安装#

依赖 PCRE 库

Copyapt-get install libpcre2-dev

下载安装

Copy$ ./configure
$ make
$ make install

介绍#

SWIG 是一个软件开发工具，能够简化不同编程语言与 C 和 C++ 程序连接的开发任务。
简而言之，SWIG 是一款编译器，它可以获取 C/C++ 声明并创建访问这些声明所需的包装器，从而可从包括 Perl、Python、Tcl、Ruby、Guile 和 Java 在内的其他语言访问这些声明。SWIG 通常不需要修改现有代码，而且通常只需几分钟即可构建一个可用的接口。

本质上，SWIG 是一个为生成代码而设计的工具，该工具可以让各种其他编程语言调用 C/C++ 代码。这些更高级的编程语言是 SWIG 代码生成器的目标语言，而 C 或 C++ 是输入语言。在运行 SWIG 时必须指定一种目标语言。这将为 C/C++ 和指定的目标语言生成代码，以便相互进行接口

这种将 C/C++ 与许多不同目标语言连接的能力是 SWIG 的核心优势和特性之一。

从一个c库开始#

假设，这里有一个c代码，我们要提供给python和golang、java等调用。

下面是代码：

Copy/* File : example.c */

double My_variable = 3.0;

/* Compute factorial of n */
int fact(int n) {
  if (n <= 1)
    return 1;
  else
    return n*fact(n-1);
}

/* Compute n mod m */
int my_mod(int n, int m) {
  return(n % m);
}

你或许会想，是不是要先学习下python扩展如何写，JNI如何搞，cgo如何弄。。

但是，当你有了SWIG，这一切就变得更简单了。开始之前，我们首先需要编写一个swig接口.

swig接口#

swig接口，你可以理解为就像pb文件一样，要先定义一套标准的接口（interface），然后swig负责根据这个swig interface，去生成连接其他语言的胶水代码。

Copy%module example

%{

/* Put headers and other declarations here */
extern double My_variable;
extern int    fact(int);
extern int    my_mod(int n, int m);

%}

extern double My_variable;
extern int    fact(int);
extern int    my_mod(int n, int m);

• 接口文件包含 C 函数原型和变量声明。
• %module 指令定义了 SWIG 将创建的模块的名称。
• {% %} 块提供了一个位置，用于将其他代码（如 C 头文件或附加 C 声明）插入到生成的 C 包装器代码中

接口就绪后，我们可以来生成对应语言的胶水代码了。

python调用c#

首先通过swig -{lang} example.i 生成wrap胶水代码。

Copyswig -python example.i

这里会生成;

• example_wrap.c 连接c代码的wrap
• example.py 可以类比GRPC生成的访问wrap的代码。

我们来看下生成的包装代码example_wrap.c：

Copy/* ----------------------------------------------------------------------------
 * This file was automatically generated by SWIG (https://www.swig.org).
 * Version 4.1.1
 *
 * Do not make changes to this file unless you know what you are doing - modify
 * the SWIG interface file instead.
 * ----------------------------------------------------------------------------- */

/* source: example.i */



#define SWIG_VERSION 0x040101
#define SWIGGO
#define SWIGMODULE example
/* -----------------------------------------------------------------------------
 *  This section contains generic SWIG labels for method/variable
 *  declarations/attributes, and other compiler dependent labels.
 * ----------------------------------------------------------------------------- */

/* template workaround for compilers that cannot correctly implement the C++ standard */
#ifndef SWIGTEMPLATEDISAMBIGUATOR
# if defined(__SUNPRO_CC) && (__SUNPRO_CC <= 0x560)
#  define SWIGTEMPLATEDISAMBIGUATOR template
# elif defined(__HP_aCC)
/* Needed even with `aCC -AA' when `aCC -V' reports HP ANSI C++ B3910B A.03.55 */
/* If we find a maximum version that requires this, the test would be __HP_aCC <= 35500 for A.03.55 */
#  define SWIGTEMPLATEDISAMBIGUATOR template
# else
#  define SWIGTEMPLATEDISAMBIGUATOR
# endif
#endif

/* inline attribute */
#ifndef SWIGINLINE
# if defined(__cplusplus) || (defined(__GNUC__) && !defined(__STRICT_ANSI__))
#   define SWIGINLINE inline
# else
#   define SWIGINLINE
# endif
#endif

/* attribute recognised by some compilers to avoid 'unused' warnings */
#ifndef SWIGUNUSED
# if defined(__GNUC__)
#   if !(defined(__cplusplus)) || (__GNUC__ > 3 || (__GNUC__ == 3 && __GNUC_MINOR__ >= 4))
#     define SWIGUNUSED __attribute__ ((__unused__))
#   else
#     define SWIGUNUSED
#   endif
# elif defined(__ICC)
#   define SWIGUNUSED __attribute__ ((__unused__))
# else
#   define SWIGUNUSED
# endif
#endif

