以新建文件为例，对比一下几个常见平台的区别。
继续看下MacOS平台的代码：
// os/file.go
// 新建文件
func Create(name string) (*File, error) {
// 跳转到下面的OpenFile
return OpenFile(name, O_RDWR|O_CREATE|O_TRUNC, 0666)
}
// OpenFile在这里还是平台无关的代码
func OpenFile(name string, flag int, perm FileMode) (*File, error) {
testlog.Open(name)
// 从openFileNolog开始，不同平台代码会有不同
f, err := openFileNolog(name, flag, perm)
if err != nil {
return nil, err
}
f.appendMode = flag&O_APPEND != 0
return f, nil
}
// os/file_unix.go
// openFileNolog的unix实现
func openFileNolog(name string, flag int, perm FileMode) (*File, error) {
setSticky := false
if !supportsCreateWithStickyBit && flag&O_CREATE != 0 && perm&ModeSticky != 0 {
if _, err := Stat(name); IsNotExist(err) {
setSticky = true
}
}
var r int
var s poll.SysFile
for {
var e error
//跳转到open
r, s, e = open(name, flag|syscall.O_CLOEXEC, syscallMode(perm))
if e == nil {
break
}
// We have to check EINTR here, per issues 11180 and 39237.
if e == syscall.EINTR {
continue
}
return nil, &PathError{Op: "open", Path: name, Err: e}
}
// open(2) itself won't handle the sticky bit on *BSD and Solaris
if setSticky {
setStickyBit(name)
}
// There's a race here with fork/exec, which we are
// content to live with. See ../syscall/exec_unix.go.
if !supportsCloseOnExec {
syscall.CloseOnExec(r)
}
kind := kindOpenFile
if unix.HasNonblockFlag(flag) {
kind = kindNonBlock
}
// 封装为File结构
f := newFile(r, name, kind)
f.pfd.SysFile = s
return f, nil
}
// os/file_open_unix.go
func open(path string, flag int, perm uint32) (int, poll.SysFile, error) {
// 跳转到syscall.Open
fd, err := syscall.Open(path, flag, perm)
return fd, poll.SysFile{}, err
}
// syscall/zsyscall_darwin_amd64.go
func Open(path string, mode int, perm uint32) (fd int, err error) {
var _p0 *byte
_p0, err = BytePtrFromString(path)
if err != nil {
return
}
// 调用syscall
r0, _, e1 := syscall(abi.FuncPCABI0(libc_open_trampoline), uintptr(unsafe.Pointer(_p0)), uintptr(mode), uintptr(perm))
fd = int(r0)
if e1 != 0 {
err = errnoErr(e1)
}
return
}
// syscall/syscall_darwin.go
func syscall(fn, a1, a2, a3 uintptr) (r1, r2 uintptr, err Errno)
// internal/abi/funcpc.go
func FuncPCABI0(f interface{}) uintptr
// syscall/zsyscall_darwin_amd64.go
func libc_open_trampoline()
//go:cgo_import_dynamic libc_open open "/usr/lib/libSystem.B.dylib"
// 先通过abi.FuncPCABI0(libc_open_trampoline)先获取到open函数的地址
// 然后通过syscall调用open函数
// open函数是libc标准库中的函数，C语言定义为
int open(const char *pathname, int flags, mode_t mode);
//syscall在这里实现
//runtime/sys_darwin.go
//go:linkname syscall_syscall syscall.syscall
//go:nosplit
func syscall_syscall(fn, a1, a2, a3 uintptr) (r1, r2, err uintptr) {
args := struct{ fn, a1, a2, a3, r1, r2, err uintptr }{fn, a1, a2, a3, r1, r2, err}
entersyscall()
//跳转到libcCall
libcCall(unsafe.Pointer(abi.FuncPCABI0(syscall)), unsafe.Pointer(&args))
exitsyscall()
return args.r1, args.r2, args.err
}
func syscall()
// runtime/sys_libc.go
func libcCall(fn, arg unsafe.Pointer) int32 {
// Leave caller's PC/SP/G around for traceback.
gp := getg()
var mp *m
if gp != nil {
mp = gp.m
}
if mp != nil && mp.libcallsp == 0 {
mp.libcallg.set(gp)
mp.libcallpc = getcallerpc()
// sp must be the last, because once async cpu profiler finds
// all three values to be non-zero, it will use them
mp.libcallsp = getcallersp()
} else {
// Make sure we don't reset libcallsp. This makes
// libcCall reentrant; We remember the g/pc/sp for the
// first call on an M, until that libcCall instance
// returns. Reentrance only matters for signals, as
// libc never calls back into Go. The tricky case is
// where we call libcX from an M and record g/pc/sp.
// Before that call returns, a signal arrives on the
// same M and the signal handling code calls another
// libc function. We don't want that second libcCall
// from within the handler to be recorded, and we
// don't want that call's completion to zero
// libcallsp.
// We don't need to set libcall* while we're in a sighandler
// (even if we're not currently in libc) because we block all
// signals while we're handling a signal. That includes the
// profile signal, which is the one that uses the libcall* info.
mp = nil
}
// 跳转到asmcgocall
res := asmcgocall(fn, arg)
if mp != nil {
mp.libcallsp = 0
}
return res
}
// 硬件平台相关代码
// runtime/asm_arm64.s
// func asmcgocall(fn, arg unsafe.Pointer) int32
// Call fn(arg) on the scheduler stack,
// aligned appropriately for the gcc ABI.
