array and point problem

Here, I have a bad program. Its outputs confusing me, anyone can tell me why ?

 #include <stdio.h>
 #include <stdlib.h>
 int main()
 {
    int i = 0;
    char *a_result[10];
    char *b_result[10];
    for (i = 0; i < 10; i++)
    {
        char a_array[10];
        char 开发者_Python百科*b_array = malloc(10*sizeof(char));
        int j = 0;
        for (j = 0; j < 9; j++)
        {
            a_array[j] = 'a' + i;
            b_array[j] = 'a' + i;
        }
        a_array[j] = '\0';
        b_array[j] = '\0';
        a_result[i] = a_array;
        b_result[i] = b_array;

    }
    for (i = 0; i < 10; i++)
        printf("a_result: %s b_result: %s\n",a_result[i],b_result[i]);
    return 0;
} 

I think the a_result and b_result should be the same, but it is not.

Here is the output on my computer.

a_result: jjjjjjjjj b_result: aaaaaaaaa
a_result: jjjjjjjjj b_result: bbbbbbbbb
a_result: jjjjjjjjj b_result: ccccccccc
a_result: jjjjjjjjj b_result: ddddddddd
a_result: jjjjjjjjj b_result: eeeeeeeee
a_result: jjjjjjjjj b_result: fffffffff
a_result: jjjjjjjjj b_result: ggggggggg
a_result: jjjjjjjjj b_result: hhhhhhhhh
a_result: jjjjjjjjj b_result: iiiiiiiii
a_result: jjjjjjjjj b_result: jjjjjjjjj

any explanation about this is appreciate!

for (i = 0; i < 10; i++)
{
    char a_array[10];
    ...
    a_result[i] = a_array;
}

You are declaring an array on stack in the for loop's scope. This means the array will become invalid once you leave the for loop. That is, the stuff inside a_result is invalid.

For on-stack vairables, the compiler will reuse the same memory area for a_array in every loop (which is not for b_array since a fresh memory will be received from heap by malloc). Hence for your case, a_result just has 10 copied of the same invalid pointer.

If you want a copy of the content that array, make the type of a_result to be

char a_result[10][10];

and use

memcpy(a_result[i], a_array, sizeof(a_result[i]));

to copy into the result.

The array

 char a_array[10];

goes out of scope at the end of every loop iteration and is then effectively recreated at the same memory location. You end up with a bunch of pointers pointing to the last re-creation, which contains the "jjjjjj" string.

You're continually writing over an array (a_array) at the same position, then storing the address into a_result[i] (a_result is an array of pointers). Thus, at the end, every element of a_result contains an identical address (of a_array), and a_array itself has the values written by the last iteration.

Worse, accessing the contents of a_array (including through the pointers in a_result) outside the for loop is undefined behavior. That storage can only be used within the loop body.

In contrast, you're allocating new heap memory for b_array (which is not in fact an array) every iteration, and storing this (new) pointer into b_result. Thus, you don't overwrite old values.