unsafe package 实际上是被编译器实现的。
- 13.1 unsafe.Sizeof, Alignof, and Offsetof
- 13.2 unsafe.Pointer
- 13.3 Example: Deep Equivalence
- 13.4 Calling C Code with cgo
- 13.5 Another Word of Caution
import "unsafe"
fmt.Println(unsafe.Sizeof(float64(0))) // "8"注意 32 位机器上一个 word 长度是 4 字节，而 64 位机器上是 8 字节。
Type Size
bool 1 byte
intN, uintN, floatN, complexN N/8 bytes
int, uint, uintptr 1 word
*T 1 word
string 2 words (data, len)
[]T 3 words (data, len, cap)
map 1 word
func 1 word
chan 1 word
interface 2 words (type, value)关于如下这个 struct 其在内存上的布局如下。
var x struct {
a bool
b int16
c []int
}
package math
import "unsafe"
func Float64bits(f float64) uint64 {
return *(*uint64)(unsafe.Pointer(&f))
}
fmt.Println(Float64bits(3.1415926)) // "0x40f918fb7a7b9f7f"Do not be tempted to introduce temporary variables of type uintptr to break this lines.
// NOTE: subtly incorrect:
tmp := uintptr(unsafe.Pointer(&x)) + unsafe.Offsetof(x.b)
pb := (*int16)(unsafe.Pointer(tmp))
*pb = 42The reason is very subtle. Some garbage collectors move variables around in memory to reduce fragmentation or bookkeeping. Garbage collectors of this kind are known as moving GCs.
再次注意， Go 是一个有垃圾回收的语言，因此不要自认为 new 就是把变量开到了堆上。实际上，变量可能被回收了。
因此如下，你用 C++ 的思维和方法去写完全不行。
pT := uintptr(unsafe.Pointer(new(T))) // NOTE: wrong!没有变量指向 new(T) ，在其一创造出来就被回收，因此其指针也是无效的。
见 equal 。
一些其他工具：
- foreign-function interfaces (FFIs)
- SWIG (swig.org)
案例参考 bzip 系列代码。
/*
#cgo CFLAGS: -I/usr/include
#cgo LDFLAGS: -L/usr/lib -lbz2
#include <bzlib.h>
int bz2compress(bz_stream *s, int action,
char *in, unsigned *inlen, char *out, unsigned *outlen);
*/
import "C"There is no package C, but this import causes go build to preprocess the file using the cgo tool before the Go compiler sees it.
During preprocessing, cgo generates a temporary package that contains Go declarations corresponding to all the C functions and types used by the file, such as C.bz_stream and C.BZ2_bzCompresInit. （具体参考代码） The cgo toll discovers these types by invoking the C compiler in a special way on the contents of the comment that precedes the import declaration.
The comment may also contain #cgo directives that specify extra options to the C toolchain. The CFLAGS and LDFLAGS values contribute extra arguments to the compiler and linker commands so that they can locate the bzlib.h header file and the libbz2.a archive library. The example assumes that these are installed beneath /usr on your system.