函数中大小为 4 的 read/write 无效

Question

我正在尝试实现一个从 int 数组中提取一些随机数的函数，而不更改原始数组。我写了这个函数

int get_random_int(int * from, int from_length, int * result, int result_length) {

    int i, pos, tmp;
    int * source = malloc(sizeof(int) * from_length); // Line 720
    /* Make sourcet to avoid to manipulate int array */
    for (i = 0; i < from_length; i++) {
        source[i] = from[i];
    };
    
    for (i = 0; i < result_length; i++) {
        /* Get random pos */
        pos = rand() % from_length;

        /* Swap first random position with last position */
        tmp = source[pos];
        source[pos] = source[from_length]; // Line 732
        source[from_length] = tmp; // Line 733

        /* Decrease last index */
        from_length--;

        /* Set random value in result array */
        result[i] = tmp;
    };

    free(source);
    return 1;
}

然后我在main中这样调用这个函数

receivers = malloc(sizeof(int) * y);
get_random_int(int_arr, x, receivers, y);
// Some useful stuff
free(receivers);

int_arr 是我要提取值的 int 数组，x 是数组的大小，receivers 是我要保存随机值的指针，y 是他的长度。

例程似乎有效，但是当我尝试运行使用 valgrind 的程序时，出现此错误。我真的不明白这是什么问题

==7== Invalid read of size 4
==7==    at 0x10BD30: get_random_int (main.c:732)
==7==    by 0x10A5A1: main (main.c:198)
==7==  Address 0x4bced08 is 0 bytes after a block of size 40 alloc'd
==7==    at 0x483877F: malloc (vg_replace_malloc.c:307)
==7==    by 0x10BC8F: get_random_int (main.c:720)
==7==    by 0x10A5A1: main (main.c:198)
==7== 
==7== Invalid write of size 4
==7==    at 0x10BD4B: get_random_int (main.c:733)
==7==    by 0x10A5A1: main (main.c:198)
==7==  Address 0x4bced08 is 0 bytes after a block of size 40 alloc'd
==7==    at 0x483877F: malloc (vg_replace_malloc.c:307)
==7==    by 0x10BC8F: get_random_int (main.c:720)
==7==    by 0x10A5A1: main (main.c:198)

Answer 1

如果您想访问索引长度为 1 的最后一个元素，C 使用从零开始的数组。例如：

    source[pos] = source[from_length - 1]; // Line 732
    source[from_length - 1] = tmp; // Line 733

函数中大小为 4 的 read/write 无效

Invalid read/write of size 4 in function

c

arrays

valgrind