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 checkfinalizers: setting checkfinalizers=1 causes the garbage collector to run 56 multiple partial non-parallel stop-the-world collections to identify common issues with 57 finalizers and cleanups, like those listed at 58 https://go.dev/doc/gc-guide#Finalizers_cleanups_and_weak_pointers. If a potential issue 59 is found, the program will terminate with a description of all potential issues, the 60 associated values, and a list of those values' finalizers and cleanups, including where 61 they were created. It also adds tracking for tiny blocks to help diagnose issues with 62 those as well. The analysis performed during the partial collection is conservative. 63 Notably, it flags any path back to the original object from the cleanup function, 64 cleanup arguments, or finalizer function as a potential issue, even if that path might 65 be severed sometime later during execution (though this is not a recommended pattern). 66 This mode also produces one line of output to stderr every GC cycle with information 67 about the finalizer and cleanup queue lengths. Lines produced by this mode start with 68 "checkfinalizers:". 69 70 decoratemappings: controls whether the Go runtime annotates OS 71 anonymous memory mappings with context about their purpose. These 72 annotations appear in /proc/self/maps and /proc/self/smaps as 73 "[anon: Go: ...]". This setting is only used on Linux. For Go 1.25, it 74 defaults to `decoratemappings=1`, enabling annotations. Using 75 `decoratemappings=0` reverts to the pre-Go 1.25 behavior. 76 77 disablethp: setting disablethp=1 on Linux disables transparent huge pages for the heap. 78 It has no effect on other platforms. disablethp is meant for compatibility with versions 79 of Go before 1.21, which stopped working around a Linux kernel default that can result 80 in significant memory overuse. See https://go.dev/issue/64332. This setting will be 81 removed in a future release, so operators should tweak their Linux configuration to suit 82 their needs before then. See https://go.dev/doc/gc-guide#Linux_transparent_huge_pages. 83 84 dontfreezetheworld: by default, the start of a fatal panic or throw 85 "freezes the world", preempting all threads to stop all running 86 goroutines, which makes it possible to traceback all goroutines, and 87 keeps their state close to the point of panic. Setting 88 dontfreezetheworld=1 disables this preemption, allowing goroutines to 89 continue executing during panic processing. Note that goroutines that 90 naturally enter the scheduler will still stop. This can be useful when 91 debugging the runtime scheduler, as freezetheworld perturbs scheduler 92 state and thus may hide problems. 93 94 efence: setting efence=1 causes the allocator to run in a mode 95 where each object is allocated on a unique page and addresses are 96 never recycled. 97 98 gccheckmark: setting gccheckmark=1 enables verification of the 99 garbage collector's concurrent mark phase by performing a 100 second mark pass while the world is stopped. If the second 101 pass finds a reachable object that was not found by concurrent 102 mark, the garbage collector will panic. 103 104 gcpacertrace: setting gcpacertrace=1 causes the garbage collector to 105 print information about the internal state of the concurrent pacer. 106 107 gcshrinkstackoff: setting gcshrinkstackoff=1 disables moving goroutines 108 onto smaller stacks. In this mode, a goroutine's stack can only grow. 109 110 gcstoptheworld: setting gcstoptheworld=1 disables concurrent garbage collection, 111 making every garbage collection a stop-the-world event. Setting gcstoptheworld=2 112 also disables concurrent sweeping after the garbage collection finishes. 113 114 gctrace: setting gctrace=1 causes the garbage collector to emit a single line to standard 115 error at each collection, summarizing the amount of memory collected and the 116 length of the pause. The format of this line is subject to change. Included in 117 the explanation below is also the relevant runtime/metrics metric for each field. 118 Currently, it is: 119 gc # @#s #%: #+#+# ms clock, #+#/#/#+# ms cpu, #->#-># MB, # MB goal, # MB stacks, #MB globals, # P 120 where the fields are as follows: 121 gc # the GC number, incremented at each GC 122 @#s time in seconds since program start 123 #% percentage of time spent in GC since program start 124 #+...