Passing Tables to Lua Functions
A use case that happens often is the passing of tables to and from Lua functions. How is that handled? There are a few idioms you see over and over again to make it happen. Before discussing the idioms, here's the code:
The Lua Program
Here's the Lua program. As you can see, function tweaktable receives a table argument, converts all values to upper case, counts the values, and puts all that info in a new table that's returned.
function tweaktable(tab_in)
local tab_out = {numfields=1}
for k,v in pairs(tab_in) do
tab_out.numfields = tab_out.numfields + 1
tab_out[tostring(k)] = string.upper(tostring(v))
end
tab_out.numfields = tostring(tab_out.numfields)
io.write("At bottom of callfuncscript.lua tweaktable(), numfields=")
io.write(tab_out.numfields)
print()
return tab_out
end
print("Priming run")
The C Program
The C program is the same as all the others except stacking arguments to Lua is a little different, and recovering the table passed back from Lua is a little different. The Lua tweaktable() function takes a table as its one and only argument and passes back one table. It passes back a completely different table so there's absolutely no question of the changes being made by reference to the args rather than passback.
Start by looking and running the code, and then we'll discuss some of the idioms that make it work...
#include <lua.h> /* Always include this when calling Lua */
#include <lauxlib.h> /* Always include this when calling Lua */
#include <lualib.h> /* Prototype for luaL_openlibs(), */
/* always include this when calling Lua */
#include <stdlib.h> /* For function exit() */
#include <stdio.h> /* For input/output */
void bail(lua_State *L, char *msg){
fprintf(stderr, "\nFATAL ERROR:\n %s: %s\n\n",
msg, lua_tostring(L, -1));
exit(1);
}
int main(void)
{
lua_State *L;
L = luaL_newstate(); /* Create Lua state variable */
luaL_openlibs(L); /* Load Lua libraries */
if (luaL_loadfile(L, "callfuncscript.lua")) /* Load but don't run the Lua script */
bail(L, "luaL_loadfile() failed"); /* Error out if file can't be read */
if (lua_pcall(L, 0, 0, 0)) /* PRIMING RUN. FORGET THIS AND YOU'RE TOAST */
bail(L, "lua_pcall() failed"); /* Error out if Lua file has an error */
printf("In C, calling Lua->tweaktable()\n");
lua_getglobal(L, "tweaktable"); /* Tell it to run callfuncscript.lua->tweaktable() */
lua_newtable(L); /* Push empty table onto stack table now at -1 */
lua_pushliteral(L, "fname"); /* Push a key onto the stack, table now at -2 */
lua_pushliteral(L, "Margie"); /* Push a value onto the stack, table now at -3 */
lua_settable(L, -3); /* Take key and value, put into table at -3, */
/* then pop key and value so table again at -1 */
lua_pushliteral(L, "lname"); /* Push a key onto the stack, table now at -2 */
lua_pushliteral(L, "Martinez"); /* Push a value onto the stack, table now at -3 */
lua_settable(L, -3); /* Take key and value, put into table at -3, */
/* then pop key and value so table again at -1 */
if (lua_pcall(L, 1, 1, 0)) /* Run function, !!! NRETURN=1 !!! */
bail(L, "lua_pcall() failed");
printf("============ Back in C again, Iterating thru returned table ============\n");
/* table is in the stack at index 't' */
lua_pushnil(L); /* Make sure lua_next starts at beginning */
const char *k, *v;
while (lua_next(L, -2)) { /* TABLE LOCATED AT -2 IN STACK */
v = lua_tostring(L, -1); /* Value at stacktop */
lua_pop(L,1); /* Remove value */
k = lua_tostring(L, -1); /* Read key at stacktop, */
/* leave in place to guide next lua_next() */
printf("Fromc k=>%s<, v=>%s<\n", k, v);
}
lua_close(L); /* Clean up, free the Lua state var */
return 0;
}
The preceding code yields the following output:
slitt@mydesk:~$ ./callfunc
Priming run
In C, calling Lua->tweaktable()
At bottom of callfuncscript.lua tweaktable(), numfields=3
============ Back in C again, Iterating thru returned table ============
Fromc k=>fname<, v=>MARGIE<
Fromc k=>lname<, v=>MARTINEZ<
Fromc k=>numfields<, v=>3<
slitt@mydesk:~$
转自：http://www.troubleshooters.com/codecorn/lua/lua_c_calls_lua.htm
