当用花括号初始化结构时，gcc会将其填充置零

Not necessarily. With gcc 10.1 on x86-64 with -O2, this code (also on godbolt)

struct foo  {
   int a;
   char b;
};

void blah(struct foo *p);

void foo(void) {
    struct foo c = { 1, 2};
    blah(&c);
}

编译为此程序集

foo:
        subq    $24, %rsp
        leaq    8(%rsp), %rdi
        movl    $1, 8(%rsp)
        movb    $2, 12(%rsp)
        call    blah
        addq    $24, %rsp
        ret

您可以看到最后三个字节未初始化。