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c - C中的指针：分配内存与赋值VS分配内存而不赋值

问题描述

仅当我没有为指针分配某些值时，才通过 calloc 分配 Char 指针的内存按预期工作。

我试图获得一些简单的示例，以更好地理解指针/双指针/三指针/ ...。在为双指针编写代码时，我偶然发现了一个非常奇怪的行为。

#define PRINT(X, ...) printf("Address of " X "\n", ##__VA_ARGS__);

...
...

    int i = 0;
    char buf[12];
    char **two_star;

    two_star = calloc(10, sizeof(char*));
    for (i = 0; i < 10 ; ++i){
        two_star[i] = calloc(10, sizeof(char));
        PRINT("two_star[%d]   = %p", i, two_star[i]);
    }

    for (i = 0; i < 10 ; ++i){
        two_star[i] = calloc(10, sizeof(char));
        snprintf(buf, 12, "pre_%d_suff", i);
        two_star[i] = buf;
        PRINT("two_star[%d]   = %p   two_star[%d]   = %s", i, two_star[i], i, two_star[i]);
    }

对于上述两个 for 循环，我得到以下结果：

对于循环 1：

Address of two_star[0]   = 0xbcc090
Address of two_star[1]   = 0xbcc0b0
Address of two_star[2]   = 0xbcc0d0
Address of two_star[3]   = 0xbcc0f0
Address of two_star[4]   = 0xbcc110
Address of two_star[5]   = 0xbcc130
Address of two_star[6]   = 0xbcc150
Address of two_star[7]   = 0xbcc170
Address of two_star[8]   = 0xbcc190
Address of two_star[9]   = 0xbcc1b0

对于循环 2：

Address of two_star[0]   = 0x7ffcd2238ab0   two_star[0]   = pre_0_suff
Address of two_star[1]   = 0x7ffcd2238ab0   two_star[1]   = pre_1_suff
Address of two_star[2]   = 0x7ffcd2238ab0   two_star[2]   = pre_2_suff
Address of two_star[3]   = 0x7ffcd2238ab0   two_star[3]   = pre_3_suff
Address of two_star[4]   = 0x7ffcd2238ab0   two_star[4]   = pre_4_suff
Address of two_star[5]   = 0x7ffcd2238ab0   two_star[5]   = pre_5_suff
Address of two_star[6]   = 0x7ffcd2238ab0   two_star[6]   = pre_6_suff
Address of two_star[7]   = 0x7ffcd2238ab0   two_star[7]   = pre_7_suff
Address of two_star[8]   = 0x7ffcd2238ab0   two_star[8]   = pre_8_suff
Address of two_star[9]   = 0x7ffcd2238ab0   two_star[9]   = pre_9_suff

这里的问题很明显。在第二个 for 循环中分配的指针都具有相同的地址值。这意味着 two_star[0] 和 two_star[9] 最终是相同的值，因为相同的内存地址最终会一次又一次地更新。

标签： cpointers

解决方案

使用strcpy :strcpy(two_star[i], buf);而不是two_star[i] = buf;，因为在您的情况下，您不复制字符串，只需重新分配指向的指针two_star[i]（buf这也是内存泄漏，因为您丢失了指向已分配内存的指针）。

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