Thread: Array of pointers - stack dumping

Hello,

I have a function that I would like to copy the contents off the pointer array into the output pointer array (and change the contents). However, it works, but I get a stack dump when I try and display the contents in the main function.

Many thanks for any suggestions,

Code:

void display_names(char **names_to_display, char **output);

int main(void)
{
    char *names[] = {"Luke", "John", "Peter", 0};
    char **my_names = names;
    char *new_output[] = {0};
    size_t i = 0;

    while(*my_names)
    {
        printf("Name: %s\n", *my_names++);
    }

    my_names = names; /* Reset */
    display_names(my_names, new_output);

    // Stack dump here.
    while(*new_output[i])
    {
        printf("Full names: %s\n", *new_output[i]);
        i++;
    }

    getchar();

    return 0;
}

void display_names(char **names_to_display, char **output)
{
    while(*names_to_display)
    {   
        *output = malloc(strlen("FullName: ") + strlen(*names_to_display) + 1); // Allocate memory

        // Copy new output
        sprintf(*output, "FullName: %s", *names_to_display++);
        printf("display_names(): Name: %s\n", *output++);
    }   
}

Hopefully you're not expecting the new_output array to be able to hold more than one element. Right?

Hello,

I thought the code below would copy the contents of each element in the array. I declare memory with enough space and use sprintf to copy the new contents over.

Code:

while(*names_to_display)
	{	
		// Stack dump here
		*output = malloc(sizeof("FullName: ") + strlen(*names_to_display) + 1); // Allocate memory
		
		// Copy new output
		sprintf(*output, "FullName: %s", *names_to_display++);
		printf("display_names(): Name: %s\n", *output++);
	}

The output in the main function should be:
FullName: Luke
FullName: John
FullName: Peter

Your code does attempt to set new_output[0], new_output[1], and new_output[2]. Since new_output[1] completely fails to exist, it is unable to do so. (You cannot realloc an array, which you aren't attempting to do in this code admittedly.) new_output is defined as an array of length 1, which it will therefore always and forever be.

Either your new_output array needs to be long enough to start with, or you need to make that dynamically-sized as well.