#ifndef SWIG_MSC_UNSUPPRESS_4505
# if defined(_MSC_VER)
#   pragma warning(disable : 4505) /* unreferenced local function has been removed */
# endif
#endif

#ifndef SWIGUNUSEDPARM
# ifdef __cplusplus
#   define SWIGUNUSEDPARM(p)
# else
#   define SWIGUNUSEDPARM(p) p SWIGUNUSED
# endif
#endif

/* internal SWIG method */
#ifndef SWIGINTERN
# define SWIGINTERN static SWIGUNUSED
#endif

/* internal inline SWIG method */
#ifndef SWIGINTERNINLINE
# define SWIGINTERNINLINE SWIGINTERN SWIGINLINE
#endif

/* exporting methods */
#if defined(__GNUC__)
#  if (__GNUC__ >= 4) || (__GNUC__ == 3 && __GNUC_MINOR__ >= 4)
#    ifndef GCC_HASCLASSVISIBILITY
#      define GCC_HASCLASSVISIBILITY
#    endif
#  endif
#endif

#ifndef SWIGEXPORT
# if defined(_WIN32) || defined(__WIN32__) || defined(__CYGWIN__)
#   if defined(STATIC_LINKED)
#     define SWIGEXPORT
#   else
#     define SWIGEXPORT __declspec(dllexport)
#   endif
# else
#   if defined(__GNUC__) && defined(GCC_HASCLASSVISIBILITY)
#     define SWIGEXPORT __attribute__ ((visibility("default")))
#   else
#     define SWIGEXPORT
#   endif
# endif
#endif

/* calling conventions for Windows */
#ifndef SWIGSTDCALL
# if defined(_WIN32) || defined(__WIN32__) || defined(__CYGWIN__)
#   define SWIGSTDCALL __stdcall
# else
#   define SWIGSTDCALL
# endif
#endif

/* Deal with Microsoft's attempt at deprecating C standard runtime functions */
#if !defined(SWIG_NO_CRT_SECURE_NO_DEPRECATE) && defined(_MSC_VER) && !defined(_CRT_SECURE_NO_DEPRECATE)
# define _CRT_SECURE_NO_DEPRECATE
#endif

/* Deal with Microsoft's attempt at deprecating methods in the standard C++ library */
#if !defined(SWIG_NO_SCL_SECURE_NO_DEPRECATE) && defined(_MSC_VER) && !defined(_SCL_SECURE_NO_DEPRECATE)
# define _SCL_SECURE_NO_DEPRECATE
#endif

/* Deal with Apple's deprecated 'AssertMacros.h' from Carbon-framework */
#if defined(__APPLE__) && !defined(__ASSERT_MACROS_DEFINE_VERSIONS_WITHOUT_UNDERSCORES)
# define __ASSERT_MACROS_DEFINE_VERSIONS_WITHOUT_UNDERSCORES 0
#endif

/* Intel's compiler complains if a variable which was never initialised is
 * cast to void, which is a common idiom which we use to indicate that we
 * are aware a variable isn't used.  So we just silence that warning.
 * See: https://github.com/swig/swig/issues/192 for more discussion.
 */
#ifdef __INTEL_COMPILER
# pragma warning disable 592
#endif


#include <stddef.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/types.h>



typedef ptrdiff_t intgo;
typedef size_t uintgo;


# if !defined(__clang__) && (defined(__i386__) || defined(__x86_64__))
#   define SWIGSTRUCTPACKED __attribute__((__packed__, __gcc_struct__))
# else
#   define SWIGSTRUCTPACKED __attribute__((__packed__))
# endif

typedef struct { char *p; intgo n; } _gostring_;
typedef struct { void* array; intgo len; intgo cap; } _goslice_;


static void Swig_free(void* p) {
  free(p);
}

static void* Swig_malloc(int c) {
  return malloc(c);
}


/* Put headers and other declarations here */
extern double My_variable;
extern int    fact(int);
extern int    my_mod(int n, int m);

#ifdef __cplusplus
extern "C" {
#endif

void _wrap_Swig_free_example_c8af3355f0aa50cc(void *_swig_go_0) {
  void *arg1 = (void *) 0 ;
  
  arg1 = *(void **)&_swig_go_0; 
  
  Swig_free(arg1);
  
}


void *_wrap_Swig_malloc_example_c8af3355f0aa50cc(intgo _swig_go_0) {
  int arg1 ;
  void *result = 0 ;
  void *_swig_go_result;
  
  arg1 = (int)_swig_go_0; 
  
  result = (void *)Swig_malloc(arg1);
  *(void **)&_swig_go_result = (void *)result; 
  return _swig_go_result;
}


void _wrap_My_variable_set_example_c8af3355f0aa50cc(double _swig_go_0) {
  double arg1 ;
  
  arg1 = (double)_swig_go_0; 
  
