Source: extern.go in package runtime

/* Package runtime contains operations that interact with Go's runtime system, such as functions to control goroutines. It also includes the low-level type information used by the reflect package; see [reflect]'s documentation for the programmable interface to the run-time type system. # Environment Variables The following environment variables ($name or %name%, depending on the host operating system) control the run-time behavior of Go programs. The meanings and use may change from release to release. The GOGC variable sets the initial garbage collection target percentage. A collection is triggered when the ratio of freshly allocated data to live data remaining after the previous collection reaches this percentage. The default is GOGC=100. Setting GOGC=off disables the garbage collector entirely. [runtime/debug.SetGCPercent] allows changing this percentage at run time. The GOMEMLIMIT variable sets a soft memory limit for the runtime. This memory limit includes the Go heap and all other memory managed by the runtime, and excludes external memory sources such as mappings of the binary itself, memory managed in other languages, and memory held by the operating system on behalf of the Go program. GOMEMLIMIT is a numeric value in bytes with an optional unit suffix. The supported suffixes include B, KiB, MiB, GiB, and TiB. These suffixes represent quantities of bytes as defined by the IEC 80000-13 standard. That is, they are based on powers of two: KiB means 2^10 bytes, MiB means 2^20 bytes, and so on. The default setting is [math.MaxInt64], which effectively disables the memory limit. [runtime/debug.SetMemoryLimit] allows changing this limit at run time. The GODEBUG variable controls debugging variables within the runtime. It is a comma-separated list of name=val pairs setting these named variables: clobberfree: setting clobberfree=1 causes the garbage collector to clobber the memory content of an object with bad content when it frees the object. cpu.*: cpu.all=off disables the use of all optional instruction set extensions. cpu.extension=off disables use of instructions from the specified instruction set extension. extension is the lower case name for the instruction set extension such as sse41 or avx as listed in internal/cpu package. As an example cpu.avx=off disables runtime detection and thereby use of AVX instructions. cgocheck: setting cgocheck=0 disables all checks for packages using cgo to incorrectly pass Go pointers to non-Go code. Setting cgocheck=1 (the default) enables relatively cheap checks that may miss some errors. A more complete, but slow, cgocheck mode can be enabled using GOEXPERIMENT (which requires a rebuild), see https://pkg.go.dev/internal/goexperiment for details. checkfinalizers: setting checkfinalizers=1 causes the garbage collector to run multiple partial non-parallel stop-the-world collections to identify common issues with finalizers and cleanups, like those listed at https://go.dev/doc/gc-guide#Finalizers_cleanups_and_weak_pointers. If a potential issue is found, the program will terminate with a description of all potential issues, the associated values, and a list of those values' finalizers and cleanups, including where they were created. It also adds tracking for tiny blocks to help diagnose issues with those as well. The analysis performed during the partial collection is conservative. Notably, it flags any path back to the original object from the cleanup function, cleanup arguments, or finalizer function as a potential issue, even if that path might be severed sometime later during execution (though this is not a recommended pattern). This mode also produces one line of output to stderr every GC cycle with information about the finalizer and cleanup queue lengths. Lines produced by this mode start with "checkfinalizers:". decoratemappings: controls whether the Go runtime annotates OS anonymous memory mappings with context about their purpose. These annotations appear in /proc/self/maps and /proc/self/smaps as "[anon: Go: ...]". This setting is only used on Linux. For Go 1.25, it defaults to `decoratemappings=1`, enabling annotations. Using `decoratemappings=0` reverts to the pre-Go 1.25 behavior. disablethp: setting disablethp=1 on Linux disables transparent huge pages for the heap. It has no effect on other platforms. disablethp is meant for compatibility with versions of Go before 1.21, which stopped working around a Linux kernel default that can result in significant memory overuse. See https://go.dev/issue/64332. This setting will be removed in a future release, so operators should tweak their Linux configuration to suit their needs before then. See https://go.dev/doc/gc-guide#Linux_transparent_huge_pages. dontfreezetheworld: by default, the start of a fatal panic or throw "freezes the world", preempting all threads to stop all running goroutines, which makes it possible to traceback all goroutines, and keeps their state close to the point of panic. Setting dontfreezetheworld=1 disables this preemption, allowing goroutines to continue executing during panic processing. Note that goroutines that naturally enter the scheduler will still stop. This can be useful when debugging the runtime scheduler, as freezetheworld perturbs scheduler state and thus may hide problems. efence: setting efence=1 causes the allocator to run in a mode where each object is allocated on a unique page and addresses are never recycled. gccheckmark: setting gccheckmark=1 enables verification of