1 前言

最近博主在后臺(tái)收到一位朋友的咨詢，說他最近參加了一場技術(shù)面試，有這么一道筆試題：

請(qǐng)使用C語言的宏定義實(shí)現(xiàn)一個(gè)功能，求得某個(gè)整型數(shù)M在N字節(jié)對(duì)齊的時(shí)，它的值大小。

說明：
1.M是一個(gè)非負(fù)整數(shù)；
2.N是2的整數(shù)倍，值可能是1，2，4，8，16等等。

要求：
1.不得使用除法(/);
2.不能使用函數(shù)實(shí)現(xiàn)，只能用宏實(shí)現(xiàn)；
3.自行設(shè)計(jì)測(cè)試用例，明確得出測(cè)試用例執(zhí)行成功與否。

2 代碼實(shí)現(xiàn)

剛好，今天比較清閑，茶余飯后，順手?jǐn)]了一把代碼：

#include 
#include 

/* max test number for aligned */
#define MAX_TEST_NUM                         1000

/* default for 8 bytes */
#define DEF_ALIGNED_BYTES                     8

/* n = 2/4/6/8/16/... */
#define GET_ALIGNED_2_POWER_NUM(num, n)        (((num) + (n) - 1) & (~((n) - 1))) 

int main(int argc, const char *argv[])
{
    int i = 0;
    int max_cnt = MAX_TEST_NUM;
    int aligned_bytes = DEF_ALIGNED_BYTES;
    int aligned_num;

    if (argc > 1) {
        aligned_bytes = atoi(argv[1]);
        if (aligned_bytes < 0) {
            printf("error aligned_bytes input !\r\n");
            return -1;
        }
    }

    /* test cases start */
    for (i = 0; i < max_cnt; i++) {
        aligned_num = GET_ALIGNED_2_POWER_NUM(i, aligned_bytes);
        //printf("%-4d ALIGNED %d => %-4d\r\n", i, aligned_bytes, aligned_num);
        if (aligned_num % aligned_bytes != 0) {
            printf("error aligned_num get: %-4d ALIGNED %d => %-4d\r\n", i, aligned_bytes, aligned_num);
            printf("test cases (0 ~ %d) ALIGNED %d [ FAIL ] !\r\n", max_cnt, aligned_bytes);
            return -1;
        }
    }

    printf("test cases (0 ~ %d) ALIGNED %d [ OK ] !\r\n", max_cnt, aligned_bytes);

    return 0;
}

3 代碼驗(yàn)證

ubuntu下面使用gcc編譯，得到可執(zhí)行文件：

gcc -o test main.c

跑了下測(cè)試用例：

recan@ubuntu:~/llc/aligned_macro_test$ ./test 2
test cases (0 ~ 1000) ALIGNED 2 [ OK ] !
recan@ubuntu:~/llc/aligned_macro_test$ 
recan@ubuntu:~/llc/aligned_macro_test$ ./test 4
test cases (0 ~ 1000) ALIGNED 4 [ OK ] !
recan@ubuntu:~/llc/aligned_macro_test$ 
recan@ubuntu:~/llc/aligned_macro_test$ ./test 8
test cases (0 ~ 1000) ALIGNED 8 [ OK ] !
recan@ubuntu:~/llc/aligned_macro_test$ 
recan@ubuntu:~/llc/aligned_macro_test$ ./test 16
test cases (0 ~ 1000) ALIGNED 16 [ OK ] !
recan@ubuntu:~/llc/aligned_macro_test$ 
recan@ubuntu:~/llc/aligned_macro_test$ ./test 32
test cases (0 ~ 1000) ALIGNED 32 [ OK ] !
recan@ubuntu:~/llc/aligned_macro_test$ 
recan@ubuntu:~/llc/aligned_macro_test$ ./test 64
test cases (0 ~ 1000) ALIGNED 64 [ OK ] !
recan@ubuntu:~/llc/aligned_macro_test$ 
recan@ubuntu:~/llc/aligned_macro_test$ ./test 128
test cases (0 ~ 1000) ALIGNED 128 [ OK ] !
recan@ubuntu:~/llc/aligned_macro_test$ 
recan@ubuntu:~/llc/aligned_macro_test$ ./test 256
test cases (0 ~ 1000) ALIGNED 256 [ OK ] !
recan@ubuntu:~/llc/aligned_macro_test$ 
recan@ubuntu:~/llc/aligned_macro_test$ 
recan@ubuntu:~/llc/aligned_macro_test$ 
recan@ubuntu:~/llc/aligned_macro_test$ ./test 3
error aligned_num get: 1    ALIGNED 3 => 1   
test cases (0 ~ 1000) ALIGNED 3 [ FAIL ] !
recan@ubuntu:~/llc/aligned_macro_test$ 
recan@ubuntu:~/llc/aligned_macro_test$ ./test 5
error aligned_num get: 1    ALIGNED 5 => 1   
test cases (0 ~ 1000) ALIGNED 5 [ FAIL ] !
recan@ubuntu:~/llc/aligned_macro_test$ 
recan@ubuntu:~/llc/aligned_macro_test$ ./test 7
error aligned_num get: 1    ALIGNED 7 => 1   
test cases (0 ~ 1000) ALIGNED 7 [ FAIL ] !
recan@ubuntu:~/llc/aligned_macro_test$ 
recan@ubuntu:~/llc/aligned_macro_test$ ./test 167
error aligned_num get: 1    ALIGNED 167 => 1   
test cases (0 ~ 1000) ALIGNED 167 [ FAIL ] !
recan@ubuntu:~/llc/aligned_macro_test$ 
recan@ubuntu:~/llc/aligned_macro_test$ 
recan@ubuntu:~/llc/aligned_macro_test$ ./test 79
error aligned_num get: 1    ALIGNED 79 => 1   
test cases (0 ~ 1000) ALIGNED 79 [ FAIL ] !
recan@ubuntu:~/llc/aligned_macro_test$ 
recan@ubuntu:~/llc/aligned_macro_test$ 

從log上看，基本實(shí)現(xiàn)了功能，對(duì)于非對(duì)齊的數(shù)值，都能做出準(zhǔn)確的判斷。

大家記住這個(gè)宏定義吧！

/* n = 2/4/6/8/16/... */
#define GET_ALIGNED_2_POWER_NUM(num, n)        (((num) + (n) - 1) & (~((n) - 1))) 

4 題外話

題外話，如果把題目中的N改為【任意非負(fù)整數(shù)】呢？

又該如何改造這個(gè)宏定義呢？

下次有時(shí)間，我們?cè)僭囋嚳矗?/p>

5 更多分享

歡迎關(guān)注我的github倉庫01workstation，日常分享一些開發(fā)筆記和項(xiàng)目實(shí)戰(zhàn)，歡迎指正問題。

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