Changing contents in array of character pointers

Question

This seems like it should be easy but i've spent way too much time on it. Hopefully someone can help.

char *string_labels[5] = { "one", "two", "three", "four", "five" };

void myFunction(void)
{

    //can print them just like expected
    for(i=0; i < 5; i++)
    {
        printf("%s\n", string_labels[i]);
    }

    //how can i change the contents of one of the elements??
    sprintf(string_labels[0], "xxx"); <-crashes

}

Answer 1

It crashes because it's in read-only memory. Try

char string_labels[][6] = { "one", "two", "three", "four", "five" };
sprintf(string_labels[0], "xxx");

Answer 2

To do this, you need to use a character array, so that you actually have some runtime-writable space to modify:

char string_labels[][20] = { "one", "two", "three", "four", "five" };

void myFunction(void)
{
    /* Printing works like before (could be improved, '5' is nasty). */
    for(i=0; i < 5; i++)
    {
        printf("%s\n", string_labels[i]);
    }

    /* Now, modifying works too (could be improved, use snprintf() for instance. */
    sprintf(string_labels[0], "xxx");
}

Answer 3

string_labels is an array of char pointers pointing to string literals. Since the string literals are read-only, any attempt to modify them leads to undefined behavior.

You can change the declaration of string_labels as below to make your sprintf work:

char string_labels[][6] = { "one", "two", "three", "four", "five" };

Answer 4

Each of string_labels[i] points to a string literal, and attempting to modify the contents of a string literal invokes undefined behavior.

You'll need to declare string_labels as an array of arrays of char , rather than as an array of pointers to char :

#define MAX_LABEL_LEN ... // however big the label can get + 0 terminator

char string_labels[][MAX_LABEL_LEN]={"one", "two", "three", "four", "five"};

This declares a 5-element array (size taken from the number of initializers) of MAX_LABEL_LEN arrays of char . Now you can write to the contents of string_labels[i] .