为什么 gcc4 发出警告以及如何避免它
我有一个函数声明为：
void event_add_card (EventAddr addr, EventType type, unsigned char card);
和 union
typedef union EventData
{
float money; /**< money info */
unsigned char card; /**< new card */
}
EventData;
当我编译以下代码时：
EventData data = {};
event_add_card (0,0, data.card);
启用警告 -Wconversion 我收到以下警告：
player-stud.c|71| warning: passing argument 3 of 'event_add_card' with different width due to prototype
为什么 gcc4 不成功以及如何修复它？？？
I have a function declared as:
void event_add_card (EventAddr addr, EventType type, unsigned char card);
and union
typedef union EventData
{
float money; /**< money info */
unsigned char card; /**< new card */
}
EventData;
When i compile following code:
EventData data = {};
event_add_card (0,0, data.card);
with enabled warning -Wconversion I receive following warning:
player-stud.c|71| warning: passing argument 3 of 'event_add_card' with different width due to prototype
Why gcc4 unsuccessful and how to fix It???
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在 4.3 之前的 gcc 版本中，当行为可能因原型是否在范围内而有所不同时，
-Wconversion会生成警告。在您给出的示例中，参数 3 (data.card) 是unsigned char；如果event_add_card()的原型在范围内，它将作为unsigned char传递，但如果原型不在范围内，它将作为传递>int由于 C 的整数提升规则 - 因此出现警告。这不是很有用，除非在将旧的 K&R 风格代码转换为使用原型的过程中。因此，如果您使用的是旧版本的 gcc，那么启用它通常不是一个有用的选项。
从 gcc 4.3 开始，行为发生了变化：它现在警告任何可能更改值的隐式转换（例如，有符号和无符号整数之间，或整数和浮点数之间）。这相当有用。 （不过，以前的功能并没有消失：它仍然可用，但重命名为
-Wtraditional-conversion。）（更多详细信息，请访问 http://gcc.gnu.org/wiki/NewWconversion 。）
In versions of gcc prior to 4.3,
-Wconversionproduces a warning when the behaviour may be different depending on whether or not a prototype is in scope. In the example you give, argument 3 (data.card) is ofunsigned char; if the prototype forevent_add_card()is in scope, it will be passed as anunsigned char, but if the prototype is not in scope it will be passed as anintdue to C's integer promotion rules - hence the warning.This isn't very useful, except during the process of converting old K&R-style code to use prototypes. As such, if you're using an older version of gcc, it's not a generally useful option to enable.
From gcc 4.3 onwards, the behaviour has changed: it now warns about any implicit conversion which might change a value (say, between signed and unsigned integers, or integers and floats). This is considerably more useful. (The previous functionality hasn't gone, though: it's still available, but renamed as
-Wtraditional-conversion.)(More details at http://gcc.gnu.org/wiki/NewWconversion .)
问题是您正在使用 -Wconversion
您的代码没有任何问题，并且您无法采取任何措施使 -Wconversion 不产生该警告。
示例：
如果您编译并使用 -Wconversion，这将产生相同的错误
-Wconversion 旨在识别 K&R 和 ISO C 之间转换时的问题，而您没有这样做。
The problem is that you're using -Wconversion
There is nothing wrong with your code, and nothing you can do to make -Wconversion not produce that warning.
Example:
This will produce the same error if you compile and use -Wconversion
-Wconversion is meant to identify issues when converting between K&R and ISO C, which you aren't doing.