Compiler warnings when dealing with huge integers

[Mooffie <mooffie@...>]

Compiler warnings when dealing with huge integers

This question pertains to Lua 5.1 and Lua 5.2 only, which always store
numbers are doubles by default (in contrast to Lua 5.3).

A project I'm contributing to gets compiled with -Wfloat-conversion
and -Wbad-function-cast (and I can't change this). The compiler is
gcc.

In a C function I need to get a huge integer (a file offset, for
example) from Lua. So I do:

    off_t a = lua_tonumber(L, -1);

But since lua_tonumber returns a float (double, to be exact), and
because of -Wfloat-conversion, the above line generates a warning:

    " warning: conversion to 'off_t {aka long long int}' from
     'lua_Number {aka double}' may alter its value [-Wfloat-conversion] "

(BTW, I can't use lua_tointeger because lua_Integer is typically small
(32 bits) and I don't want to ship a copy of Lua (with a bigger
lua_Integer) with the project.)

On the other hand, if I write that line as:

  off_t a = (off_t)lua_tonumber(L, -1);

I inhibit that warning but get a different one because of the
-Wbad-function-cast:

    " warning: cast from function call of type 'lua_Number {aka double}'
      to non-matching type 'long long int' [-Wbad-function-cast] "

Is there an obvious way to get around this problem? That is, a way to
write code that doesn't trigger these warnings?

I know I can solve this problem by doing instead:

    lua_Number num = lua_tonumber(L, -1);
    off_t a = (off_t) num;

or by defining a function encapsulating this (say,
get_lua_huge_integer() returning a int64_t), but I was wondering if
there's something obvious I'm missing.