  My_variable = arg1;
  
}


double _wrap_My_variable_get_example_c8af3355f0aa50cc() {
  double result;
  double _swig_go_result;
  
  
  result = (double)My_variable;
  _swig_go_result = result; 
  return _swig_go_result;
}


intgo _wrap_fact_example_c8af3355f0aa50cc(intgo _swig_go_0) {
  int arg1 ;
  int result;
  intgo _swig_go_result;
  
  arg1 = (int)_swig_go_0; 
  
  result = (int)fact(arg1);
  _swig_go_result = result; 
  return _swig_go_result;
}


intgo _wrap_my_mod_example_c8af3355f0aa50cc(intgo _swig_go_0, intgo _swig_go_1) {
  int arg1 ;
  int arg2 ;
  int result;
  intgo _swig_go_result;
  
  arg1 = (int)_swig_go_0; 
  arg2 = (int)_swig_go_1; 
  
  result = (int)my_mod(arg1,arg2);
  _swig_go_result = result; 
  return _swig_go_result;
}


#ifdef __cplusplus
}
#endif

可以看到，自动生成的wrap代码为interface里的函数，变量都生成了包装函数，通过extern "C"实现wrap函数导出，后续的其他编程语言可以通过访问这些导出的wrap函数，实现访问原来的c函数。

编译模块#

我们现在来生成一个python代码模块。

Copy

[root@dev swig_demo]#gcc -c -fpic example.c example_wrap.c -I/root/anaconda3/include/python3.9
[root@dev swig_demo]#gcc -shared example.o example_wrap.o -o _example.so

[root@dev swig_demo]#ll
total 244
drwxr-xr-x 3 root root   4096 Jan 18 19:40 ./
drwxr-xr-x 4 root root   4096 Jan 18 19:28 ../
-rw-r--r-- 1 root root    203 Jan 18 19:28 example.c
-rw-r--r-- 1 root root    244 Jan 18 19:29 example.i
-rw-r--r-- 1 root root   1560 Jan 18 19:39 example.o
-rw-r--r-- 1 root root   2101 Jan 18 19:36 example.py
-rwxr-xr-x 1 root root  52480 Jan 18 19:39 _example.so*
-rw-r--r-- 1 root root 110727 Jan 18 19:36 example_wrap.c
-rw-r--r-- 1 root root  53240 Jan 18 19:39 example_wrap.o

[root@dev swig_demo]#python3
Python 3.9.13 (main, Aug 25 2022, 23:26:10) 
[GCC 11.2.0] :: Anaconda, Inc. on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import example
>>> example.fact(4)
24
>>> example.my_mod(23, 7)
2
>>> example.cvar.My_variable + 4.5
7.5
>>> 

• swig -python example.i 会生成一个c包装代码example_wrap.c 和python 库代码example.py
• example.py 里的函数定义和interface一致，实际上里面有代码去访问之前生成的wrap接口。
• gcc -c -fpic example.c example_wrap.c -I/root/anaconda3/include/python3.9 这里编译代码，需要指定下python include代码，根据实际位置指定

与CMake等build system集成#

swig支持与cmake等集成，我们编写一个cmakelists.txt文件

CopyFIND_PACKAGE(SWIG REQUIRED)
INCLUDE(${SWIG_USE_FILE})
FIND_PACKAGE(PythonLibs)
INCLUDE_DIRECTORIES(${PYTHON_INCLUDE_PATH})

INCLUDE_DIRECTORIES(${CMAKE_CURRENT_SOURCE_DIR})

SET(CMAKE_SWIG_FLAGS "")

SET_SOURCE_FILES_PROPERTIES(example.i PROPERTIES CPLUSPLUS ON)
SET_SOURCE_FILES_PROPERTIES(example.i PROPERTIES SWIG_FLAGS "-includeall")
SWIG_ADD_MODULE(example python example.i example.c)
SWIG_LINK_LIBRARIES(example ${PYTHON_LIBRARIES})

编译：

Copy[root@dev swig_demo]#mkdir build
[root@dev swig_demo]#cd build/
[root@dev build]#cmake ..
[root@dev build]#make
Scanning dependencies of target example_swig_compilation
[ 25%] Swig compile example.i for python
[ 25%] Built target example_swig_compilation
[ 50%] Building CXX object CMakeFiles/_example.dir/CMakeFiles/_example.dir/examplePYTHON_wrap.o
[ 75%] Building C object CMakeFiles/_example.dir/example.o
[100%] Linking CXX shared module _example.so
[100%] Built target _example
[root@dev build]#ls
CMakeCache.txt  CMakeFiles  cmake_install.cmake  example.py  _example.so  Makefile
[root@dev build]#ll
total 104
drwxr-xr-x 3 root root  4096 Jan 18 19:51 ./
drwxr-xr-x 4 root root  4096 Jan 18 19:50 ../
-rw-r--r-- 1 root root 15949 Jan 18 19:50 CMakeCache.txt
drwxr-xr-x 7 root root  4096 Jan 18 19:51 CMakeFiles/
-rw-r--r-- 1 root root  1662 Jan 18 19:51 cmake_install.cmake
-rw-r--r-- 1 root root  2101 Jan 18 19:51 example.py
-rwxr-xr-x 1 root root 57472 Jan 18 19:51 _example.so*
-rw-r--r-- 1 root root  6688 Jan 18 19:51 Makefile

