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// Copyright 2016 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
package runtime
import (
// For historical reasons these functions are called as though they
// were in the syscall package.
//go:linkname Cgocall syscall.Cgocall
//go:linkname CgocallDone syscall.CgocallDone
//go:linkname CgocallBack syscall.CgocallBack
//go:linkname CgocallBackDone syscall.CgocallBackDone
// A routine that may be called by SWIG.
//go:linkname _cgo_panic _cgo_panic
// iscgo is set to true if the cgo tool sets the C variable runtime_iscgo
// to true.
var iscgo bool
// cgoHasExtraM is set on startup when an extra M is created for cgo.
// The extra M must be created before any C/C++ code calls cgocallback.
var cgoHasExtraM bool
// cgoAlwaysFalse is a boolean value that is always false.
// The cgo-generated code says if cgoAlwaysFalse { cgoUse(p) }.
// The compiler cannot see that cgoAlwaysFalse is always false,
// so it emits the test and keeps the call, giving the desired
// escape analysis result. The test is cheaper than the call.
var cgoAlwaysFalse bool
// Cgocall prepares to call from code written in Go to code written in
// C/C++. This takes the current goroutine out of the Go scheduler, as
// though it were making a system call. Otherwise the program can
// lookup if the C code blocks. The idea is to call this function,
// then immediately call the C/C++ function. After the C/C++ function
// returns, call cgocalldone. The usual Go code would look like
// syscall.Cgocall()
// defer syscall.Cgocalldone()
// cfunction()
func Cgocall() {
mp := getg().m
mp.incgo = true
// CgocallDone prepares to return to Go code from C/C++ code.
func CgocallDone() {
gp := getg()
if gp == nil {
throw("no g in CgocallDone")
gp.m.incgo = false
// If we are invoked because the C function called _cgo_panic,
// then _cgo_panic will already have exited syscall mode.
if readgstatus(gp)&^_Gscan == _Gsyscall {
// CgocallBack is used when calling from C/C++ code into Go code.
// The usual approach is
// syscall.CgocallBack()
// defer syscall.CgocallBackDone()
// gofunction()
func CgocallBack() {
gp := getg()
if gp == nil || gp.m == nil {
gp = getg()
mp := gp.m
mp.dropextram = true
// This is a C-created stack.
// Record the outermost Go frame to help stack scan.
gp.entrysp = getcallersp()
gp.m.incgo = false
if gp.m.ncgo == 0 {
// The C call to Go came from a thread created by C.
// The C call to Go came from a thread not currently running
// any Go. In the case of -buildmode=c-archive or c-shared,
// this call may be coming in before package initialization
// is complete. Wait until it is.
mp := gp.m
if mp.needextram || atomic.Load(&extraMWaiters) > 0 {
mp.needextram = false
// CgocallBackDone prepares to return to C/C++ code that has called
// into Go code.
func CgocallBackDone() {
// We are going to stop running in Go mode and return to C mode.
// We were almost certainly called by defer; if so, clean up
// the defer struct now, before we leave Go mode. But don't
// leave Go mode if we are panicing or called from Goexit,
// since in those cases we will continue executing deferred functions.
gp := getg()
mp := gp.m
drop := false
if gp.deferring && gp._panic == nil && !gp.goexiting {
d := gp._defer
if d == nil {
throw("no defer struct when deferring")
gp._defer =
// If we are the top level Go function called from C,
// then we need to release the m.
if mp.dropextram && mp.ncgo == 0 {
drop = true
// Don't go back to C mode if we are panicing. Just let the
// panic walk up through the Go stack.
if gp._panic == nil && !gp.goexiting {
gp.m.incgo = true
if drop {
mp.dropextram = false
} else if gp.deferring && gp._panic == nil && !gp.goexiting {
gp.ranCgocallBackDone = true
// _cgo_panic may be called by SWIG code to panic.
func _cgo_panic(p *byte) {
// cgo_yield exists in the gc toolchain to let TSAN deliver a signal.
// gccgo does not need this.
var cgo_yield = &_cgo_yield
var _cgo_yield unsafe.Pointer