Turning a string of numbers into an array of individual integers

I know there is a clear solution to this, but I cant seem to find it.
In my program, I am fed a string that contains integers such as

Code:

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*str = "45678"

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and my program is simply supposed to read the number in and store each given number in a separate spot in an integer array. So basically, when my program has finished running it should be stored like:

Code:

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arr[0] = 4
arr[1] = 5
arr[2] = 6
arr[3] = 7
arr[4] = 8

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the way I have been attempting to do this was just copying it to the array like so:

Code:

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arr = malloc(sizeof(int) *(strlen(str));

for (i=0; i<strlen(str); i++)
{
a->digits[i] = str[i];
}

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however, this just seems to return an impossibly high garbage value when I do. I'm assuming the way I'm trying to store it is 'illegal', but I cant seem to find online a proper way to do it.

Is the string a predefined string? As in its always going to be the same, or is it something the user inputs and can be changed?

It is a predefined string given in the program's main function