我编写了简单的示例代码（也可以在 ideone 上找到）来显示我发现问题的地方。Sizeof 在常量的情况下返回错误的结果s。波纹管是文件main.c。

#include <stdint.h>
#include <stdio.h>

// comment this option if you know what is UB and want to avoit it 
#define I_DONT_KNOW_WHAT_IS_UB ( 1 )

struct _sa {
    uint32_t w;
    uint8_t const a[];
};

uint8_t const a[] = { 7,6,5,4,3,2,1,0 };

struct _sa const s = {
    .w = 8,
    .a = { 7,6,5,4,3,2,1,0 },
};

char const b[] = "line";

int main(void)
{
    #ifdef I_DONT_KNOW_WHAT_IS_UB
    printf("sizeof(a) = %d n", (int)sizeof(a)); // = 8
    printf("sizeof(s) = %d n", (int)sizeof(s)); // = 4
    printf("sizeof(b) = %d n", (int)sizeof(b)); // = 5
    #else
    printf("sizeof(a) = %zu n", sizeof(a)); // = 8
    printf("sizeof(s) = %zu n", sizeof(s)); // = 4
    printf("sizeof(b) = %zu n", sizeof(b)); // = 5
    #endif

    return 0;
}

我使用旧的 ubuntu：

uname -a
Linux imbearr 4.4.0-148-generic #174~14.04.1-Ubuntu SMP Thu May 9 08:17:37 UTC 2019 x86_64 x86_64 x86_64 GNU/Linux

gcc --version
gcc (Ubuntu 9.3.0-11ubuntu0~14.04) 9.3.0
Copyright (C) 2019 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

gcc main.c -o szof -Xlinker -Map=szof.map
grep -C 1 "bsb" ./szof.map 
                0x00000000004005e8                a
                0x00000000004005f0                s
                0x00000000004005fc                b

所以我通过链接器映射文件检查了大小：0x4005fc - 0x4005f0 = 12, 0x4005f0 - 0x4005e8 = 8。

为什么我有这样的结果，这是gcc的限制还是错误，可能是我的错误？

回答