use malloc to set an array of strings then clear it

问题

I want to make an array of string using malloc, and then clear ALL the allocated memory, I believe that I use malloc correct but I can't understand what I'm doing wrong when I try to clear it:

#include <stdlib.h>
#include <stdio.h>
#include <string.h>
void print_arr(char* arr[]);
void clear_arr(char* arr[], int size);
int main()
{
    int number = 10;
    char **myarr = malloc((number+1)*sizeof(char*));
    for (int i = 0; i <= number; i++)
    {
        myarr[i]=NULL;
    }
    for (int i = 0; i < number; i++)
    {
        char str[] = "word";
        int len = strlen(str);
        myarr[i]=malloc((len+1)*sizeof(char));
        strcpy(myarr[i], str);
    }
    print_arr(myarr);
    clear_arr(myarr, number);
    print_arr(myarr);
    free(myarr);

   return 0; 
}
void print_arr(char* arr[])
{
    for (int i = 0; arr[i] != NULL; i++)
    {
        printf("%s\n",arr[i]);
    }

}
void clear_arr(char* arr[], int size)
{
    for (int i = 0; i < size; i++)
    {
        free(arr[i]);
    }

}

but my output for number = 3 is:

word
word
word
Pi╛
word
word

it's look like that only the first "cell" of the array is getting free and the other doesn't effected. what wrong with my clear_arr function? I'm using VScode to compile if that matters...

回答1:

That's because you try to print something that you free'd. The free function deallocate the memory but you still have the address in the array.

What you can do is:

void clear_arr(char* arr[], int size)
{
    for (int i = 0; i < size; i++)
    {
        free(arr[i]);
        arr[i] = NULL;
    }

}

This way, the print function won't loop through an array of free'd pointers.

There is a better way to malloc and copy a string, you can use the function strdup.

Your code could be optimized this way:

int number = 10;
char **myarr = malloc((number+1)*sizeof(char*));
for (int i = 0; i < number; i++)
{
  myarr[i] = strdup("word");
}

myarr[number] = NULL;

来源：https://stackoverflow.com/questions/60475182/use-malloc-to-set-an-array-of-strings-then-clear-it

标签

arrays

malloc