3.6 – Pushing Values onto the Stack

The API has the following functions to push C values onto the stack:

       void lua_pushboolean       (lua_State *L, int b);
       void lua_pushnumber        (lua_State *L, lua_Number n);
       void lua_pushlstring       (lua_State *L, const char *s, size_t len);
       void lua_pushstring        (lua_State *L, const char *s);
       void lua_pushnil           (lua_State *L);
       void lua_pushcfunction     (lua_State *L, lua_CFunction f);
       void lua_pushlightuserdata (lua_State *L, void *p);

These functions receive a C value, convert it to a corresponding Lua value, and push the result onto the stack. In particular, lua_pushlstring and lua_pushstring make an internal copy of the given string. lua_pushstring can only be used to push proper C strings (that is, strings that end with a zero and do not contain embedded zeros); otherwise, you should use the more general lua_pushlstring, which accepts an explicit size.

You can also push “formatted” strings:

       const char *lua_pushfstring  (lua_State *L, const char *fmt, ...);
       const char *lua_pushvfstring (lua_State *L, const char *fmt, va_list argp);

These functions push onto the stack a formatted string and return a pointer to that string. They are similar to sprintf and vsprintf, but with some important differences:

• You do not have to allocate the space for the result: The result is a Lua string and Lua takes care of memory allocation (and deallocation, through garbage collection).
• The conversion specifiers are quite restricted. There are no flags, widths, or precisions. The conversion specifiers can be simply `%%´ (inserts a `%´ in the string), `%s´ (inserts a zero-terminated string, with no size restrictions), `%f´ (inserts a lua_Number), `%d´ (inserts an int), and `%c´ (inserts an int as a character).

The function

       void lua_concat (lua_State *L, int n);

concatenates the n values at the top of the stack, pops them, and leaves the result at the top. If n is 1, the result is that single string (that is, the function does nothing); if n is 0, the result is the empty string. Concatenation is done following the usual semantics of Lua (see 2.5.4).