读取变量时出错，无法访问地址 X 处的内存

Question

可能只是 C 新手的另一个愚蠢的指针问题。虽然想不通这个。似乎不知何故我的堆栈帧已损坏。分配似乎无关紧要，但它是一个相当基本的 I/O 练习。尝试通过单次读取读取结构数组（不能使用高级 I/O 函数，例如 fread()）。

#include "A2_Phase2.h"

void read_directory(Cdir directory[], int cnt) 
{
    int fd;
    char filename[] = "RandomStructDir.bin";

    fd = open(filename, O_RDONLY, S_IRWXU);
    if (fd < 0)
        perror(strcat(filename, " failed to open."));

    if (read(fd, &(directory[0].code[0]), sizeof(Cdir) * cnt) < 0) {
        perror(strcat(filename, " could not be accessed."));
    }

    close(fd);
}

int binary_search(Cdir directory[], char *key, int l, int r) {

    int mid = (int) r / 2;

    if (strncmp(key, directory[mid].code, 3) < 0)
        return binary_search(directory, key, l, mid - 1);
    else if (strncmp(key, directory[mid].code, 3) > 0)
        return binary_search(directory, key, mid + 1, r);
    else
        return mid;
}

int main(int argc, char *argv[]) 
{
    int COUNTRY_COUNT = atoi(argv[1]);
    printf("%d", COUNTRY_COUNT);

    Cdir *directory = (Cdir *) malloc(sizeof(Cdir) * COUNTRY_COUNT);
    read_directory(directory, COUNTRY_COUNT);
    binary_search(directory, "ZWE", 0, 238);
    free(directory);
}

我通过 GDB 收到此错误：

Program received signal SIGSEGV, Segmentation fault.
0x0000000000400940 in binary_search (
    directory=<error reading variable: Cannot access memory at address 0x7fffff7feff8>, 
    key=<error reading variable: Cannot access memory at address 0x7fffff7feff0>, l=<error reading variable: Cannot access memory at address 0x7fffff7fefec>, 
    r=<error reading variable: Cannot access memory at address 0x7fffff7fefe8>)
    at A2_Phase2.c:19
19  int binary_search(Cdir directory[], char *key, int l, int r) { 

谢谢！

Answer 1

int COUNTRY_COUNT = atoi(argv[1]);

读取国家数量作为程序的参数，但您稍后硬编码假设这是>= 238当您调用

binary_search(directory, "ZWE", 0, 238);

你能试一下吗

binary_search(directory, "ZWE", 0, COUNTRY_COUNT-1);

反而？您的binary_search函数中还有一些错误可以重写为

int binary_search(Cdir directory[], const char *key, int l, int r)
{
    int mid = (r + l) / 2;
    int cmp = strncmp(key, directory[mid].code, 3);
    if (l >= r) {
        if (cmp == 0)
            return l;
        return -1;
    }
    if (cmp < 0) 
        return binary_search(directory, key, l, mid - 1);
    else if (cmp > 0)
        return binary_search(directory, key, mid + 1, r);
    else
        return mid;
}

主要的变化是

• 的计算mid考虑l以及r
• （正如基里连科所指出的）认识到可能找不到匹配项。在这种情况下返回 -1
• 减少调用次数strcmp。非常小，但它使我的代码更清晰，并将提高搜索性能

不太重要的是，有一些风格问题使您的代码难以阅读

• 函数内部大量不必要的空白
• COUNTRY_COUNT对变量使用大写（例如）是不常见的。所有大写通常非正式地保留用于使用小写或驼峰命名的变量定义