luaopen functions

Question

I found in the lua sample code these calls:

luaopen_table(L);
luaopen_io(L);
luaopen_string(L);
luaopen_math(L);

I searched in lua header files and I found other functions with luaopen:

LUALIB_API int (luaopen_base) (lua_State *L);
LUALIB_API int (luaopen_table) (lua_State *L);
LUALIB_API int (luaopen_io) (lua_State *L);
LUALIB_API int (luaopen_os) (lua_State *L);
LUALIB_API int (luaopen_string) (lua_State *L);
LUALIB_API int (luaopen_math) (lua_State *L);
LUALIB_API int (luaopen_debug) (lua_State *L);
LUALIB_API int (luaopen_package) (lua_State *L);

Can you please explain what these functions mean? For example, may I use tables if I don't call luaopen_table? I didn't find any documentation about this!

Not quite a dupe of stackoverflow.com/questions/4551101/…

Mooing Duck
– Mooing Duck

2014-08-14 21:30:35 +00:00
Commented Aug 14, 2014 at 21:30 — Mooing Duck
– Mooing Duck, Commented Aug 14, 2014 at 21:30

fouronnes · Accepted Answer · 2011-03-09 08:44:55Z

If you're using Lua 5.1, which is the latest version, the Reference Manual has an answer :

To have access to these libraries, the C host program should call the luaL_openlibs function, which opens all standard libraries. Alternatively, it can open them individually by calling luaopen_base (for the basic library), luaopen_package (for the package library), luaopen_string (for the string library), luaopen_table (for the table library), luaopen_math (for the mathematical library), luaopen_io (for the I/O library), luaopen_os (for the Operating System library), and luaopen_debug (for the debug library). These functions are declared in lualib.h and should not be called directly: you must call them like any other Lua C function, e.g., by using lua_call.

[...]

The luaopen_* functions (to open libraries) cannot be called directly, like a regular C function. They must be called through Lua, like a Lua function.

And yes, you can still use tables if you don't import the table library, they are built-in. You just don't have access to the table manipulation functions.

Dan McKinnon · Accepted Answer · 2011-06-18 21:12:03Z

1

You have to push them onto the stack to call them like you would a normal lua c function.

lua_State *l = lua_open();
lua_pushcfunction(l,luaopen_base);
lua_call(l,0,0);
lua_pushcfunction(l,luaopen_math);
lua_call(l,0,0);
lua_pushcfunction(l,luaopen_string);
lua_call(l,0,0);
lua_pushcfunction(l,luaopen_table);
lua_call(l,0,0);

answered Jun 18, 2011 at 21:12

Dan McKinnon

451 bronze badge

1 Comment

Sqeaky Over a year ago

The OP asked two questions, you answered neither and provided sample code of dubious value without explanation. -1

Johann Oskarsson · Accepted Answer · 2019-07-24 19:40:35Z

For those who browse this question looking for how to use these functions in modern Lua:

As of Lua 5.3 you need to luaL_requiref these, based on the source code in luaL_openlibs. I found no reference to that in any manual. So here is an example that opens up only the base library which allows lua to print to standard output.

#include <lua.h>
#include <lualib.h>
#include <lauxlib.h>

int main( int argc, char *argv[] ) {

  lua_State *lua = luaL_newstate();
  luaL_requiref( lua, "_G", luaopen_base, 1 );
  lua_pop( lua, 1 );

  luaL_dostring( lua, "print \"Hello, lua\"" );

  lua_close( lua );

  return 0;
}

I cannot say what exactly all these libraries mean, nor how not loading them will affect you, but you can load them individually with the example given above. For example,

luaL_requiref( lua, LUA_IOLIBNAME, luaopen_io, 1 );
lua_pop( lua, 1 );

would load only the I/O library. See also the manual.

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