Memory leak with string arrays in c - c

Question

char **add_string(char **existing, const char *string){
size_t size = 0;
while (NULL != existing[size])
{
++size;
}
char **arr = realloc(existing, (size + 2) * sizeof *arr);
arr[size] = malloc(strlen(string) + 1);
strcpy(arr[size], string);
arr[size+1] = '\0';
return arr;
}
void free_strings(char **strings)
{
size_t size = 0;
while (NULL != strings[size])
{
free(strings[size]);
++size;
}
}
I am having a memory leak at line
char **arr = realloc(existing, (size + 2) * sizeof *arr);
I thought existing memory was suppose to be free'd when using realloc? How do I fix this memory leak?
edit: added my free_string function and the main I am using to run the program.

Answer 1

You did not append the array pointed to by the pointer existing with null pointer. Thus in this loop
while (NULL != existing[size])
{
++size;
}
the function has undefined behavior.
It seems you mean the following
char ** add_string( char **existing, const char *string )
{
size_t size = 0;
while ( NULL != existing[size] ) ++size;
char **arr = realloc( existing, ( size + 2 ) * sizeof *arr );
if ( arr != NULL )
{
arr[size] = malloc( strlen( string ) + 1 );
strcpy( arr[size], string );
arr[size+1] = NULL;
}
return arr;
}
Also the function free_strings is incorrect. It should look like
void free_strings( char **strings )
{
size_t size` = 0;
do
{
free( strings[size] );
} while ( strings[size++] != NULL );
free( strings );
}