将未知类型与负值进行比较

Question

我有一个变量 id，其类型可能因平台而异。在几个地方，现有代码(我无法更改)将 id 设置为“负”值，例如

id = -ETIMEDOUT;

在某些平台上，id 可能被签名；在其他人身上，它可能没有签名。我想测试一下 id 是否设置为 -ETIMEDOUT。当 id 未签名时，下面的天真尝试巧妙地失败了:

if(id == -ETIMEDOUT)

我该如何简洁地测试这种情况？

Answer 1

@jlahd 发布了一个很好的答案。不过，我想提供一个替代方案。

if (id == (typeof(id)) -ETIMEDOUT)

我认为这会做类似的事情，但它有一个问题。它不在 C 标准中，而是 GCC 扩展。

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逐字引用@rici 的评论:

Since you probably know some typedef which works as the type of id, you can presumably do this without typeof, although that is cleaner. However, it should be

(typeof(id))(-ETIMEDOUT) 

(the parentheses around -ETIMEDOUT are just cosmetic but the unary minus is required.) Alternatively, the following should be pretty well foolproof:

if (-id == ETIMEDOUT)

since if id is signed, that obviously works; if id is unsigned, then unary minus has well-defined behaviour.