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 pagetrace: setting pagetrace=/path/to/file will write out a trace of page events 175 that can be viewed, analyzed, and visualized using the x/debug/cmd/pagetrace tool. 176 Build your program with GOEXPERIMENT=pagetrace to enable this functionality. Do not 177 enable this functionality if your program is a setuid binary as it introduces a security 178 risk in that scenario. Currently not supported on Windows, plan9 or js/wasm. Setting this 179 option for some applications can produce large traces, so use with care. 180 181 panicnil: setting panicnil=1 disables the runtime error when calling panic with nil 182 interface value or an untyped nil. 183 184 invalidptr: invalidptr=1 (the default) causes the garbage collector and stack 185 copier to crash the program if an invalid pointer value (for example, 1) 186 is found in a pointer-typed location. Setting invalidptr=0 disables this check. 187 This should only be used as a temporary workaround to diagnose buggy code. 188 The real fix is to not store integers in pointer-typed locations. 189 190 sbrk: setting sbrk=1 replaces the memory allocator and garbage collector 191 with a trivial allocator that obtains memory from the operating system and 192 never reclaims any memory. 193 194 scavtrace: setting scavtrace=1 causes the runtime to emit a single line to standard 195 error, roughly once per GC cycle, summarizing the amount of work done by the 196 scavenger as well as the total amount of memory returned to the operating system 197 and an estimate of physical memory utilization. The format of this line is subject 198 to change, but currently it is: 199 scav # KiB work (bg), # KiB work (eager), # KiB total, #% util 200 where the fields are as follows: 201 # KiB work (bg) the amount of memory returned to the OS in the background since 202 the last line 203 # KiB work (eager) the amount of memory returned to the OS eagerly since the last line 204 # KiB now the amount of address space currently returned to the OS 205 #% util the fraction of all unscavenged heap memory which is in-use 206 If the line ends with "(forced)", then scavenging was forced by a 207 debug.FreeOSMemory() call. 208 209 scheddetail: setting schedtrace=X and scheddetail=1 causes the scheduler to emit 210 detailed multiline info every X milliseconds, describing state of the scheduler, 211 processors, threads and goroutines. 212 213 schedtrace: setting schedtrace=X causes the scheduler to emit a single line to standard 214 error every X milliseconds, summarizing the scheduler state. 215 216 tracebackancestors: setting tracebackancestors=N extends tracebacks with the stacks at 217 which goroutines were created, where N limits the number of ancestor goroutines to 218 report. This also extends the information returned by runtime.Stack. 219 Setting N to 0 will report no ancestry information. 220 221 tracefpunwindoff: setting tracefpunwindoff=1 forces the execution tracer to 222 use the runtime's default stack unwinder instead of frame pointer unwinding. 223 This increases tracer overhead, but could be helpful as a workaround or for 224 debugging unexpected regressions caused by frame pointer unwinding. 225 226 traceadvanceperiod: the approximate period in nanoseconds between trace generations. Only 227 applies if a program is built with GOEXPERIMENT=exectracer2. Used primarily for testing 228 and debugging the execution tracer. 229 230 tracecheckstackownership: setting tracecheckstackownership=1 enables a debug check in the 231 execution tracer to double-check stack ownership before taking a stack trace. 232 233 asyncpreemptoff: asyncpreemptoff=1 disables signal-based 234 asynchronous goroutine preemption. This makes some loops 235 non-preemptible for long periods, which may delay GC and 236 goroutine scheduling. This is useful for debugging GC issues 237 because it also disables the conservative stack scanning used 238 for asynchronously preempted goroutines. 239 240 The [net] and [net/http] packages also refer to debugging variables in GODEBUG. 241 See the documentation for those packages for details. 242 243 The GOMAXPROCS variable limits the number of operating system threads that 244 can execute user-level Go code simultaneously. There is no limit to the number of threads 245 that can be blocked in system calls on behalf of Go code; those do not count against 246 the GOMAXPROCS limit. This package's [GOMAXPROCS] function queries and changes 247 the limit. 248 249 The GORACE variable configures the race detector, for programs built using -race. 250 See the [Race Detector article] for details. 251 252 The GOTRACEBACK variable controls the amount of output generated when a Go 253 program fails due to an unrecovered panic or an unexpected runtime condition. 254 By default, a failure prints a stack trace for the current goroutine, 255 eliding functions internal to the run-time system, and then exits with exit code 2. 256 The failure prints stack traces for all goroutines if there is no current goroutine 257 or the failure is internal to the run-time. 258 GOTRACEBACK=none omits the goroutine stack traces entirely. 259 GOTRACEBACK=single (the default) behaves as described above. 260 GOTRACEBACK=all adds stack traces for all user-created goroutines. 261 GOTRACEBACK=system is like “all” but adds stack frames for run-time functions 262 and shows goroutines created internally by the run-time. 263 GOTRACEBACK=crash is like “system” but crashes in an operating system-specific 264 manner instead of exiting. For example, on Unix systems, the crash raises 265 SIGABRT to trigger a core dump. 266 GOTRACEBACK=wer is like “crash” but doesn't disable Windows Error Reporting (WER). 