取消引用类型双关指针将破坏严格别名规则 [英] Dereferencing type-punned pointer will break strict-aliasing rules

问题描述

我使用以下代码从文件中读取数据，作为更大程序的一部分.

I used the following piece of code to read data from files as part of a larger program.

double data_read(FILE *stream,int code) {
        char data[8];
        switch(code) {
        case 0x08:
            return (unsigned char)fgetc(stream);
        case 0x09:
            return (signed char)fgetc(stream);
        case 0x0b:
            data[1] = fgetc(stream);
            data[0] = fgetc(stream);
            return *(short*)data;
        case 0x0c:
            for(int i=3;i>=0;i--)
                data[i] = fgetc(stream);
            return *(int*)data;
        case 0x0d:
            for(int i=3;i>=0;i--)
                data[i] = fgetc(stream);
            return *(float*)data;
        case 0x0e:
            for(int i=7;i>=0;i--)
                data[i] = fgetc(stream);
            return *(double*)data;
        }
        die("data read failed");
        return 1;
    }

现在我被告知使用 -O2 并且我收到以下 gcc 警告:警告:取消引用类型双关指针会破坏严格的别名规则

Now I am told to use -O2 and I get following gcc warning: warning: dereferencing type-punned pointer will break strict-aliasing rules

谷歌搜索我发现了两个正交的答案:

Googleing I found two orthogonal answers:

• 结论:无需担心；gcc 试图比法律更服从实际的法律.

对比

• 所以基本上如果你有一个 int* 和一个 float* 他们不允许指向相同的内存位置.如果您的代码不遵守这一点，那么编译器的优化器很可能会破坏您的代码.

最后我不想忽略警告.你会推荐什么?

In the end I don't want to ignore the warnings. What would you recommend?

[更新] 我用真实的函数替换了玩具示例.

[update] I substituted the toy example with the real function.

推荐答案

看起来你真的很想用 fread:

It looks a lot as if you really want to use fread:

int data;
fread(&data, sizeof(data), 1, stream);

也就是说，如果您确实想走读取字符的路线，然后将它们重新解释为 int，那么在 C 中执行此操作的安全方法(但在 C++ 中不是)是使用 a工会:

That said, if you do want to go the route of reading chars, then reinterpreting them as an int, the safe way to do it in C (but not in C++) is to use a union:

union
{
    char theChars[4];
    int theInt;
} myunion;

for(int i=0; i<4; i++)
    myunion.theChars[i] = fgetc(stream);
return myunion.theInt;

我不知道为什么原始代码中 data 的长度是 3.我假设你想要 4 个字节；至少我不知道任何 int 为 3 个字节的系统.

I'm not sure why the length of data in your original code is 3. I assume you wanted 4 bytes; at least I don't know of any systems where an int is 3 bytes.

请注意，您的代码和我的代码都高度不可移植.

Note that both your code and mine are highly non-portable.

如果您想从文件中读取各种长度的整数，请尝试如下操作:

If you want to read ints of various lengths from a file, portably, try something like this:

unsigned result=0;
for(int i=0; i<4; i++)
    result = (result << 8) | fgetc(stream);

(注意:在实际程序中，您还需要针对 EOF 测试 fgetc() 的返回值.)

(Note: In a real program, you would additionally want to test the return value of fgetc() against EOF.)

这会以小端格式从文件中读取一个 4 字节的无符号文件，不管系统的字节序是什么.它应该适用于任何无符号至少为 4 个字节的系统.

This reads a 4-byte unsigned from the file in little-endian format, regardless of what the endianness of the system is. It should work on just about any system where an unsigned is at least 4 bytes.

如果你想要字节序中立，不要使用指针或联合；改用位移.

If you want to be endian-neutral, don't use pointers or unions; use bit-shifts instead.

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