CGO

Note CGO is not GO

• ebitengine/purego
  • 不需要 go 调用 lib
  • 通过 asm 调用
• 避免 CGO
  • hashicorp/go-plugin
  • notti/nocgo - dlopen without cgo
    • dlopen, dlclose, dlerror, dlsym
    • ffi 汇编, 支持 386 和 amd64
  • rainycape/dl - dlopen / dlsym
  • Isolating problematic Cgo code
• 参考
  • golang/go#18296 - runtime: dlopen/dlsym without CGo
  • 类型转换
• C -> CGO
  • modernc.org/ccgo/v3
  • modernc.org/libc
  • 案例
    • modernc.org/sqlite
    • PCRE2 go.arsenm.dev/pcre
    • uber/h3 akhenakh/goh3

类型转换

• cmd/cgo

Go

type InteropFunctions interface {
	// CString malloc - 调用者 C.free
	CString(s string) *C.char
	// CBytes malloc - 调用者 C.free
	CBytes([]byte) unsafe.Pointer
	GoString(*C.char) string
	GoStringN(*C.char, C.int) string
	GoBytes(unsafe.Pointer, C.int) []byte
}

• C.malloc 不回直接调用 C 的 malloc
• C.malloc 确保不会返回 NULL - 如果申请失败则直接 crash

func GetString(in *C.char, l C.int) *C.char {
	input := C.GoBytes(unsafe.Pointer(in), C.int(l))
  // 使用者需要 free 返回的指针
	return C.CString(string(input))
}

// #include <stdio.h>
// #include <errno.h>
// #cgo CFLAGS: -DPNG_DEBUG=1
// #cgo amd64 386 CFLAGS: -DX86=1
// #cgo LDFLAGS: -lpng
// #include <png.h>
// #cgo pkg-config: png cairo
// #cgo LDFLAGS: -L${SRCDIR}/libs -lfoo
import "C"

size_t _GoStringLen(_GoString_ s);
const char *_GoStringPtr(_GoString_ s);

• SRCDIR -> x.go 的位置

C

typedef void *GoMap;
typedef void *GoChan;
typedef struct { void *t; void *v; } GoInterface;
typedef struct { void *data; GoInt len; GoInt cap; } GoSlice;

typedef struct { const char *p; ptrdiff_t n; } _GoString_;
typedef _GoString_ GoString;

• GODEBUG=cgocheck=1
  • 0 - 不检查
  • 2 - 更严格的检查
• runtime/cgo
  • 安全传递 Go 内数据
  • interface{} -> cgo.Handle - uintptr - C.uintptr_t

go tool cgo

dlopen

# 查看 so symbol
# 没有地址的 symbol 是动态的
nm -gDC /usr/lib/libsqlite3.so
objdump -TC libz.so
readelf -Ws libz.so
# 只看 symbol
readelf -Ws /usr/lib/libsqlite3.so | awk '{print $8}'

// +build !cgo

package dlopen

// we have to use the 3 argument format here :( - 2 argument format is only allowed from inside cgo

//go:cgo_import_dynamic libc_dlopen_x dlopen "libdl.so.2"
//go:cgo_import_dynamic libc_dlclose_x dlclose "libdl.so.2"
//go:cgo_import_dynamic libc_dlsym_x dlsym "libdl.so.2"
//go:cgo_import_dynamic libc_dlerror_x dlerror "libdl.so.2"

// on amd64 we don't need the following line - on 386 we do...
// anyway - with those lines the output is better (but doesn't matter) - without it on amd64 we get multiple DT_NEEDED with "libc.so.6" etc

//go:cgo_import_dynamic _ _ "libdl.so.2"

sqlite

• cznic/sqlite
  • CGo-free port of SQLite/SQLite3 v3.37.0
  • C to Go
  • glebarez/go-sqlite
    • driver
• mattn/go-sqlite3
  • -tags libsqlite3 可 link libsqlite3.so
• No CGO
  • iamacarpet/go-sqlite3-win64
    • sqlite3.dll wrapper
  • cvilsmeier/sqinn-go
    • 基于 sqlite 命令行进行 IO 操作
  • zombiezen/go-sqlite
    • 可执行 SQL
    • 不提供 database/sql driver
    • fork crawshaw/sqlite
  • crawshaw/sqlite
    • low-level Go interface to SQLite 3
    • [Go and SQLite: when database/sql chafes](crawshaw.io/blog/go-and-sqlite
  • alicebob/sqlittle
    • 直接读取文件 - 不支持 SQL
    • 只读
    • incompatible database version
      • 要求 journal mode
      • 不支持 WAL
      • schema format > 1
      • UTF8 encoding

FAQ

查找用到了 cgo 的模块

go list -f "{{if .CgoFiles}}{{.ImportPath}}{{end}}" $(go list -f "{{.ImportPath}}{{range .Deps}} {{.}}{{end}}" ./...)

zig cgo cross compile

# Go 1.18+
CGO_ENABLED=1 GOOS=linux GOARCH=amd64 CC="zig cc -target x86_64-linux-musl" CXX="zig c++ -target x86_64-linux-musl" go build --tags extended

• amd64 -> x86_64
• Zig Makes Go Cross Compilation Just Work