+# wall-clock/CPU times for the phases of the GC 125 #->#-># MB heap size at GC start, at GC end, and live heap, or /gc/scan/heap:bytes 126 # MB goal goal heap size, or /gc/heap/goal:bytes 127 # MB stacks estimated scannable stack size, or /gc/scan/stack:bytes 128 # MB globals scannable global size, or /gc/scan/globals:bytes 129 # P number of processors used, or /sched/gomaxprocs:threads 130 The phases are stop-the-world (STW) sweep termination, concurrent 131 mark and scan, and STW mark termination. The CPU times 132 for mark/scan are broken down in to assist time (GC performed in 133 line with allocation), background GC time, and idle GC time. 134 If the line ends with "(forced)", this GC was forced by a 135 runtime.GC() call. 136 137 harddecommit: setting harddecommit=1 causes memory that is returned to the OS to 138 also have protections removed on it. This is the only mode of operation on Windows, 139 but is helpful in debugging scavenger-related issues on other platforms. Currently, 140 only supported on Linux. 141 142 inittrace: setting inittrace=1 causes the runtime to emit a single line to standard 143 error for each package with init work, summarizing the execution time and memory 144 allocation. No information is printed for inits executed as part of plugin loading 145 and for packages without both user defined and compiler generated init work. 146 The format of this line is subject to change. Currently, it is: 147 init # @#ms, # ms clock, # bytes, # allocs 148 where the fields are as follows: 149 init # the package name 150 @# ms time in milliseconds when the init started since program start 151 # clock wall-clock time for package initialization work 152 # bytes memory allocated on the heap 153 # allocs number of heap allocations 154 155 madvdontneed: setting madvdontneed=0 will use MADV_FREE 156 instead of MADV_DONTNEED on Linux when returning memory to the 157 kernel. This is more efficient, but means RSS numbers will 158 drop only when the OS is under memory pressure. On the BSDs and 159 Illumos/Solaris, setting madvdontneed=1 will use MADV_DONTNEED instead 160 of MADV_FREE. This is less efficient, but causes RSS numbers to drop 161 more quickly. 162 163 memprofilerate: setting memprofilerate=X will update the value of runtime.MemProfileRate. 164 When set to 0 memory profiling is disabled. Refer to the description of 165 MemProfileRate for the default value. 166 167 profstackdepth: profstackdepth=128 (the default) will set the maximum stack 168 depth used by all pprof profilers except for the CPU profiler to 128 frames. 169 Stack traces that exceed this limit will be truncated to the limit starting 170 from the leaf frame. Setting profstackdepth to any value above 1024 will 171 silently default to 1024. Future versions of Go may remove this limitation 172 and extend profstackdepth to apply to the CPU profiler and execution tracer. 173 174 panicnil: setting panicnil=1 disables the runtime error when calling panic with nil 175 interface value or an untyped nil. 176 177 invalidptr: invalidptr=1 (the default) causes the garbage collector and stack 178 copier to crash the program if an invalid pointer value (for example, 1) 179 is found in a pointer-typed location. Setting invalidptr=0 disables this check. 180 This should only be used as a temporary workaround to diagnose buggy code. 181 The real fix is to not store integers in pointer-typed locations. 182 183 sbrk: setting sbrk=1 replaces the memory allocator and garbage collector 184 with a trivial allocator that obtains memory from the operating system and 185 never reclaims any memory. 186 187 scavtrace: setting scavtrace=1 causes the runtime to emit a single line to standard 188 error, roughly once per GC cycle, summarizing the amount of work done by the 189 scavenger as well as the total amount of memory returned to the operating system 190 and an estimate of physical memory utilization. The format of this line is subject 191 to change, but currently it is: 192 scav # KiB work (bg), # KiB work (eager), # KiB total, #% util 193 where the fields are as follows: 194 # KiB work (bg) the amount of memory returned to the OS in the background since 195 the last line 196 # KiB work (eager) the amount of memory returned to the OS eagerly since the last line 197 # KiB now the amount of address space currently returned to the OS 198 #% util the fraction of all unscavenged heap memory which is in-use 199 If the line ends with "(forced)", then scavenging was forced by a 200 debug.FreeOSMemory() call. 201 202 scheddetail: setting schedtrace=X and scheddetail=1 causes the scheduler to emit 203 detailed multiline info every X milliseconds, describing state of the scheduler, 204 processors, threads and goroutines. 