From 754bbf7a25a8dda49b5d08ef0d0443bbf5af0e36 Mon Sep 17 00:00:00 2001 From: Craig Jennings Date: Sun, 7 Apr 2024 13:41:34 -0500 Subject: new repository --- devdocs/go/runtime%2Fcgo%2Findex.html | 96 +++++++++++++++++++++++++++++++++++ 1 file changed, 96 insertions(+) create mode 100644 devdocs/go/runtime%2Fcgo%2Findex.html (limited to 'devdocs/go/runtime%2Fcgo%2Findex.html') diff --git a/devdocs/go/runtime%2Fcgo%2Findex.html b/devdocs/go/runtime%2Fcgo%2Findex.html new file mode 100644 index 00000000..1f504a91 --- /dev/null +++ b/devdocs/go/runtime%2Fcgo%2Findex.html @@ -0,0 +1,96 @@ +

Package cgo

import "runtime/cgo"
Overview
Index

Overview

Package cgo contains runtime support for code generated by the cgo tool. See the documentation for the cgo command for details on using cgo.

Package files

callbacks.go callbacks_traceback.go cgo.go handle.go iscgo.go linux.go mmap.go setenv.go sigaction.go

type Handle

Handle provides a way to pass values that contain Go pointers (pointers to memory allocated by Go) between Go and C without breaking the cgo pointer passing rules. A Handle is an integer value that can represent any Go value. A Handle can be passed through C and back to Go, and Go code can use the Handle to retrieve the original Go value.

The underlying type of Handle is guaranteed to fit in an integer type that is large enough to hold the bit pattern of any pointer. The zero value of a Handle is not valid, and thus is safe to use as a sentinel in C APIs.

For instance, on the Go side:

package main
+
+/*
+#include <stdint.h> // for uintptr_t
+
+extern void MyGoPrint(uintptr_t handle);
+void myprint(uintptr_t handle);
+*/
+import "C"
+import "runtime/cgo"
+
+//export MyGoPrint
+func MyGoPrint(handle C.uintptr_t) {
+	h := cgo.Handle(handle)
+	val := h.Value().(string)
+	println(val)
+	h.Delete()
+}
+
+func main() {
+	val := "hello Go"
+	C.myprint(C.uintptr_t(cgo.NewHandle(val)))
+	// Output: hello Go
+}
+

and on the C side:

#include <stdint.h> // for uintptr_t
+
+// A Go function
+extern void MyGoPrint(uintptr_t handle);
+
+// A C function
+void myprint(uintptr_t handle) {
+    MyGoPrint(handle);
+}
+

Some C functions accept a void* argument that points to an arbitrary data value supplied by the caller. It is not safe to coerce a cgo.Handle (an integer) to a Go unsafe.Pointer, but instead we can pass the address of the cgo.Handle to the void* parameter, as in this variant of the previous example:

package main
+
+/*
+extern void MyGoPrint(void *context);
+static inline void myprint(void *context) {
+    MyGoPrint(context);
+}
+*/
+import "C"
+import (
+	"runtime/cgo"
+	"unsafe"
+)
+
+//export MyGoPrint
+func MyGoPrint(context unsafe.Pointer) {
+	h := *(*cgo.Handle)(context)
+	val := h.Value().(string)
+	println(val)
+	h.Delete()
+}
+
+func main() {
+	val := "hello Go"
+	h := cgo.NewHandle(val)
+	C.myprint(unsafe.Pointer(&h))
+	// Output: hello Go
+}
+

type Handle uintptr

func NewHandle

func NewHandle(v any) Handle

NewHandle returns a handle for a given value.

The handle is valid until the program calls Delete on it. The handle uses resources, and this package assumes that C code may hold on to the handle, so a program must explicitly call Delete when the handle is no longer needed.

The intended use is to pass the returned handle to C code, which passes it back to Go, which calls Value.

func (Handle) Delete

func (h Handle) Delete()

Delete invalidates a handle. This method should only be called once the program no longer needs to pass the handle to C and the C code no longer has a copy of the handle value.

The method panics if the handle is invalid.

func (Handle) Value

func (h Handle) Value() any

Value returns the associated Go value for a valid handle.

The method panics if the handle is invalid.

type Incomplete

Incomplete is used specifically for the semantics of incomplete C types.

type Incomplete struct {
+    // contains filtered or unexported fields
+}
+

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