需要帮助查找 C 程序中的分段错误

Question

我正在为一个个人学习项目在 C 中实现数据加密标准，但在过去的 3 天里，我遇到了一个段错误，这个错误一直困扰着我。我知道这不是“为我修复我的代码”类型问题的地方，但我需要第二双眼睛来检查这个：

/*we must define our own modulo, as the C modulo returns unexpected results:*/

#define MOD(x, n) ((x % n + n) % n)

/*example: the 12th bit should be in the second byte so return 1 (the first byte being 0)*/

#define GET_BYTE_NUM(bit_index) (bit_index/8)

/*example: a bit index of 12 means this bit is the 4th bit of the second byte so return 4*/ 

#define GET_BIT_NUM(bit_index) MOD(bit_index, 8)

typedef unsigned char byte;
 
/*each row represents a byte, at the bit to be place in the position
 * for example for the first row (first byte) we will place bits 31, 0, 1, 2, 3, 4 in
 * in bit positions 0-6, respectively. The last two bits will be left blank. Since this is supposed to be a crypto implementation, static prevents this value from being accessed outside the file.*/
const static byte e_box[8][6] = { {31, 0, 1, 2, 3, 4}, {3, 4, 5, 6, 7, 8}, {7, 8, 9, 10, 11, 12}, {11, 12, 13, 14, 15, 16}, {12, 16, 17, 18, 19, 20}, {19, 20, 21, 22, 23, 24},
{23, 24, 25, 26, 27, 28}, {27, 28, 29, 30, 31, 0} }


void e(byte **four_byte_block)
{
    int i, n, l = 0, four_bit_num, four_byte_num;
    /*create the new byte_block and initialize all values to 0, we will have 4 spaces of bytes, so 32 bits in total*/
    byte *new_byte_block = (byte*)calloc(4, sizeof(byte));
    byte bit;
    for(i = 0 i < 8; i++)
    {
        for(n = 0; n < 6; n++)
        {
            /*get the byte number of the bit at l*/
            four_byte_num = GET_BYTE_NUM(e_box[i][n]);
            /*find what index the bit at l is in its byte*/
            half_bit_num = GET_BIT_NUM(e_box[i][n]);
   
            bit = *four_byte_block[half_byte_num]; /*SEG FAULT!*/

        }
    }
    /*finally, set four_byte_block equal to new_byte_block*/
    /*four_byte_block = NULL;
     * four_byte_block = new_byte_block;*/
}

我已将问题缩小到标记为 /SEG FAULT!/ 的那一行，但我看不出是什么问题。当我打印 half_byte_num 时，我得到一个在 half_block 范围内的数字，当我打印 half_block 的值时，我可以确认这些值存在。

我相信我可能对指针做错了什么，即通过传递 **four_byte_block，（指向指针的指针）并且它的操作可能导致段错误。

Answer 1

你试过这个吗：

bit = (*four_byte_block)[half_byte_num];

而不是这个：

bit = *four_byte_block[half_byte_num];

这些不一样，举个例子，拿下面的代码：

#include <string.h>
#include <stdio.h>
#include <stdlib.h>

int main()
{
    char **lines = malloc(sizeof(char *) * 8);

    for (int i = 0; i < 8l; i++)
        lines[i] = strdup("hey");

    printf("%c\n", *lines[1]);
    printf("%c\n",(*lines)[1]);
    return 0;
}

前者会输出h。后者会输出 e.

这是因为operators precedence，[]会在*之前被求值，所以如果你想去*foo的第n个索引，你需要输入 (*foo)[n].

需要帮助查找 C 程序中的分段错误

Need Help Finding Segmentation fault in C Program

c

gcc

c89