205 206 schedtrace: setting schedtrace=X causes the scheduler to emit a single line to standard 207 error every X milliseconds, summarizing the scheduler state. 208 209 tracebackancestors: setting tracebackancestors=N extends tracebacks with the stacks at 210 which goroutines were created, where N limits the number of ancestor goroutines to 211 report. This also extends the information returned by runtime.Stack. 212 Setting N to 0 will report no ancestry information. 213 214 tracefpunwindoff: setting tracefpunwindoff=1 forces the execution tracer 215 and block and mutex profilers to use the runtime's default stack 216 unwinder instead of frame pointer unwinding. This increases their 217 overhead, but could be helpful as a workaround or for debugging 218 unexpected regressions caused by frame pointer unwinding. 219 220 traceadvanceperiod: the approximate period in nanoseconds between trace generations. Only 221 applies if a program is built with GOEXPERIMENT=exectracer2. Used primarily for testing 222 and debugging the execution tracer. 223 224 tracecheckstackownership: setting tracecheckstackownership=1 enables a debug check in the 225 execution tracer to double-check stack ownership before taking a stack trace. 226 227 asyncpreemptoff: asyncpreemptoff=1 disables signal-based 228 asynchronous goroutine preemption. This makes some loops 229 non-preemptible for long periods, which may delay GC and 230 goroutine scheduling. This is useful for debugging GC issues 231 because it also disables the conservative stack scanning used 232 for asynchronously preempted goroutines. 233 234 The [net] and [net/http] packages also refer to debugging variables in GODEBUG. 235 See the documentation for those packages for details. 236 237 The GOMAXPROCS variable limits the number of operating system threads that 238 can execute user-level Go code simultaneously. There is no limit to the number of threads 239 that can be blocked in system calls on behalf of Go code; those do not count against 240 the GOMAXPROCS limit. This package's [GOMAXPROCS] function queries and changes 241 the limit. 242 243 The GORACE variable configures the race detector, for programs built using -race. 244 See the [Race Detector article] for details. 245 246 The GOTRACEBACK variable controls the amount of output generated when a Go 247 program fails due to an unrecovered panic or an unexpected runtime condition. 248 By default, a failure prints a stack trace for the current goroutine, 249 eliding functions internal to the run-time system, and then exits with exit code 2. 250 The failure prints stack traces for all goroutines if there is no current goroutine 251 or the failure is internal to the run-time. 252 GOTRACEBACK=none omits the goroutine stack traces entirely. 253 GOTRACEBACK=single (the default) behaves as described above. 254 GOTRACEBACK=all adds stack traces for all user-created goroutines. 255 GOTRACEBACK=system is like “all” but adds stack frames for run-time functions 256 and shows goroutines created internally by the run-time. 257 GOTRACEBACK=crash is like “system” but crashes in an operating system-specific 258 manner instead of exiting. For example, on Unix systems, the crash raises 259 SIGABRT to trigger a core dump. 260 GOTRACEBACK=wer is like “crash” but doesn't disable Windows Error Reporting (WER). 261 For historical reasons, the GOTRACEBACK settings 0, 1, and 2 are synonyms for 262 none, all, and system, respectively. 263 The [runtime/debug.SetTraceback] function allows increasing the 264 amount of output at run time, but it cannot reduce the amount below that 265 specified by the environment variable. 266 267 The GOARCH, GOOS, GOPATH, and GOROOT environment variables complete 268 the set of Go environment variables. They influence the building of Go programs 269 (see [cmd/go] and [go/build]). 270 GOARCH, GOOS, and GOROOT are recorded at compile time and made available by 271 constants or functions in this package, but they do not influence the execution 272 of the run-time system. 