As a more advanced example, we will build a wrapper for calling Lua functions, using the vararg facility in C. Our wrapper function (let us call it call_va) receives the name of the function to be called, a string describing the types of the arguments and results, then the list of arguments, and finally a list of pointers to variables to store the results; it handles all the details of the API. With this function, we could write our previous example simply as
call_va("f", "dd>d", x, y, &z);
where the string "dd>d" means "two arguments of type double, one result of type double". This descriptor can use the letters `d´ for double, `i´ for integer, and `s´ for strings; a `>´ separates arguments from the results. If the function has no results, the `>´ is optional.
#include <stdarg.h>
void call_va (const char *func, const char *sig, ...) {
va_list vl;
int narg, nres; /* number of arguments and results */
va_start(vl, sig);
lua_getglobal(L, func); /* get function */
/* push arguments */
narg = 0;
while (*sig) { /* push arguments */
switch (*sig++) {
case 'd': /* double argument */
lua_pushnumber(L, va_arg(vl, double));
break;
case 'i': /* int argument */
lua_pushnumber(L, va_arg(vl, int));
break;
case 's': /* string argument */
lua_pushstring(L, va_arg(vl, char *));
break;
case '>':
goto endwhile;
default:
error(L, "invalid option (%c)", *(sig - 1));
}
narg++;
luaL_checkstack(L, 1, "too many arguments");
} endwhile:
/* do the call */
nres = strlen(sig); /* number of expected results */
if (lua_pcall(L, narg, nres, 0) != 0) /* do the call */
error(L, "error running function `%s': %s",
func, lua_tostring(L, -1));
/* retrieve results */
nres = -nres; /* stack index of first result */
while (*sig) { /* get results */
switch (*sig++) {
case 'd': /* double result */
if (!lua_isnumber(L, nres))
error(L, "wrong result type");
*va_arg(vl, double *) = lua_tonumber(L, nres);
break;
case 'i': /* int result */
if (!lua_isnumber(L, nres))
error(L, "wrong result type");
*va_arg(vl, int *) = (int)lua_tonumber(L, nres);
break;
case 's': /* string result */
if (!lua_isstring(L, nres))
error(L, "wrong result type");
*va_arg(vl, const char **) = lua_tostring(L, nres);
break;
default:
error(L, "invalid option (%c)", *(sig - 1));
}
nres++;
}
va_end(vl);
}
