Copy a string from a pointer to an index within an array of pointers

Question

As stated in the title, I wanted to copy a string from a char pointer to location within an array of char pointers. When doing strcpy(), the output results in seg fault, but don't understand why this occurs.

The abbreviated code has the following:

void make_history(char *entry) {
    //static char past_entries[10][10];
    static char *past_entries[10];
    static int index = 0;
    static int array_index = 0;

    char *input;
    if((input = strchr(entry, '\n')) != NULL)
        *input = '\0';

    if(strcmp(entry, "history") != 0) {
        strcpy(past_entries[index], entry);
        *past_entries[index] = &entry;
        index = (index + 1) % 10;
        array_index++;
    }
}

Instead of trying to return a 2d array (which is also very tricky), I thought it would be easier to copy the date from entry to the location within the array of pointers past_entries. Again, strcpy does not work, is there a valid reason as to why this occurs, and a possible workaround or solution to this fix?

Thank you

Please show how you call make_history. Read this: minimal reproducible example — Jabberwocky
– Jabberwocky, Commented Sep 26, 2018 at 12:29
I mean, the call to the function is just make_history("message") where "message is the string which I want to add to the array. There is not much to it. — nice_remark
– nice_remark, Commented Sep 26, 2018 at 12:45

Qubit · Accepted Answer · 2018-09-26 12:42:43Z

1

In your example past_entries is just an array of pointers, but you don't allocate any memory to them (initially pointing to NULL). You then try to write to those locations, hence the crash.

To solve this, just allocate some memory before you try to copy your string to it:

past_entries[index] = malloc(strlen(entry) + 1);

Of course you should not forget to free all of this in the end.

Oh and, remove that line: *past_entries[index] = &entry;. It tries to assign a pointer to a character array to a character.

edited Sep 26, 2018 at 12:42

answered Sep 26, 2018 at 12:29

Qubit

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6 Comments

aschepler Over a year ago

Since past_entries is declared static, it is in fact guaranteed the elements will initially be null pointers. Still bad news.

Qubit Over a year ago

@aschepler Thank you, I've fixed that part of the answer.

nice_remark Over a year ago

@Qubit I wanted to have the values be static as I want the array to update for each call to the function. The general idea is to create a circular array that shows that last 10 entries made -- kind of a like a history

Qubit Over a year ago

I think you misunderstand what this does. Sure, you can have a static array of 10 character pointers and the addresses will be saved between function calls. But you still need them to point somewhere meaningful for them to store data (your entries). You can either do that with the line you have commented out (if you know an upper bound to the size of the entries) or dynamically using malloc (if you do not know an upper bound).

nice_remark Over a year ago

@Qubit I guess I just am having trouble understanding how this new pointer is returned

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