273 274 # Security 275 276 On Unix platforms, Go's runtime system behaves slightly differently when a 277 binary is setuid/setgid or executed with setuid/setgid-like properties, in order 278 to prevent dangerous behaviors. On Linux this is determined by checking for the 279 AT_SECURE flag in the auxiliary vector, on the BSDs and Solaris/Illumos it is 280 determined by checking the issetugid syscall, and on AIX it is determined by 281 checking if the uid/gid match the effective uid/gid. 282 283 When the runtime determines the binary is setuid/setgid-like, it does three main 284 things: 285 - The standard input/output file descriptors (0, 1, 2) are checked to be open. 286 If any of them are closed, they are opened pointing at /dev/null. 287 - The value of the GOTRACEBACK environment variable is set to 'none'. 288 - When a signal is received that terminates the program, or the program 289 encounters an unrecoverable panic that would otherwise override the value 290 of GOTRACEBACK, the goroutine stack, registers, and other memory related 291 information are omitted. 292 293 [Race Detector article]: https://go.dev/doc/articles/race_detector 294 */ 295 package runtime 296 297 import ( 298 "internal/goarch" 299 "internal/goos" 300 ) 301 302 // Caller reports file and line number information about function invocations on 303 // the calling goroutine's stack. The argument skip is the number of stack frames 304 // to ascend, with 0 identifying the caller of Caller. (For historical reasons the 305 // meaning of skip differs between Caller and [Callers].) The return values report 306 // the program counter, the file name (using forward slashes as path separator, even 307 // on Windows), and the line number within the file of the corresponding call. 308 // The boolean ok is false if it was not possible to recover the information. 309 func Caller(skip int) (pc uintptr, file string, line int, ok bool) { 310 rpc := make([]uintptr, 1) 311 n := callers(skip+1, rpc) 312 if n < 1 { 313 return 314 } 315 frame, _ := CallersFrames(rpc).Next() 316 return frame.PC, frame.File, frame.Line, frame.PC != 0 317 } 318 319 // Callers fills the slice pc with the return program counters of function invocations 320 // on the calling goroutine's stack. The argument skip is the number of stack frames 321 // to skip before recording in pc, with 0 identifying the frame for Callers itself and 322 // 1 identifying the caller of Callers. 323 // It returns the number of entries written to pc. 324 // 325 // To translate these PCs into symbolic information such as function 326 // names and line numbers, use [CallersFrames]. CallersFrames accounts 327 // for inlined functions and adjusts the return program counters into 328 // call program counters. Iterating over the returned slice of PCs 329 // directly is discouraged, as is using [FuncForPC] on any of the 330 // returned PCs, since these cannot account for inlining or return 331 // program counter adjustment. 332 func Callers(skip int, pc []uintptr) int { 333 // runtime.callers uses pc.array==nil as a signal 334 // to print a stack trace. Pick off 0-length pc here 335 // so that we don't let a nil pc slice get to it. 336 if len(pc) == 0 { 337 return 0 338 } 339 return callers(skip, pc) 340 } 341 342 var defaultGOROOT string // set by cmd/link 343 344 // GOROOT returns the root of the Go tree. It uses the 345 // GOROOT environment variable, if set at process start, 346 // or else the root used during the Go build. 347 // 348 // Deprecated: The root used during the Go build will not be 349 // meaningful if the binary is copied to another machine. 350 // Use the system path to locate the “go” binary, and use 351 // “go env GOROOT” to find its GOROOT. 352 func GOROOT() string { 353 s := gogetenv("GOROOT") 354 if s != "" { 355 return s 356 } 357 return defaultGOROOT 358 } 359 360 // buildVersion is the Go tree's version string at build time. 361 // 362 // If any GOEXPERIMENTs are set to non-default values, it will include 363 // "X:<GOEXPERIMENT>". 364 // 365 // This is set by the linker. 366 // 367 // This is accessed by "go version <binary>". 368 var buildVersion string 369 370 // Version returns the Go tree's version string. 371 // It is either the commit hash and date at the time of the build or, 372 // when possible, a release tag like "go1.3". 373 func Version() string { 374 return buildVersion 375 } 376 377 // GOOS is the running program's operating system target: 378 // one of darwin, freebsd, linux, and so on. 379 // To view possible combinations of GOOS and GOARCH, run "go tool dist list". 380 const GOOS string = goos.GOOS 381 382 // GOARCH is the running program's architecture target: 383 // one of 386, amd64, arm, s390x, and so on. 384 const GOARCH string = goarch.GOARCH 385