267 For historical reasons, the GOTRACEBACK settings 0, 1, and 2 are synonyms for 268 none, all, and system, respectively. 269 The [runtime/debug.SetTraceback] function allows increasing the 270 amount of output at run time, but it cannot reduce the amount below that 271 specified by the environment variable. 272 273 The GOARCH, GOOS, GOPATH, and GOROOT environment variables complete 274 the set of Go environment variables. They influence the building of Go programs 275 (see [cmd/go] and [go/build]). 276 GOARCH, GOOS, and GOROOT are recorded at compile time and made available by 277 constants or functions in this package, but they do not influence the execution 278 of the run-time system. 279 280 # Security 281 282 On Unix platforms, Go's runtime system behaves slightly differently when a 283 binary is setuid/setgid or executed with setuid/setgid-like properties, in order 284 to prevent dangerous behaviors. On Linux this is determined by checking for the 285 AT_SECURE flag in the auxiliary vector, on the BSDs and Solaris/Illumos it is 286 determined by checking the issetugid syscall, and on AIX it is determined by 287 checking if the uid/gid match the effective uid/gid. 288 289 When the runtime determines the binary is setuid/setgid-like, it does three main 290 things: 291 - The standard input/output file descriptors (0, 1, 2) are checked to be open. 292 If any of them are closed, they are opened pointing at /dev/null. 293 - The value of the GOTRACEBACK environment variable is set to 'none'. 294 - When a signal is received that terminates the program, or the program 295 encounters an unrecoverable panic that would otherwise override the value 296 of GOTRACEBACK, the goroutine stack, registers, and other memory related 297 information are omitted. 298 299 [Race Detector article]: https://go.dev/doc/articles/race_detector 300 */ 301 package runtime 302 303 import ( 304 "internal/goarch" 305 "internal/goos" 306 ) 307 308 // Caller reports file and line number information about function invocations on 309 // the calling goroutine's stack. The argument skip is the number of stack frames 310 // to ascend, with 0 identifying the caller of Caller. (For historical reasons the 311 // meaning of skip differs between Caller and [Callers].) The return values report 312 // the program counter, the file name (using forward slashes as path separator, even 313 // on Windows), and the line number within the file of the corresponding call. 314 // The boolean ok is false if it was not possible to recover the information. 315 func Caller(skip int) (pc uintptr, file string, line int, ok bool) { 316 rpc := make([]uintptr, 1) 317 n := callers(skip+1, rpc) 318 if n < 1 { 319 return 320 } 321 frame, _ := CallersFrames(rpc).Next() 322 return frame.PC, frame.File, frame.Line, frame.PC != 0 323 } 324 325 // Callers fills the slice pc with the return program counters of function invocations 326 // on the calling goroutine's stack. The argument skip is the number of stack frames 327 // to skip before recording in pc, with 0 identifying the frame for Callers itself and 328 // 1 identifying the caller of Callers. 329 // It returns the number of entries written to pc. 330 // 331 // To translate these PCs into symbolic information such as function 332 // names and line numbers, use [CallersFrames]. CallersFrames accounts 333 // for inlined functions and adjusts the return program counters into 334 // call program counters. Iterating over the returned slice of PCs 335 // directly is discouraged, as is using [FuncForPC] on any of the 336 // returned PCs, since these cannot account for inlining or return 337 // program counter adjustment. 338 func Callers(skip int, pc []uintptr) int { 339 // runtime.callers uses pc.array==nil as a signal 340 // to print a stack trace. Pick off 0-length pc here 341 // so that we don't let a nil pc slice get to it. 342 if len(pc) == 0 { 343 return 0 344 } 345 return callers(skip, pc) 346 } 347 348 var defaultGOROOT string // set by cmd/link 349 350 // GOROOT returns the root of the Go tree. It uses the 351 // GOROOT environment variable, if set at process start, 352 // or else the root used during the Go build. 353 // 354 // Deprecated: The root used during the Go build will not be 355 // meaningful if the binary is copied to another machine. 356 // Use the system path to locate the “go” binary, and use 357 // “go env GOROOT” to find its GOROOT. 358 func GOROOT() string { 359 s := gogetenv("GOROOT") 360 if s != "" { 361 return s 362 } 363 return defaultGOROOT 364 } 365 366 // buildVersion is the Go tree's version string at build time. 367 // 368 // If any GOEXPERIMENTs are set to non-default values, it will include 369 // "X:<GOEXPERIMENT>". 370 // 371 // This is set by the linker. 372 // 373 // This is accessed by "go version <binary>". 374 var buildVersion string 375 376 // Version returns the Go tree's version string. 377 // It is either the commit hash and date at the time of the build or, 378 // when possible, a release tag like "go1.3". 379 func Version() string { 380 return buildVersion 381 } 382 383 // GOOS is the running program's operating system target: 384 // one of darwin, freebsd, linux, and so on. 385 // To view possible combinations of GOOS and GOARCH, run "go tool dist list". 386 const GOOS string = goos.GOOS 387 388 // GOARCH is the running program's architecture target: 389 // one of 386, amd64, arm, s390x, and so on. 390 const GOARCH string = goarch.GOARCH 391