c - 取消引用指向结构的指针以访问其第一个成员

Question

由于特定原因，我只想通过取消引用结构指针来访问结构的第一个成员。

我想知道这是否合法，或者在某些情况下会导致 UB；如果这个有任何问题，什么是正确的解决方案。

谢谢。

#include <stdio.h>
#include <stdlib.h>

typedef struct test_s
{
    void * data ;
    struct test_s * next ;

} test_t ;


int main( void )
{
    test_t * t = calloc( 1 , sizeof( test_t ) ) ;

    int n = 123;

    t->data = &n ; //int is used only for an address, this could be anything, an object for example
    void ** v = ( void* )t ;
    printf("Address of  n: %p\nAddress of *t: %p\n\n" , &n , *v ) ; //dereference the pointer to struct to access its first member

return 0;
}

Answer 1

是的，这是合法的。来自 C99，6.7.2.1.13:

A pointer to a structure object, suitably converted, points to its initial member (or if that member is a bit-field, then to the unit in which it resides), and vice versa. There may be unnamed padding within a structure object, but not at its beginning.