读/写联合以在C＆＃x2b;＆＃x2B中文件安全是否安全。 11？

Question

在C ++ 11中，可以安全地将固定的大小结构读取到文件，但是我找不到任何引用是否可以安全地读取/编写一个联合文件。如果有以下结构和联合：

struct typeA {
  char val[10];
};

struct typeB {
  int val;
};

union u {
  struct typeA a;
  struct typeB b;
};

struct u_wrapper {
  char type[10]; /* indicate which union member is initialized */
  union u;
};

在这种情况下，在struct u_wrapper上执行以下写（）操作是安全的吗？

/* following code are pseudocode */

struct u_wrapper w;
w.u.b.val = 1; /* init b */
strcpy(w.type, "b");

/* write struct w to a file */
write(fd, &w, sizeof(w));      -----> is using sizeof(w) correct?
...

/* get file size from stat() */
...
size_t file_size = st.st_size;

/* read file content to buf */
void *buf = malloc(file_size);
read(fd, buf, file_size);
...

struct u_wrapper *w_read = reinterpret_cast<u_wrapper *>(buf); ----> is this correct and safe?

/* does it guarrantee w_read->b has the correct value? */
if (strncmp(w_read->type, "b", 1) == 0) {
  printf("Value of b: %d\n", w_read->b.val);   ----> will this always works correctly?
}

感谢您的帮助！