go share library

http://blog.ralch.com/tutorial/golang-sharing-libraries/

 

Sharing Golang packages to C and Go

Sun, Aug 23, 2015

programming languages tutorial

The latest Go 1.5 version is out. As part of the new features, Go compiler can compile packages as a shared libraries.

It accepts -buildmode argument that determines how a package is compiled. These are the following options:

• archive: Build the listed non-main packages into .a files. Packages named main are ignored.
• c-archive: Build the listed main package, plus all packages it imports, into a C archive file.
• c-shared: Build the listed main packages, plus all packages that they import, into C shared libraries.
• shared: Combine all the listed non-main packages into a single shared library.
• exe: Build the listed main packages and everything they import into executables. Packages not named main are ignored.

By default, listed main packages are built into executables and listed non-main packages are built into .a files.

In this article we will explore two major ways to share libraries between Go and C:

Using shared library in Go

Assume that GOPATH contains this structure:

.
├── calc
│   └── calc.go
└── cashier
    └── main.go

The calc package contains a set of functions that do arithmetic opertaions:

// filename: calc.go
package calc

func Sum(x, y int) int { return x + y } 

Before compile any shared library, the standard builtin packages should be installed as shared library. This will allow any other shared library to link with them.

$ go install -buildmode=shared -linkshared std

Then the calc package can be compiled as shared library linked to std libraries:

$ go install -buildmode=shared -linkshared calc

Due to a issue, building and installing shared library should be from $GOPATH/src.

Lets use the shared library calc in the cashier application:

// package: cashier
// filename: main.go
package main

import "calc" import "fmt" func main() { fmt.Println("Cashier Application") fmt.Printf("Result: %d\n", calc.Sum(5, 10)) } 

The application should be compiled and linked with calc library with the following command:

$ go build -linkshared -o app cashier

The output of executing the application is:

$ ./app
Cashier Application
Result: 15 

Note that this feature is available on linux/amd64 platform or when gccgo compiler is used.

Using shared Go library in C

Go functions can be executed from C applications. They should be exported by using the following comment line:

//export <your_function_name>

In the code snippet below, the function SayHello and SayBye are exported:

// package name: nautilus
package main

import "C"
import "fmt" //export SayHello func SayHello(name string) { fmt.Printf("Nautilus says: Hello, %s!\n", name) } //export SayBye func SayBye() { fmt.Println("Nautilus says: Bye!") } func main() { // We need the main function to make possible // CGO compiler to compile the package as C shared library } 

The packaged should be compiled with buildmode flags c-shared or c-archive:

// as c-shared library
$ go build -buildmode=c-shared -o nautilus.a nautilus.go

// as c-archive 
$ go build -buildmode=c-archive -o nautilus.a nautilus.go

As result the GO compiler will produce a static/dynamic C library nautilus.a and header file nautilus.h. The header file contains type definitions that marshall and unmarshall data between Go and C:

typedef signed char GoInt8;
typedef unsigned char GoUint8; typedef short GoInt16; typedef unsigned short GoUint16; typedef int GoInt32; typedef unsigned int GoUint32; typedef long long GoInt64; typedef unsigned long long GoUint64; typedef GoInt64 GoInt; typedef GoUint64 GoUint; typedef __SIZE_TYPE__ GoUintptr; typedef float GoFloat32; typedef double GoFloat64; typedef __complex float GoComplex64; typedef __complex double GoComplex128; typedef struct { char *p; GoInt n; } GoString; typedef void *GoMap; typedef void *GoChan; typedef struct { void *t; void *v; } GoInterface; typedef struct { void *data; GoInt len; GoInt cap; } GoSlice; #endif /* End of boilerplate cgo prologue. */ #ifdef __cplusplus extern "C" { #endif extern void SayHello(GoString p0); extern void SayBye(); #ifdef __cplusplus } #endif 

The header file nautilus.h shoulde be imported from every C application that executed SayHello and SayBye functions.

In the example below, the SayHello function is called with parameter of type GoString. It includes char* field and its length.

// filename: _wale.c
#include "nautilus.h"
#include <stdio.h> int main() { printf("This is a C Application.\n"); GoString name = {"Jack", 4}; SayHello(name); SayBye(); return 0; } 

The _wale.c file is compiled with the following command:

$ gcc -o _wale _wale.c nautilus.a

Execution produce the following output:

$ ./wale
This is a C Application.
Nautilus says: Hello, Jack!
Nautilus says: Bye!

Conclusion

Sharing libraries between C and Go gives opportunity to build greater and better application by using the best from both worlds. This provides to a legacy system a modern language that can improve their maintainance costs and business needs. It maximize code reusability in the Go ecosystem.

转载于:https://www.cnblogs.com/jvava/p/5109584.html