the garbage collector's concurrent mark phase by performing a second mark pass while the world is stopped. If the second pass finds a reachable object that was not found by concurrent mark, the garbage collector will panic. gcpacertrace: setting gcpacertrace=1 causes the garbage collector to print information about the internal state of the concurrent pacer. gcshrinkstackoff: setting gcshrinkstackoff=1 disables moving goroutines onto smaller stacks. In this mode, a goroutine's stack can only grow. gcstoptheworld: setting gcstoptheworld=1 disables concurrent garbage collection, making every garbage collection a stop-the-world event. Setting gcstoptheworld=2 also disables concurrent sweeping after the garbage collection finishes. gctrace: setting gctrace=1 causes the garbage collector to emit a single line to standard error at each collection, summarizing the amount of memory collected and the length of the pause. The format of this line is subject to change. Included in the explanation below is also the relevant runtime/metrics metric for each field. Currently, it is: gc # @#s #%: #+#+# ms clock, #+#/#/#+# ms cpu, #->#-># MB, # MB goal, # MB stacks, #MB globals, # P where the fields are as follows: gc # the GC number, incremented at each GC @#s time in seconds since program start #% percentage of time spent in GC since program start #+...+# wall-clock/CPU times for the phases of the GC #->#-># MB heap size at GC start, at GC end, and live heap, or /gc/scan/heap:bytes # MB goal goal heap size, or /gc/heap/goal:bytes # MB stacks estimated scannable stack size, or /gc/scan/stack:bytes # MB globals scannable global size, or /gc/scan/globals:bytes # P number of processors used, or /sched/gomaxprocs:threads The phases are stop-the-world (STW) sweep termination, concurrent mark and scan, and STW mark termination. The CPU times for mark/scan are broken down in to assist time (GC performed in line with allocation), background GC time, and idle GC time. If the line ends with "(forced)", this GC was forced by a runtime.GC() call. harddecommit: setting harddecommit=1 causes memory that is returned to the OS to also have protections removed on it. This is the only mode of operation on Windows, but is helpful in debugging scavenger-related issues on other platforms. Currently, only supported on Linux. inittrace: setting inittrace=1 causes the runtime to emit a single line to standard error for each package with init work, summarizing the execution time and memory allocation. No information is printed for inits executed as part of plugin loading and for packages without both user defined and compiler generated init work. The format of this line is subject to change. Currently, it is: init # @#ms, # ms clock, # bytes, # allocs where the fields are as follows: init # the package name @# ms time in milliseconds when the init started since program start # clock wall-clock time for package initialization work # bytes memory allocated on the heap # allocs number of heap allocations madvdontneed: setting madvdontneed=0 will use MADV_FREE instead of MADV_DONTNEED on Linux when returning memory to the kernel. This is more efficient, but means RSS numbers will drop only when the OS is under memory pressure. On the BSDs and Illumos/Solaris, setting madvdontneed=1 will use MADV_DONTNEED instead of MADV_FREE. This is less efficient, but causes RSS numbers to drop more quickly. memprofilerate: setting memprofilerate=X will update the value of runtime.MemProfileRate. When set to 0 memory profiling is disabled. Refer to the description of MemProfileRate for the default value. profstackdepth: profstackdepth=128 (the default) will set the maximum stack depth used by all pprof profilers except for the CPU profiler to 128 frames. Stack traces that exceed this limit will be truncated to the limit starting from the leaf frame. Setting profstackdepth to any value above 1024 will silently default to 1024. Future versions of Go may remove this limitation and extend profstackdepth to apply to the CPU profiler and execution tracer. pagetrace: setting pagetrace=/path/to/file will write out a trace of page events that can be viewed, analyzed, and visualized using the x/debug/cmd/pagetrace tool. Build your program with GOEXPERIMENT=pagetrace to enable this functionality. Do not enable this functionality if your program is a setuid binary as it introduces a security risk in that scenario. Currently not supported on Windows, plan9 or js/wasm. Setting this option for some applications can produce large traces, so use with care. panicnil: setting panicnil=1 disables the runtime error when calling panic with nil interface value or an untyped nil. invalidptr: invalidptr=1 (the default) causes the garbage collector and stack copier to crash the program if an invalid pointer value (for example, 1) is found in a pointer-typed location. Setting invalidptr=0 disables this check. This should only be used as a temporary workaround to diagnose buggy code. The real fix is to not store integers in pointer-typed locations. sbrk: setting sbrk=1 replaces the memory allocator and garbage collector with a trivial allocator that obtains memory from the operating system and never reclaims any memory. scavtrace: setting scavtrace=1 causes the runtime to emit a single line to standard error, roughly once per GC cycle, summarizing the amount of work done by the scavenger as well as the total amount of memory returned to the operating system and an estimate of physical memory utilization. The format of this line is