// See cgocall.go for more details.
TEXT ·asmcgocall(SB),NOSPLIT,$0-20
MOVD fn+0(FP), R1
MOVD arg+8(FP), R0
MOVD RSP, R2 // save original stack pointer
CBZ g, nosave
MOVD g, R4
// Figure out if we need to switch to m->g0 stack.
// We get called to create new OS threads too, and those
// come in on the m->g0 stack already. Or we might already
// be on the m->gsignal stack.
MOVD g_m(g), R8
MOVD m_gsignal(R8), R3
CMP R3, g
BEQ nosave
MOVD m_g0(R8), R3
CMP R3, g
BEQ nosave
// Switch to system stack.
MOVD R0, R9 // gosave_systemstack_switch<> and save_g might clobber R0
BL gosave_systemstack_switch<>(SB)
MOVD R3, g
BL runtime·save_g(SB)
MOVD (g_sched+gobuf_sp)(g), R0
MOVD R0, RSP
MOVD (g_sched+gobuf_bp)(g), R29
MOVD R9, R0
// Now on a scheduling stack (a pthread-created stack).
// Save room for two of our pointers /*, plus 32 bytes of callee
// save area that lives on the caller stack. */
MOVD RSP, R13
SUB $16, R13
MOVD R13, RSP
MOVD R4, 0(RSP) // save old g on stack
MOVD (g_stack+stack_hi)(R4), R4
SUB R2, R4
MOVD R4, 8(RSP) // save depth in old g stack (can't just save SP, as stack might be copied during a callback)
BL (R1)
MOVD R0, R9
// Restore g, stack pointer. R0 is errno, so don't touch it
MOVD 0(RSP), g
BL runtime·save_g(SB)
MOVD (g_stack+stack_hi)(g), R5
MOVD 8(RSP), R6
SUB R6, R5
MOVD R9, R0
MOVD R5, RSP
MOVW R0, ret+16(FP)
RET
nosave:
// Running on a system stack, perhaps even without a g.
// Having no g can happen during thread creation or thread teardown
// (see needm/dropm on Solaris, for example).
// This code is like the above sequence but without saving/restoring g
// and without worrying about the stack moving out from under us
// (because we're on a system stack, not a goroutine stack).
// The above code could be used directly if already on a system stack,
// but then the only path through this code would be a rare case on Solaris.
// Using this code for all "already on system stack" calls exercises it more,
// which should help keep it correct.
MOVD RSP, R13
SUB $16, R13
MOVD R13, RSP
MOVD $0, R4
MOVD R4, 0(RSP) // Where above code stores g, in case someone looks during debugging.
MOVD R2, 8(RSP) // Save original stack pointer.
BL (R1)
// Restore stack pointer.
MOVD 8(RSP), R2
MOVD R2, RSP
MOVD R0, ret+16(FP)
RET
// 然后回到openFileNolog中
// 在openFileNolog中，继续调用newFile，整体封装为File结构，原路返回
func newFile(fd int, name string, kind newFileKind) *File {
f := &File{&file{
pfd: poll.FD{
Sysfd: fd,
IsStream: true,
ZeroReadIsEOF: true,
},
name: name,
stdoutOrErr: fd == 1 || fd == 2,
}}
pollable := kind == kindOpenFile || kind == kindPipe || kind == kindNonBlock
// If the caller passed a non-blocking filedes (kindNonBlock),
// we assume they know what they are doing so we allow it to be
// used with kqueue.
if kind == kindOpenFile {
switch runtime.GOOS {
case "darwin", "ios", "dragonfly", "freebsd", "netbsd", "openbsd":
var st syscall.Stat_t
err := ignoringEINTR(func() error {
return syscall.Fstat(fd, &st)
})
typ := st.Mode & syscall.S_IFMT
// Don't try to use kqueue with regular files on *BSDs.
// On FreeBSD a regular file is always
// reported as ready for writing.
// On Dragonfly, NetBSD and OpenBSD the fd is signaled
// only once as ready (both read and write).
// Issue 19093.
// Also don't add directories to the netpoller.
if err == nil && (typ == syscall.S_IFREG || typ == syscall.S_IFDIR) {
pollable = false
}
// In addition to the behavior described above for regular files,
// on Darwin, kqueue does not work properly with fifos:
// closing the last writer does not cause a kqueue event
// for any readers. See issue #24164.
if (runtime.GOOS == "darwin" || runtime.GOOS == "ios") && typ == syscall.S_IFIFO {
pollable = false
}
}
}
clearNonBlock := false
if pollable {
if kind == kindNonBlock {
// The descriptor is already in non-blocking mode.
// We only set f.nonblock if we put the file into
// non-blocking mode.
} else if err := syscall.SetNonblock(fd, true); err == nil {
f.nonblock = true
clearNonBlock = true
} else {
pollable = false
}
}
// An error here indicates a failure to register
// with the netpoll system. That can happen for
// a file descriptor that is not supported by
// epoll/kqueue; for example, disk files on
// Linux systems. We assume that any real error
// will show up in later I/O.
// We do restore the blocking behavior if it was set by us.
if pollErr := f.pfd.Init("file", pollable); pollErr != nil && clearNonBlock {
if err := syscall.SetNonblock(fd, false); err == nil {
f.nonblock = false
}
}
runtime.SetFinalizer(f.file, (*file).close)
return f
}