github.com/sitano/gsysint@v0.0.0-20190607084937-69a4f3233e4e/g/cgocall.go (about) 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 // Cgo call and callback support. 6 // 7 // To call into the C function f from Go, the cgo-generated code calls 8 // runtime.cgocall(_cgo_Cfunc_f, frame), where _cgo_Cfunc_f is a 9 // gcc-compiled function written by cgo. 10 // 11 // runtime.cgocall (below) calls entersyscall so as not to block 12 // other goroutines or the garbage collector, and then calls 13 // runtime.asmcgocall(_cgo_Cfunc_f, frame). 14 // 15 // runtime.asmcgocall (in asm_$GOARCH.s) switches to the m->g0 stack 16 // (assumed to be an operating system-allocated stack, so safe to run 17 // gcc-compiled code on) and calls _cgo_Cfunc_f(frame). 18 // 19 // _cgo_Cfunc_f invokes the actual C function f with arguments 20 // taken from the frame structure, records the results in the frame, 21 // and returns to runtime.asmcgocall. 22 // 23 // After it regains control, runtime.asmcgocall switches back to the 24 // original g (m->curg)'s stack and returns to runtime.cgocall. 25 // 26 // After it regains control, runtime.cgocall calls exitsyscall, which blocks 27 // until this m can run Go code without violating the $GOMAXPROCS limit, 28 // and then unlocks g from m. 29 // 30 // The above description skipped over the possibility of the gcc-compiled 31 // function f calling back into Go. If that happens, we continue down 32 // the rabbit hole during the execution of f. 33 // 34 // To make it possible for gcc-compiled C code to call a Go function p.GoF, 35 // cgo writes a gcc-compiled function named GoF (not p.GoF, since gcc doesn't 36 // know about packages). The gcc-compiled C function f calls GoF. 37 // 38 // GoF calls crosscall2(_cgoexp_GoF, frame, framesize). Crosscall2 39 // (in cgo/gcc_$GOARCH.S, a gcc-compiled assembly file) is a two-argument 40 // adapter from the gcc function call ABI to the 6c function call ABI. 41 // It is called from gcc to call 6c functions. In this case it calls 42 // _cgoexp_GoF(frame, framesize), still running on m->g0's stack 43 // and outside the $GOMAXPROCS limit. Thus, this code cannot yet 44 // call arbitrary Go code directly and must be careful not to allocate 45 // memory or use up m->g0's stack. 46 // 47 // _cgoexp_GoF calls runtime.cgocallback(p.GoF, frame, framesize, ctxt). 48 // (The reason for having _cgoexp_GoF instead of writing a crosscall3 49 // to make this call directly is that _cgoexp_GoF, because it is compiled 50 // with 6c instead of gcc, can refer to dotted names like 51 // runtime.cgocallback and p.GoF.) 52 // 53 // runtime.cgocallback (in asm_$GOARCH.s) switches from m->g0's 54 // stack to the original g (m->curg)'s stack, on which it calls 55 // runtime.cgocallbackg(p.GoF, frame, framesize). 56 // As part of the stack switch, runtime.cgocallback saves the current 57 // SP as m->g0->sched.sp, so that any use of m->g0's stack during the 58 // execution of the callback will be done below the existing stack frames. 59 // Before overwriting m->g0->sched.sp, it pushes the old value on the 60 // m->g0 stack, so that it can be restored later. 61 // 62 // runtime.cgocallbackg (below) is now running on a real goroutine 63 // stack (not an m->g0 stack). First it calls runtime.exitsyscall, which will 64 // block until the $GOMAXPROCS limit allows running this goroutine. 65 // Once exitsyscall has returned, it is safe to do things like call the memory 66 // allocator or invoke the Go callback function p.GoF. runtime.cgocallbackg 67 // first defers a function to unwind m->g0.sched.sp, so that if p.GoF 68 // panics, m->g0.sched.sp will be restored to its old value: the m->g0 stack 69 // and the m->curg stack will be unwound in lock step. 70 // Then it calls p.GoF. Finally it pops but does not execute the deferred 71 // function, calls runtime.entersyscall, and returns to runtime.cgocallback. 72 // 73 // After it regains control, runtime.cgocallback switches back to 74 // m->g0's stack (the pointer is still in m->g0.sched.sp), restores the old 75 // m->g0.sched.sp value from the stack, and returns to _cgoexp_GoF. 76 // 77 // _cgoexp_GoF immediately returns to crosscall2, which restores the 78 // callee-save registers for gcc and returns to GoF, which returns to f. 79 80 package g 81 82 // Addresses collected in a cgo backtrace when crashing. 83 // Length must match arg.Max in x_cgo_callers in runtime/cgo/gcc_traceback.c. 84 type CgoCallers [32]uintptr 85