handling input and output of a 16 char hex integer in C++

Okay, so I have to process a very large data file.  This file contains 16 character long hex numbers (such as 090027248000289b).

I need to read these into a program to compare with a separate list of similar values (looking for those which are missing from the list.  This will require a lot of comparison checks.

Rather than comparing with strcmp() or strncmp() it would be far quicker to compare the unsigned long values directly.  This reduces all the steps present in either of these.

The plan was to read in the list of values we do have into an array of unsigned long values and then process the other file against this.

Before putting in all the logic I've just created a quick loop to read the file in and print the values out and I'm having issues.  This is where I need help.  I'm not familiar with converting back and forth between strings representing unsigned longs and actual unsigned longs.

I'm using an fgets to read each line into a buffer (and replacing the \n with a \0 to terminate).  I'm printing out this buffer first to confirm it's good so far.

Then I'm trying an sscanf from the buffer into an element of unsigned longs and printing that with a printf to confirm it converted into the integer and this is where I've run into the issue.

The declaration for my array is:
   unsigned long   parent_ids[200000];

Of course, there are two statements required to get this output; the sscanf, and the printf.

I'm trying to use:
   sscanf(input_buffer, "%lx", &parent_ids[num_parent_ids]);
for the scan and
   printf("%lx", parent_ids[num_parent_ids]);
for the print.

The problem is that the value output is always just the last 8 hex characters of the original string.  So, for 090027248000289b above what prints out is 8000289b.  In other words, I appear to only be reading the last 8 hex chars or the unsigned long is playing games with me or even the print is the root cause.  In short though, I'm struggling to know exactly which of these is happening and how to do it properly.

I would expect an unsigned long should support numbers of the above format (which is 8 bytes long).

I'm using Visual Studio for this on Windows 10.

Could someone help by pointing out which step is in error or which assumptions are wrong and how to correct?