Using Lua to call C (in Linux)

Question

I made a C program and wrote the following two function in that

1. int c_add(lua_State* L)  
   {...some code here...  
   }  
2. int main(int argc, char* argv)  
   {...some code here...  
   lua_register(L, "c_add", c_add);  
   }

And compiled it by following command successfully.

gcc -o test.exe test.c -I /home/ec2-user/install/lua-5.1.5/src -llua-5.1

But the following error showed after using a lua program to call it.

lua: func2.lua:2: attempt to call global 'c_add' (a nil value)

How to solve this problem?`

@cyclaminist thank you for reply. The code is here. -- func2.lua print(c_add(3,4)) — Allen Chen
– Allen Chen, Commented Dec 20, 2018 at 6:27
lua.exe is not the test.exe. You have c_add function defined by the code in test.exe, so you must run func2.lua from within the test.exe — Vlad
– Vlad, Commented Dec 20, 2018 at 9:40

PaulR · Accepted Answer · 2018-12-20 17:45:57Z

3

You need to compile the code as a shared library to access the C functions from an external Lua instance. You don't need the main function and you pass the -shared flag to gcc. Then you need to let Lua know what to do when require is called by implementing the following function:

//             same name as the so/dll
//                      v
LUALIB_API int luaopen_test(lua_State *L) {
    lua_register(L, "c_add", c_add);
    return 0;
}

This creates a single, global function. It's better to register a luaL_Reg array of functions with luaL_register:

static const luaL_Reg function_list[] = {
    {"c_add",   c_add},
    {NULL, NULL}
};

LUALIB_API int luaopen_test(lua_State *L) {
    luaL_register(L, "test", function_list);
    return 1;
}

So that a table of functions is returned by require:

local my_c_functions = require("test")
print(my_c_functions.c_add(42, 42))

edited Dec 20, 2018 at 17:45

answered Dec 20, 2018 at 17:18

PaulR

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4 Comments

Allen Chen Over a year ago

Thank you. I now have a deeper understanding of this. But a new error appeared when I using the code above. lua: error loading module 'test' from file '/usr/local/lib/lua/5.3/test.so': /usr/local/lib/lua/5.3/test.so: undefined symbol: luaL_register Does it means I have to include come library in C program?

cyclaminist Over a year ago

luaL_register is from Lua 5.1. In Lua 5.3, luaL_register is not defined unless you have compiled Lua with LUA_COMPAT_MODULE defined, so if you are compiling your library for Lua 5.3, it would be better to replace luaL_register(L, "test", function_list) with luaL_newlib(L, function_list) (and to make sure you are using the Lua 5.3 versions of the Lua headers).

Allen Chen Over a year ago

@cyclaminist Thank you for detailed explanation. I've replaced the function by following your advice, but the next error appeared. lua: error loading module 'test' from file '/usr/local/lib/lua/5.3/test.so': /usr/local/lib/lua/5.3/test.so: undefined symbol: luaopen_test But there is no "luaopen_test" in my source code.

cyclaminist Over a year ago

As PaulR explained above, your library needs a function for the Lua VM to call when you require the library from Lua. In this case, the function needs to be named luaopen_test. See the documentation for package.searchers for more information.

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