner instead of exiting. For example, on Unix systems, the crash raises 245 SIGABRT to trigger a core dump. 246 GOTRACEBACK=wer is like “crash” but doesn't disable Windows Error Reporting (WER). 247 For historical reasons, the GOTRACEBACK settings 0, 1, and 2 are synonyms for 248 none, all, and system, respectively. 249 The [runtime/debug.SetTraceback] function allows increasing the 250 amount of output at run time, but it cannot reduce the amount below that 251 specified by the environment variable. 252 253 The GOARCH, GOOS, GOPATH, and GOROOT environment variables complete 254 the set of Go environment variables. They influence the building of Go programs 255 (see [cmd/go] and [go/build]). 256 GOARCH, GOOS, and GOROOT are recorded at compile time and made available by 257 constants or functions in this package, but they do not influence the execution 258 of the run-time system. 259 260 # Security 261 262 On Unix platforms, Go's runtime system behaves slightly differently when a 263 binary is setuid/setgid or executed with setuid/setgid-like properties, in order 264 to prevent dangerous behaviors. On Linux this is determined by checking for the 265 AT_SECURE flag in the auxiliary vector, on the BSDs and Solaris/Illumos it is 266 determined by checking the issetugid syscall, and on AIX it is determined by 267 checking if the uid/gid match the effective uid/gid. 268 269 When the runtime determines the binary is setuid/setgid-like, it does three main 270 things: 271 - The standard input/output file descriptors (0, 1, 2) are checked to be open. 272 If any of them are closed, they are opened pointing at /dev/null. 273 - The value of the GOTRACEBACK environment variable is set to 'none'. 274 - When a signal is received that terminates the program, or the program 275 encounters an unrecoverable panic that would otherwise override the value 276 of GOTRACEBACK, the goroutine stack, registers, and other memory related 277 information are omitted. 278 279 [Race Detector article]: https://go.dev/doc/articles/race_detector 280 */ 281 package runtime 282 283 import ( 284 "internal/goarch" 285 "internal/goos" 286 ) 287 288 // Caller reports file and line number information about function invocations on 289 // the calling goroutine's stack. The argument skip is the number of stack frames 290 // to ascend, with 0 identifying the caller of Caller. (For historical reasons the 291 // meaning of skip differs between Caller and [Callers].) The return values report the 292 // program counter, file name, and line number within the file of the corresponding 293 // call. The boolean ok is false if it was not possible to recover the information. 294 func Caller(skip int) (pc uintptr, file string, line int, ok bool) { 295 rpc := make([]uintptr, 1) 296 n := callers(skip+1, rpc) 297 if n < 1 { 298 return 299 } 300 frame, _ := CallersFrames(rpc).Next() 301 return frame.PC, frame.File, frame.Line, frame.PC != 0 302 } 303 304 // Callers fills the slice pc with the return program counters of function invocations 305 // on the calling goroutine's stack. The argument skip is the number of stack frames 306 // to skip before recording in pc, with 0 identifying the frame for Callers itself and 307 // 1 identifying the caller of Callers. 308 // It returns the number of entries written to pc. 309 // 310 // To translate these PCs into symbolic information such as function 311 // names and line numbers, use [CallersFrames]. CallersFrames accounts 312 // for inlined functions and adjusts the return program counters into 313 // call program counters. Iterating over the returned slice of PCs 314 // directly is discouraged, as is using [FuncForPC] on any of the 315 // returned PCs, since these cannot account for inlining or return 316 // program counter adjustment. 317 func Callers(skip int, pc []uintptr) int { 318 // runtime.callers uses pc.array==nil as a signal 319 // to print a stack trace. Pick off 0-length pc here 320 // so that we don't let a nil pc slice get to it. 321 if len(pc) == 0 { 322 return 0 323 } 324 return callers(skip, pc) 325 } 326 327 var defaultGOROOT string // set by cmd/link 328 329 // GOROOT returns the root of the Go tree. It uses the 330 // GOROOT environment variable, if set at process start, 331 // or else the root used during the Go build. 332 func GOROOT() string { 333 s := gogetenv("GOROOT") 334 if s != "" { 335 return s 336 } 337 return defaultGOROOT 338 } 339 340 // buildVersion is the Go tree's version string at build time. 341 // 342 // If any GOEXPERIMENTs are set to non-default values, it will include 343 // "X:<GOEXPERIMENT>". 344 // 345 // This is set by the linker. 346 // 347 // This is accessed by "go version <binary>". 348 var buildVersion string 349 350 // Version returns the Go tree's version string. 351 // It is either the commit hash and date at the time of the build or, 352 // when possible, a release tag like "go1.3". 353 func Version() string { 354 return buildVersion 355 } 356 357 // GOOS is the running program's operating system target: 358 // one of darwin, freebsd, linux, and so on. 359 // To view possible combinations of GOOS and GOARCH, run "go tool dist list". 360 const GOOS string = goos.GOOS 361 362 // GOARCH is the running program's architecture target: 363 // one of 386, amd64, arm, s390x, and so on. 364 const GOARCH string = goarch.GOARCH 365