Overlaying array of strings over char array - c

Question

I am working a function that needs to be re-entrant - the function is given a memory buffer as an argument and should use such buffer for all its memory needs. In other words, it can't use malloc, but rather should draw the memory the supplied buffer.
The challenge that I ran into is how to overlay an array of strings over a char array of given size (the buffer is supplied as char *), but my result is array of strings (char **).
Below is a repro:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#define BUFFER_SIZE 100
#define INPUT_ARRAY_SIZE 3
char *members[] = {
"alex",
"danny",
"max"
};
int main() {
// this simulates a buffer that is presented to my func
char *buffer = malloc(BUFFER_SIZE);
char *orig = buffer;
memset(buffer, NULL, BUFFER_SIZE);
// pointers will be stored at the beginning of the buffer
char **pointers = &buffer;
// strings will be stored after the pointers
char *strings = buffer + (sizeof(char *) * INPUT_ARRAY_SIZE);
for(int i = 0; i < INPUT_ARRAY_SIZE; i++) {
strncpy(strings, members[i], (strlen(members[i]) + 1));
// Need to store pointer to string in the pointers section
// pointers[i] = strings; // This does not do what I expect
strings += ((strlen(members[i]) + 1));
}
for (int i=0; i < BUFFER_SIZE; i++) {
printf("%c", orig[i]);
}
// Need to return pointers
}
With the problematic line commented out, the code above prints:
alex danny max
However, I need some assistance in figuring out how to write addresses of the strings at the beginning.
Of course, if there an easier way of accomplishing this task, please, let me know.

Answer 1

Here take a look at this.
/* conditions :
*
* 'buffer' should be large enough, 'arr_length','arr' should be valid.
*
*/
char ** pack_strings(char *buffer, char * arr[], int arr_length)
{
char **ptr = (char**) buffer;
char *string;
int index = 0;
string = buffer + (sizeof(char *) * (arr_length+1)); /* +1 for NULL */
while(index < arr_length)
{
size_t offset;
ptr[index] = string;
offset = strlen(arr[index])+1;
strcpy(string,arr[index]);
string += offset;
++index;
}
ptr[index] = NULL;
return ptr;
}
usage
char **ptr = pack_strings(buffer,members,INPUT_ARRAY_SIZE);
for (int i=0; ptr[i] != NULL; i++)
puts(ptr[i]);