lua_pushfstring

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

Pushes onto the stack a formatted string and returns a pointer to this string. It is similar to the C function sprintf, but has some important differences:

You do not have to allocate 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 only be '=%%=' (inserts a '=%=' in the string), '=%s=' (inserts a zero-terminated string, with no size restrictions), '=%f=' (inserts a lua_Number), '=%p=' (inserts a pointer as a hexadecimal numeral), '=%d=' (inserts an int), and '=%c=' (inserts an int as a character).