How to return a list of values from Lua to C and print them one by one?

Question

I am using C API embedded with Lua. My goal is that: pass an array of integers into Lua and calculate their factorials, then the results are passed back to C and printed out.

To realize the goal, my C code is:

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

int main(void){
  int status, result, i;
  double fac;

  lua_State *L;  // set Lua state
  L = luaL_newstate();

  luaL_openlibs(L); 

  status = luaL_loadfile(L, "factorial.lua");  // load the Lua script for factorial calculation
  if (status) {
    fprintf(stderr, "Couldn't load file: %s\n", lua_tostring(L, -1));
    exit(1);
  }

  lua_newtable(L);  

  for (i = 1; i <= 10; i++) {
    lua_pushnumber(L, i);   /* Push the table index */
    lua_pushnumber(L, i*2); /* Push the cell value */
    lua_rawset(L, -3);      /* Stores the pair in the table */
  }

  lua_setglobal(L, "foo");

  result = lua_pcall(L, 0, LUA_MULTRET, 0);
  if (result) {
    fprintf(stderr, "Failed to run script: %s\n", lua_tostring(L, -1));
    exit(1);
  }

  // the following loop is for factorial print-out
  while (lua_next(L, -1) != 0) {  
    fac = lua_tonumber(L, -1);
    printf("%.0f\n", fac);
    lua_pop(L, 1);
  }

  lua_close(L);    

  return 0;
}

And my Lua script is like:

-- this is the function to calculate the factorial
function fact(n)
if n == 0 then
       return 1
else
   return n * fact(n-1)
end
end

io.write("We calculate the factorial of the following numbers: \n")

return_table = {}
for i = 1, #foo do
 n = foo[i]
 factorial_result = fact(n)
 print(n)
 table.insert(return_table, factorial_result)
end

io.write("Here we show the results: \n")
for i=1,10 do 
return(return_table[i])
end

The compilation goes well but when I run it on terminal, I got:

We calculate the factorial of the following numbers:
2.0
4.0
6.0
8.0
10.0
12.0
14.0
16.0
18.0
20.0
Here we show the results:
Segmentation fault (core dumped)

I don't know why the result is like this. It seems that no problem with the passing from C to Lua but there is problem from Lua to C. Can anyone help me with this?

Answer 1

Your code has two problems:

1. You have a return statement inside a loop in your Lua code. Once a return statement is hit, your Lua script is done executing. It'll return that one value to the C code and it's going to be a number, not the table your C code is expecting.
2. You're calling lua_next without first pushing a starting key onto the stack.

In your Lua code, change this:

for i=1,10 do 
    return(return_table[i])
end

To this:

return(return_table) -- don't need parentheses here, btw

In your C code, add lua_pushnil(L) before your while loop.