Why doesn't passing an address to a pointer array store that unique value?

Question

I'm trying to create a pointer array that holds strings of various sizes. I want to prompt the user for input, store the values in the array, and then print them using the following code:

#include <stdio.h>

int main() {

    char *string_array[10];

    for (int x = 0; x < 2; x++)
    {
        char string[50];
        printf("Input Name of Fruit number %d\n", x);
        fgets(string, 50, stdin);
        string_array[x] = string;
    }

    for (int x = 0; x < 2; x++)
    {
        printf("%s\n", string_array[x]);
    }
    return 0;
}

I expected this code to take in two user prompts and store the address of each string. However, when I run the code, the final user prompt represents the value stored at each address in the pointer array. On inspection, I found that every address is the same.

I presume this is because the string variable is instantiated to the same address every time, or is never changed despite being defined in each for loop. What's happening here with the addresses to prevent this code from working?

Answer 1

for (int x = 0; x < 2; x++)
{
        char string[50];
        printf("Input Name of Fruit number %d\n", x);
        fgets(string, 50, stdin);
        string_array[x] = string;
}

The array string in this loop has block scope. It ceases to exist when the block ends. So string_array[0] and string_array[1] will be pointing to memory that no longer exists, and accessing that memory would invoke undefined behaviour.

1. Objects have a lifetime outside of which they can't be accessed.

2. Referring to an object outside of its lifetime has undefined behaviour.

3. Automatic variables have a lifetime corresponding to the execution of the their block of definition.

---

Fix:

As commented by @Weather Vane:

You are storing the same address of the same buffer, but when you report the strings, that buffer has gone out of scope. What you can do is to make a copy:

string_array[x] = strdup(string);

Another solution would be to allocate memory with malloc() and use standard strcpy(). The memory can later be resized with realloc() if the input exceeds the original length of the block.

Answer 2

Why doesn't passing an address to a pointer array store that unique value?

Question is amiss.
The same unique value address (of string[]) was stored successfully 3 times in string_array[x].

Is simply that after each iteration of the first for() block, that address was invalid.

Answer 3

I slightly modified your code. Use strcpy to pass a string to another. Note that 50 is the maximum string size you can read because of char string[50].

#include <stdio.h>
#include <string.h>

int main() {

    char string_array[10][50];
    char string[50];

    for (int x = 0; x < 2; x++)
    {
        printf("Input Name of Fruit number %d\n", x);
        fgets(string, 50, stdin);
        strcpy(string_array[x], string);
    }

    for (int x = 0; x < 2; x++)
    {
        printf("%s\n", string_array[x]);
    }
    return 0;
}