C编译器可以在一个结构中的第一个元素之前增加填充？ [英] Can a C compiler add padding before the first element in a structure?

问题描述

具体来说，我在想，如果我能做出这样的事情：

Specifically, I was wondering if I could make something like this:

typedef struct {
    char *s; /* still a cstr, with '0' bit at end */
    size_t len;
} str;
str *newstr(char *s) {/*...*/};
void freestr(str *s) {/*...*/};

和做这样的事情（把它与 STDLIB /串功能CSTR）：

and do things like this (treat it as a cstr with stdlib/string functions):

int main() {
    str *s = newstr("hello");
    printf("The first character of '%s' is '%c'", *s, (*s)[0]);
    freestr(s);
}

如果不是，它是不是一个大问题 - 当然，我真的不关心浪费一个字节。

If not, it's not a big deal—and, of course, I'm not really concerned about wasting a byte.

推荐答案

没有，这就是标准明确禁止将任何填充的地方。一个结构的第一构件和结构的地址的地址必须是相同的。

No, that's a place where the standard explicitly forbids placing any padding. The address of the first member of a struct and the address of the struct must be the same.

在C2011标准规定的n1570草案第6.7.2.1（15）：

Section 6.7.2.1 (15) in the n1570 draft of the C2011 standard states:

在一个结构物中，非位字段部件和单元，其中位字段驻留有在其声明的顺序增加的地址。 一个指向结构对象，合适的转换，指向其初始成员（如该成员是一个位域，然后在它所在的单元），反之亦然。有可能是结构对象内无名填充，但不是在其开头。

Within a structure object, the non-bit-field members and the units in which bit-fields reside have addresses that increase in the order in which they are declared. 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.

（重点煤矿）

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