英文:
Can a union be larger than the the largest member when the largest member is a primitive type?
问题
有没有可能使一个union的大小大于最大成员的大小，当最大成员是整数、浮点类型、_Complex类型、_Atomic类型、数据指针或函数指针，而不是数组、结构体或联合体*？例如，是否可以（或者是否有）一个系统，下面的代码会打印两个不同的大小（假设打印不会失败）。如果不行，是否有其他方法可以生成一个比最大成员更大的union，即使最大成员是一个"原始类型"**？
#include <complex.h>
#include <stdint.h>
#include <stdio.h>
#define MAX(a,b) ( (a)>(b) ? (a) : (b) )
#define MAX4(a,b,c,d) MAX( MAX( (a), (b) ), MAX( (c), (d) ) )
union Foo
{
uintmax_t a;
long double f;
void (*fp)(void);
void *data;
double complex c;
};
int main(void)
{
size_t l = MAX4( sizeof(void*), sizeof(void(*)(void)), sizeof(long double), sizeof(uintmax_t) );
l = MAX( l, sizeof(double complex) );
printf( "%zu %zu\n", sizeof(union Foo), l );
}
这（可能）不会改变任何生产代码，只是为了好奇。
相关但不同的问题（在union内部使用struct）：https://stackoverflow.com/questions/8453881/sizeof-union-larger-than-expected-how-does-type-alignment-take-place-here?noredirect=1&lq=1 https://stackoverflow.com/questions/76075501/size-of-union-changes-with-unexpected-word-alignment
*还有其他类型类别我忘了吗？
**这些类型有没有合适的名称？
英文:
Is there a possibility to make the size of a union larger than the largest member when the largest member is a integer, floating point type, _Complex type, _Atomic type, data pointer or function pointer and not a array, struct nor union*?
For example, can there be (or is there) a system where the following code will print 2 different sizes (assuming printing doesn't fail). If not, is there a other way to generate a union that is larger then the largest member even when the largest member is a "primitive type"**?
#include <complex.h>
#include <stdint.h>
#include <stdio.h>
#define MAX(a,b) ( (a)>(b) ? (a) : (b) )
#define MAX4(a,b,c,d) MAX( MAX( (a), (b) ), MAX( (c), (d) ) )
union Foo
{
uintmax_t a;
long double f;
void (*fp)(void);
void *data;
double complex c;
};
int main(void)
{
size_t l = MAX4( sizeof(void*), sizeof(void(*)(void)), sizeof(long double), sizeof(uintmax_t) );
l = MAX( l, sizeof(double complex) );
printf( "%zu %zu\n", sizeof(union Foo), l );
}
It (probably) doesn't change any productive code, it is just for curiosity.
Related but not the same question (use of a struct inside the union): https://stackoverflow.com/questions/8453881/sizeof-union-larger-than-expected-how-does-type-alignment-take-place-here?noredirect=1&lq=1 https://stackoverflow.com/questions/76075501/size-of-union-changes-with-unexpected-word-alignment
*Are there any other classes of types i forgot?
**Is there a proper name for this types?
答案1
得分: 4
有，您可以使用_Alignas：
union Foo {
_Alignas(32) uintmax_t a;
long double f;
void (*fp)(void);
void *data;
double complex c;
};
//
printf("%zu\n", sizeof(union Foo)); // 32
英文:
> is there a other way to generate a union that is larger then the largest member even when the largest member is a "primitive type"?
Yes, you can use _Alignas:
union Foo {
_Alignas(32) uintmax_t a;
long double f;
void (*fp)(void);
void *data;
double complex c;
};
//
printf("%zu\n", sizeof(union Foo)); // 32
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