golang调用c#

Go 并不支持直接调用用 C/C++ 编写的函数。cgo 程序可用于生成调用 C 代码的包装器，但目前没有直接调用 C++ 代码的简单方式。SWIG 填补了这一空白。

默认情况下 SWIG 会生成可以直接被 go build 使用的文件。 Go 需要大于1.2 版本。

同样上面的example.i， 我们可以

Copyswig -go example.i
go install

上面的命令，会生成example.go

Copy/* ----------------------------------------------------------------------------
 * This file was automatically generated by SWIG (https://www.swig.org).
 * Version 4.1.1
 *
 * Do not make changes to this file unless you know what you are doing - modify
 * the SWIG interface file instead.
 * ----------------------------------------------------------------------------- */

// source: example.i

package example

/*
#define intgo swig_intgo
typedef void *swig_voidp;

#include <stddef.h>
#include <stdint.h>


typedef ptrdiff_t intgo;
typedef size_t uintgo;



typedef struct { char *p; intgo n; } _gostring_;
typedef struct { void* array; intgo len; intgo cap; } _goslice_;


extern void _wrap_Swig_free_example_c8af3355f0aa50cc(uintptr_t arg1);
extern uintptr_t _wrap_Swig_malloc_example_c8af3355f0aa50cc(swig_intgo arg1);
extern void _wrap_My_variable_set_example_c8af3355f0aa50cc(double arg1);
extern double _wrap_My_variable_get_example_c8af3355f0aa50cc(void);
extern swig_intgo _wrap_fact_example_c8af3355f0aa50cc(swig_intgo arg1);
extern swig_intgo _wrap_my_mod_example_c8af3355f0aa50cc(swig_intgo arg1, swig_intgo arg2);
#undef intgo
*/
import "C"

import "unsafe"
import _ "runtime/cgo"
import "sync"


type _ unsafe.Pointer

var Swig_escape_always_false bool
var Swig_escape_val interface{}

type _swig_fnptr *byte
type _swig_memberptr *byte


func getSwigcptr(v interface { Swigcptr() uintptr }) uintptr {
	if v == nil {
		return 0
	}
	return v.Swigcptr()
}


type _ sync.Mutex

func Swig_free(arg1 uintptr) {
	_swig_i_0 := arg1
	C._wrap_Swig_free_example_c8af3355f0aa50cc(C.uintptr_t(_swig_i_0))
}

func Swig_malloc(arg1 int) (_swig_ret uintptr) {
	var swig_r uintptr
	_swig_i_0 := arg1
	swig_r = (uintptr)(C._wrap_Swig_malloc_example_c8af3355f0aa50cc(C.swig_intgo(_swig_i_0)))
	return swig_r
}

func SetMy_variable(arg1 float64) {
	_swig_i_0 := arg1
	C._wrap_My_variable_set_example_c8af3355f0aa50cc(C.double(_swig_i_0))
}

func GetMy_variable() (_swig_ret float64) {
	var swig_r float64
	swig_r = (float64)(C._wrap_My_variable_get_example_c8af3355f0aa50cc())
	return swig_r
}

func Fact(arg1 int) (_swig_ret int) {
	var swig_r int
	_swig_i_0 := arg1
	swig_r = (int)(C._wrap_fact_example_c8af3355f0aa50cc(C.swig_intgo(_swig_i_0)))
	return swig_r
}

func My_mod(arg1 int, arg2 int) (_swig_ret int) {
	var swig_r int
	_swig_i_0 := arg1
	_swig_i_1 := arg2
	swig_r = (int)(C._wrap_my_mod_example_c8af3355f0aa50cc(C.swig_intgo(_swig_i_0), C.swig_intgo(_swig_i_1)))
	return swig_r
}

可以看到，生成的代码默认还是通过cgo去调用swig生成的包装器wrap代码。

本质上，SWIG和我们常用的GRPC起到类似的作用:-)

总结#

SWIG 建立起java、python等其他高级编程语言调用c/c++ 代码的桥梁，可以不用了解JNI、cgo等复杂的跨语言调用知识，实现一次编写接口，同步生成多语言接口。

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