C - 在二维数组之后释放内存

Question

I'm having a problem. I created a 2D array, and in the end, i would like to free it. Where's the problem in my code? If i run my program everything works fine, until the end. It never prints out Succes!, so I think it bugs out before that point. Any help? Here's my code :

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

int main()
{
    int n=5;
    int size=51;
    char **a_name=(char**)malloc(sizeof(char*)*n);
    for(int i=0;i<n;i++){
        a_name[i]=(char*)malloc(sizeof(char)*size);
        scanf("%s",a_name[i]);
    }
    for(int i=0;i<n;i++){
        printf("%s\n",a_name[i]);
    }
    n=6;
    *a_name=realloc(*a_name,sizeof(char*)*n);
    scanf("%s",a_name[n-1]);
    printf("\n");
    for(int i=0;i<n;i++){
        printf("%s\n",a_name[i]);
    }
    for(int i=0;i<n;i++){
        free(a_name[i]);
    }
    free(a_name);
    printf("Succes!\n");
    return 0;
}

Answer 1

Firstly, change

*a_name = realloc(*a_name, sizeof(char*) * n);

to

a_name = (char**)realloc(a_name, sizeof(char*) * n);

You don't want to reallocate the contents of the array of strings ( a_name ) to one of the strings itself ( *a_name ).

Secondly, the source of the problem is that after you called the realloc, you didn't call malloc on the newly added elements of a_name like you did here:

for(int i=0;i<n;i++) {
    a_name[i] = (char*)malloc(sizeof(char) * size); // here you allocated a 51 byte block for each char*
    scanf("%s", a_name[i]);
}

The sixth element of the array is a dynamically allocated pointer, but the array of chars it points to hasn't been allocated the 51 bytes you allocated for the first 5 strings.

The solution is to add this line before the second scanf call

a_name[n - 1] = (char*)malloc(sizeof(char) * size);

Your final code should look like this

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

int main()
{
    int n = 5;
    int size = 51;

    char **a_name = (char**)malloc(sizeof(char*)*n); 
    
    for (int i = 0; i < n; i++) {
        a_name[i] = (char*)malloc(sizeof(char)*size);
        scanf("%s", a_name[i]);
    }
    
    for (int i = 0; i < n; i++) {
        printf("%s\n", a_name[i]);
    }

    n = 6;
    a_name = (char**)realloc(a_name, sizeof(char*) * n);
    a_name[n - 1] = (char*)malloc(sizeof(char) * size); // allocate memory for the string first
    scanf("%s", a_name[n - 1]);
    printf("\n");
    for (int i = 0; i < n; i++) {
        printf("%s\n", a_name[i]);
    }
    for (int i = 0; i < n; i++) {
        free(a_name[i]);
    }
    free(a_name);
    printf("Success!\n");
    return 0;
}