lua c api

一 概述

很多情况下有这种需求：在特定的框架中针对不同的业务做少量修改。使用 C/C++ 开发稳定的框架，调用针对不同业务开发的 Lua 函数可以实现需求。其实从 C/C++ 调用 Lua 函数非常简单，调用时将 Lua 函数压入栈、将函数参数压入栈，调用 lua_pcall 完成调用；调用后从栈获得调用函数返回结果。

二 示例

1. 程序示例

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>

#include "lua.h"
#include "lauxlib.h"
#include "lualib.h"

/******************************************************************************
 * 函数结构体定义
 ******************************************************************************/
typedef struct function_buf_s {
    char *name;  // 函数名
    int nargs;   // 参数数量
    int nret;    // 返回值数量
    char *chunk; // 函数体
} function_buf;
// 此处 function 不应该使用 function f() … end 方式编写，写出的函数其实不会被执行

static function_buf

lua_function[] = {

{"sayhi", 1, 1,

"local name = …n"

"return string.format('hi %s!', name)n"

},

{"echo", 1, 1,

"function echo(msg)n"

"    return msgn"

"endn"

"local msg = …n"

"return echo(msg)n"

},

{NULL, 0, 0,NULL}

};
static int

sayHi(lua_State *L) {
<span class="n">function_buf</span> <span class="o">*</span><span class="n">function</span> <span class="o">=</span> <span class="o">&amp;</span><span class="n">lua_function</span><span class="p">[</span><span class="mi">0</span><span class="p">];</span>

<span class="kt">int</span> <span class="n">ret</span> <span class="o">=</span> <span class="n">luaL_loadbuffer</span><span class="p">(</span><span class="n">L</span><span class="p">,</span> <span class="n">function</span><span class="o">-&gt;</span><span class="n">chunk</span><span class="p">,</span> <span class="n">strlen</span><span class="p">(</span><span class="n">function</span><span class="o">-&gt;</span><span class="n">chunk</span><span class="p">),</span> 
                          <span class="n">function</span><span class="o">-&gt;</span><span class="n">name</span><span class="p">);</span>

<span class="k">if</span> <span class="p">(</span><span class="n">ret</span><span class="p">)</span> <span class="p">{</span>
    <span class="k">return</span> <span class="n">luaL_error</span><span class="p">(</span><span class="n">L</span><span class="p">,</span> <span class="s">&#34;call loadbuffer fail ret:%d, code:%s&#34;</span><span class="p">,</span> 
                      <span class="n">ret</span><span class="p">,</span> <span class="n">function</span><span class="o">-&gt;</span><span class="n">chunk</span><span class="p">);</span>
<span class="p">}</span>

<span class="n">lua_pushstring</span><span class="p">(</span><span class="n">L</span><span class="p">,</span> <span class="s">&#34;lua&#34;</span><span class="p">);</span>
<span class="n">ret</span> <span class="o">=</span> <span class="n">lua_pcall</span><span class="p">(</span><span class="n">L</span><span class="p">,</span> <span class="mi">1</span><span class="p">,</span> <span class="mi">1</span><span class="p">,</span> <span class="mi">0</span><span class="p">);</span>
<span class="k">if</span> <span class="p">(</span><span class="n">ret</span> <span class="o">!=</span> <span class="mi">0</span><span class="p">)</span> <span class="p">{</span>
    <span class="k">return</span> <span class="n">luaL_error</span><span class="p">(</span><span class="n">L</span><span class="p">,</span> <span class="s">&#34;lua_pcall fail ret:%d %s&#34;</span><span class="p">,</span> 
                      <span class="n">ret</span><span class="p">,</span> <span class="n">lua_tostring</span><span class="p">(</span><span class="n">L</span><span class="p">,</span> <span class="o">-</span><span class="mi">1</span><span class="p">));</span>
<span class="p">}</span>

<span class="c1">// 输出类型

    printf("typename %sn", luaL_typename(L, -1));
<span class="k">if</span> <span class="p">(</span><span class="o">!</span><span class="n">lua_isstring</span><span class="p">(</span><span class="n">L</span><span class="p">,</span> <span class="o">-</span><span class="mi">1</span><span class="p">))</span> <span class="p">{</span>
    <span class="k">return</span> <span class="n">luaL_error</span><span class="p">(</span><span class="n">L</span><span class="p">,</span> <span class="s">&#34;lua_pcall ret not string&#34;</span><span class="p">);</span>
<span class="p">}</span>

<span class="c1">// 函数执行结果在栈中，取出来再放入栈

    const char *msg = lua_tostring(L, -1);

lua_pop(L, -1);

lua_pushfstring(L, "sayhi say:%s", msg);

return 1;

}
/******************************************************************************


通用入口函数


调用时首先将函数名压入栈，之后是其他参数

****************************************************************************/

static int

common_entry(lua_State L) {

if (!lua_isstring(L, 1)) {

return luaL_error(L, "first arg should be function name, string type");

}

const char  function_name = lua_tostring(L, 1);
function_buf *function = lua_function;
for (;function->name; function++) {

if (!strcmp(function->name, function_name)) break;

}

if (!function->name) {

return luaL_error(L, "not found:%s function in array", function_name);

}
int ret = luaL_loadbuffer(L, function->chunk, strlen(function->chunk),

function->name);
if (ret) {

return luaL_error(L, "call loadbuffer fail ret:%d, code:%s",

ret, function->chunk);

}
int i;

for(i=2; i <= function->nargs+1; i++) {

lua_pushstring(L, lua_tostring(L, i));

}
ret = lua_pcall(L, function->nargs, function->nret, 0);

if (ret != 0) {

return luaL_error(L, "lua_pcall fail ret:%d %s",

ret, lua_tostring(L, -1));

}
int nret = function->nret;
for (i=-1; i >= -nret; i–){

printf("ret:%d is:%sn", -i, lua_tostring(L, i));

}
return nret;

}


static const

struct luaL_reg call_function_lib[] = {

{"sayHi", sayHi},

{"common_entry", common_entry},

{NULL, NULL}

};
/******************************************************************************


注册函数

*****************************************************************************/

int

luaopen_call_function(lua_State l) {

luaL_openlib(l, "call_function", call_function_lib, 0);

return 1;

}

程序中有两个示例，sayHi 函数接收一个 string 参数并返回一个 string 参数；common_entry 函数可以根据函数名选择相应的函数体进行执行，更加灵活。

在函数定义中需要注意，luaL_loadbuffer 系列函数会将字符串当做程序段载入并放在栈顶，但是并未执行函数。如果载入的代码段是 function f()…end 格式，使用 lua_pcall 调用代码段时函数 f 仅做了定义，未执行函数体。

2. 调用示例

$ /usr/local/lua5.1.5/bin/lua -e "require 'call_function'; ret = call_function.sayHi('lua'); print(ret)"

typename string

sayhi say:hi lua!



$ /usr/local/lua5.1.5/bin/lua -e "require 'call_function'; ret = call_function.common_entry('echo','lua is cool'); print(ret)"

ret:1 is:lua is cool

lua is cool

三 函数说明

1. luaL_error

int luaL_error (lua_State L, const char fmt, …);

触发错误，并使用 fmt 与后续参数格式化输出错误消息字符串。luaL_error 函数并不会退出，通常的是否用法为：

return luaL_error(args);

四 参考资料

-programming in lua

-lua 5.1 手册