lua api call, on lua functions int the stack

Question

following code is not running, i get 0 when i print(lua_getstack()). i do not want to change the code's reasoning, i need it as simple as it seems to do wht i need. Any suggestion?

int main() {
   lua_State *L = luaL_newstate();
   luaL_openlibs(L);

   const char *luaCode = "function jj() print('Hello from Lua!') end";
   if (luaL_dostring(L, luaCode) != LUA_OK) {
    fprintf(stderr, "Error executing Lua code: %s\n", lua_tostring(L, -1));
    lua_close(L);
    return 1;
    }

   // Call a Lua function to populate the stack
   lua_getglobal(L, "jj");
   lua_pcall(L, 0, 0, 0);

   // Try to get stack information
   lua_Debug ar;
   int gg = lua_getstack(L, 0, &ar);

   if (gg == 0) {
      fprintf(stderr, "No stack information available\n");
   } else {
    printf("Stack level: %d\n", 0);
    printf("  Function: %s\n", ar.name ? ar.name : "(no name)");
    printf("  Source: %s\n", ar.short_src);
    printf("  Line: %d\n", ar.currentline);
   }

   lua_close(L);
   return 0;
  }

Probably there is no Lua stack because you have already exited lua_pcall. — ESkri
– ESkri, Commented Feb 4, 2024 at 10:33

Oka · Accepted Answer · 2024-02-05 16:14:35Z

After lua_pcall returns, jj is no longer on the call stack.

You can use lua_getinfo:

int lua_getinfo (lua_State *L, const char *what, lua_Debug *ar);

Starting the what string with > pops a function from the stack, and fills out the debug structure with information based on the rest of the characters in the what string (see: lua_Debug for details).

#include <lauxlib.h>                                                        
#include <lua.h>                                                           
#include <lualib.h>                                                         
#include <stdio.h>                                                          
                                                                           
int main(void)                                           
{                                                                           
    lua_State *L = luaL_newstate();                                        
    luaL_openlibs(L);                                             
                                                                             
    const char *luaCode = "function jj() print('Hello from Lua!') end";       
    luaL_dostring(L, luaCode);                                                                                         
    lua_getglobal(L, "jj");                                                   
                                                                              
    lua_Debug ar;                                                          

    if (lua_getinfo(L, ">Sn", &ar)) {
        printf("Function (%s): %s\n", ar.what, ar.name ? ar.name : "(no name)");
        printf("Source: %s\n", ar.short_src);
        printf("Line: %d\n", ar.linedefined);
    }
              
    lua_close(L);
}

Function (Lua): (no name)
Source: [string "function jj() print('Hello from Lua!') end"]
Line: 1

Or set a hook with lua_sethook. Here is a hook that runs every time a function is called:

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

static void hook(lua_State *L, lua_Debug *ar)
{
    if (LUA_HOOKCALL == ar->event && lua_getinfo(L, "Snl", ar)) {
        printf("Function (%s): %s\n", ar->what, ar->name ? ar->name : "(no name)");
        printf("Source: %s\n", ar->short_src);
        printf("Line: %d\n", ar->currentline);
    }
}

int main(void)
{
    lua_State *L = luaL_newstate();
    luaL_openlibs(L);
    const char *luaCode = "function jj() print('Hello from Lua!') end";
    luaL_dostring(L, luaCode);
    lua_sethook(L, hook, LUA_MASKCALL, 0);
    lua_getglobal(L, "jj");
    lua_pcall(L, 0, 0, 0);
    lua_close(L);
}

Function (Lua): (no name)
Source: [string "function jj() print('Hello from Lua!') end"]
Line: 1
Function (C): print
Source: [C]
Line: -1
Hello from Lua!

Whereas lua_getstack is for collecting the activation record for a currently active function call (which you can then use with lua_getinfo to collect debug information).

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

static int trace(lua_State *L)
{
    lua_Debug ar;

    if (lua_getstack(L, 1, &ar) && lua_getinfo(L, "Snl", &ar)) {
        printf("Function (%s): %s\n", ar.what, ar.name ? ar.name : "(no name)");
        printf("Source: %s\n", ar.short_src);
        printf("Line: %d\n", ar.currentline);
    }

    return 0;
}

int main(void)
{
    lua_State *L = luaL_newstate();
    luaL_openlibs(L);
    const char *luaCode = "function jj()\nprint('Hello from Lua!')\ntrace()\nend";
    luaL_dostring(L, luaCode);

    lua_register(L, "trace", trace);

    lua_getglobal(L, "jj");
    lua_pcall(L, 0, 0, 0);
    lua_close(L);
}

Hello from Lua!
Function (Lua): (no name)
Source: [string "function jj()..."]
Line: 3

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