Question: While reading the [pigeonhole principle](http://en.wikipedia.org/wiki/Pigeonhole_principle) on Wikipedia, I come across - "collisions are inevitable in a hash table because the number of possible keys exceeds the number of indices in the array. No hashing algorithm, no matter how clever, can avoid these collisions". But isn't [gperf](http://www.gnu.org/software/gperf/) doing this exactly?

Please enlighten.

Answer: 
`gperf` creates the hash function and the hash table basing on a **predefined** list of strings.

It therefore appears to me that `gperf` creates the hashes long enough so that there are enough possibilities.  

That's what you can do only if you know every possible key upfront - which is an assumption which didn't hold for the description in the wikipedia's entry, which was apparently related to a "non-constant" hash-table.
From the gperf's website: "For a given list of strings, it produces a hash function and hash table,..." - which means it has to know all the strings previously in order to prepare a function, that works without collisions.

Usual hash functions you are using in general programming languages are able to handle any strings as input one after another (the list is not given at once) and therefore they can produce collisions.