c - 在 C 中使用 free() 时出错

Question

我正在尝试用 C 语言编写一个程序，它具有一个函数 split_array。它接受一个指向动态分配数组的指针，然后将其分成两半，即左和右。

程序示例代码为:

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

void split_array(int *A, int x)
{
    int *left=NULL, *right=NULL, i, mid=0;

    mid=x/2;
    left=(int*)malloc(sizeof(int)*mid);
        if(left==NULL)
        {
            printf("\nError allocating memory. Aborting!\n\n");
            exit(1);
        }

    if(x%2==0)
    {
            right=(int*)malloc(sizeof(int)*mid);
            if(right==NULL)
            {
                    printf("\nError allocating memory. Aborting!\n\n");
                    exit(1);
            }
    }
    else
    {
        right=(int*)malloc(sizeof(int)*(mid+1));
            if(right==NULL)
            {
                    printf("\nError allocating memory. Aborting!\n\n");
                    exit(1);
            }
    }

        for(i=0;i<mid;i++)
           left[i]=A[i];

        for(i=mid;i<x;i++)
           right[i]=A[i];

        printf("\nleft: ");
        for(i=0;i<mid;i++)
            printf("%d ", left[i]);
        printf("\n");
        printf("\nright: ");
        for(i=mid;i<x;i++)
            printf("%d ", right[i]);
        printf("\n");

    free(left);
    free(right);

    left=NULL;
    right=NULL;
}

int main()
{
    int *arr=NULL, i, l=0;

    printf("Enter number of elements required in array: ");
    scanf("%d", &l);

    arr=(int*)malloc(sizeof(int)*l);
    if(arr==NULL)
    {
        printf("\nError allocating memory. Aborting!\n\n");
        exit(1);
    }

    printf("Enter the elements: ");
    for(i=0;i<l;i++)
        scanf("%d", &arr[i]);

    printf("\nYou entered the elements: ");
    for(i=0;i<l;i++)
        printf("%d ", arr[i]);
    printf("\n");

    split_array(arr, l);

    free(arr);
    arr=NULL;

return 0;
}

只要数组长度小于等于6，就正常执行。但是当长度大于或等于 7 时，它开始抛出错误消息。

输出示例是:

Enter number of elements required in array: 7 Enter the elements: 1 2 3 4 5 6 7

You entered the elements: 1 2 3 4 5 6 7

left: 1 2 3 

right: 4 5 6 7 
*** glibc detected *** ./a.out: free(): invalid next size (fast): 0x0000000002451060 ***
======= Backtrace: ========= /lib/x86_64-linux-gnu/libc.so.6(+0x76a16)[0x7f1d2d616a16] /lib/x86left: 1 2 3 

right: 4 5 6 7 
*** glibc detected *** ./a.out: free(): invalid next size (fast): 0x00000_64-linux-gnu/libc.so.6(cfree+0x6c)[0x7f1d2d61b7bc] ./a.out[0x400910] ./a.out[0x400a3e] /lib/x86_64-linux-gnu/libc.so.6(__libc_start_main+0xfd)[0x7f1d2d5beead] ./a.out[0x400619]
======= Memory map: ======== 00400000-00401000 r-xp 00000000 08:01 25954902                           /home/ralph/Desktop/a.out 00600000-00601000 rw-p 00000000 08:01 25954902                         /home/ralph/Desktop/a.out 02451000-02472000 rw-p 00000000 00:00 0      [heap] 7f1d28000000-7f1d28021000 rw-p 00000000 00:00 0  7f1d28021000-7f1d2c000000 ---p 00000000 00:00 0  7f1d2d38a000-7f1d2d39f000 r-xp 00000000 08:01 17957772                 /lib/x86_64-linux-gnu/libgcc_s.so.1 7f1d2d39f000-7f1d2d59f000 ---p 00015000 08:01 17957772                   /lib/x86_64-linux-gnu/libgcc_s.so.1 7f1d2d59f000-7f1d2d5a0000 rw-p 00015000 08:01 17957772                   /lib/x86_64-linux-gnu/libgcc_s.so.1 7f1d2d5a0000-7f1d2d722000 r-xp 00000000 08:01 17956950                   /lib/x86_64-linux-gnu/libc-2.13.so 7f1d2d722000-7f1d2d922000 ---p 00182000 08:01 17956950                   /lib/x86_64-linux-gnu/libc-2.13.so 7f1d2d922000-7f1d2d926000 r--p 00182000 08:01 17956950                   /lib/x86_64-linux-gnu/libc-2.13.so 7f1d2d926000-7f1d2d927000 rw-p 00186000 08:01 17956950                   /lib/x86_64-linux-gnu/libc-2.13.so 7f1d2d927000-7f1d2d92c000 rw-p 00000000 00:00 0  7f1d2d92c000-7f1d2d94c000 r-xp 00000000 08:01 17957749                   /lib/x86_64-linux-gnu/ld-2.13.so 7f1d2db2c000-7f1d2db2f000 rw-p 00000000 00:00 0  7f1d2db47000-7f1d2db4b000 rw-p 00000000 00:00 0  7f1d2db4b000-7f1d2db4c000 r--p 0001f000 08:01 17957749                 /lib/x86_64-linux-gnu/ld-2.13.so 7f1d2db4c000-7f1d2db4d000 rw-p 00020000 08:01 17957749                   /lib/x86_64-linux-gnu/ld-2.13.so 7f1d2db4d000-7f1d2db4e000 rw-p 00000000 00:00 0  7fff59345000-7fff59366000 rw-p 00000000 00:00 0      [stack]

7fff593ff000-7fff59400000 r-xp 00000000 00:00 0                        [vdso] ffffffffff600000-ffffffffff601000 r-xp 00000000 00:00 0         [vsyscall] Aborted

我在 gcc 4.7.2 中使用 GNU/Linux amd64。关于为什么会发生这种情况的任何说明？

谢谢

Answer 1

这些行

for(i=mid;i<x;i++)
   right[i]=A[i];

和这些行

for(i=mid;i<x;i++)
   printf("%d ", right[i]);

错了。您正在越界访问 right，并导致未定义的行为。

他们需要:

for(i=mid;i<x;i++)
   right[i-mid]=A[i];

和

for(i=mid;i<x;i++)
   printf("%d ", right[i-mid]);