subject to change, but currently it is: scav # KiB work (bg), # KiB work (eager), # KiB total, #% util where the fields are as follows: # KiB work (bg) the amount of memory returned to the OS in the background since the last line # KiB work (eager) the amount of memory returned to the OS eagerly since the last line # KiB now the amount of address space currently returned to the OS #% util the fraction of all unscavenged heap memory which is in-use If the line ends with "(forced)", then scavenging was forced by a debug.FreeOSMemory() call. scheddetail: setting schedtrace=X and scheddetail=1 causes the scheduler to emit detailed multiline info every X milliseconds, describing state of the scheduler, processors, threads and goroutines. schedtrace: setting schedtrace=X causes the scheduler to emit a single line to standard error every X milliseconds, summarizing the scheduler state. tracebackancestors: setting tracebackancestors=N extends tracebacks with the stacks at which goroutines were created, where N limits the number of ancestor goroutines to report. This also extends the information returned by runtime.Stack. Setting N to 0 will report no ancestry information. tracefpunwindoff: setting tracefpunwindoff=1 forces the execution tracer to use the runtime's default stack unwinder instead of frame pointer unwinding. This increases tracer overhead, but could be helpful as a workaround or for debugging unexpected regressions caused by frame pointer unwinding. traceadvanceperiod: the approximate period in nanoseconds between trace generations. Only applies if a program is built with GOEXPERIMENT=exectracer2. Used primarily for testing and debugging the execution tracer. tracecheckstackownership: setting tracecheckstackownership=1 enables a debug check in the execution tracer to double-check stack ownership before taking a stack trace. asyncpreemptoff: asyncpreemptoff=1 disables signal-based asynchronous goroutine preemption. This makes some loops non-preemptible for long periods, which may delay GC and goroutine scheduling. This is useful for debugging GC issues because it also disables the conservative stack scanning used for asynchronously preempted goroutines. The [net] and [net/http] packages also refer to debugging variables in GODEBUG. See the documentation for those packages for details. The GOMAXPROCS variable limits the number of operating system threads that can execute user-level Go code simultaneously. There is no limit to the number of threads that can be blocked in system calls on behalf of Go code; those do not count against the GOMAXPROCS limit. This package's [GOMAXPROCS] function queries and changes the limit. The GORACE variable configures the race detector, for programs built using -race. See the [Race Detector article] for details. The GOTRACEBACK variable controls the amount of output generated when a Go program fails due to an unrecovered panic or an unexpected runtime condition. By default, a failure prints a stack trace for the current goroutine, eliding functions internal to the run-time system, and then exits with exit code 2. The failure prints stack traces for all goroutines if there is no current goroutine or the failure is internal to the run-time. GOTRACEBACK=none omits the goroutine stack traces entirely. GOTRACEBACK=single (the default) behaves as described above. GOTRACEBACK=all adds stack traces for all user-created goroutines. GOTRACEBACK=system is like “all” but adds stack frames for run-time functions and shows goroutines created internally by the run-time. GOTRACEBACK=crash is like “system” but crashes in an operating system-specific manner instead of exiting. For example, on Unix systems, the crash raises SIGABRT to trigger a core dump. GOTRACEBACK=wer is like “crash” but doesn't disable Windows Error Reporting (WER). For historical reasons, the GOTRACEBACK settings 0, 1, and 2 are synonyms for none, all, and system, respectively. The [runtime/debug.SetTraceback] function allows increasing the amount of output at run time, but it cannot reduce the amount below that specified by the environment variable. The GOARCH, GOOS, GOPATH, and GOROOT environment variables complete the set of Go environment variables. They influence the building of Go programs (see [cmd/go] and [go/build]). GOARCH, GOOS, and GOROOT are recorded at compile time and made available by constants or functions in this package, but they do not influence the execution of the run-time system. # Security On Unix platforms, Go's runtime system behaves slightly differently when a binary is setuid/setgid or executed with setuid/setgid-like properties, in order to prevent dangerous behaviors. On Linux this is determined by checking for the AT_SECURE flag in the auxiliary vector, on the BSDs and Solaris/Illumos it is determined by checking the issetugid syscall, and on AIX it is determined by checking if the uid/gid match the effective uid/gid. When the runtime determines the binary is setuid/setgid-like, it does three main things: - The standard input/output file descriptors (0, 1, 2) are checked to be open. If any of them are closed, they are opened pointing at /dev/null. - The value of the GOTRACEBACK environment variable is set to 'none'. - When a signal is received that terminates the program, or the program encounters an unrecoverable panic that would otherwise override the value of GOTRACEBACK, the goroutine stack, registers, and other memory related information are omitted. [Race Detector article]: https://go.dev/doc/articles/race_detector */