# Compute the factorial of almost any number Mainly an excuse to use GMP

fact.c

#include <stdlib.h>
#include <stdio.h>

#include <gmp.h>

/**
 * Compute a Factorial
 * 
 * returns a string representation of the number `n` in base 10. 
 */
char* fact(unsigned int n)
{
	mpz_t res;
	char* str;

	mpz_init_set_ui(res, 1);
	
	while (n > 1) {
		mpz_mul_ui(res, res, n--);
	}
	
	str = mpz_get_str(NULL, 10, res);
	
	mpz_clear(res);
	
	return str;
}

int main (int argc, char* argv[])
{
	unsigned int n;
	char* f;
	
	for (int i = 1; i < argc; i++) {
		n = atoi(argv[i]);
		f = fact(n);
		printf("%d! = %s\n", n, f);
		free(f);
	}
}

Computes the following in < 1 second:
100000! = 28242294079603478742934215780245355184774949260912248505789180865429779509010630178725517714138311636107136117373619629514749961831239180227260734090938324220055569688667840380377379444961268380147875111966906386044926144538111370090160766866405407170565952261298041958356778909047541512871140836924251535293096260672271038744246088635454363982931747761775532621851126474855864918180381519877161219681514129902304463824068896508357500229649939642364256635271614935207801331202943393059481996043539694202547610187382521727119665242224629786132218975049740195171653153048987483605056695271548017651216213800410981680797345354785175202462194504834501377326310693909350359885988263210528414140015756786096090291650746966135452625307325475302156830715404249700297210424646577137505616581728515517216288676066448180981858456491209123026827681473512252959123566976223847532826157700469481149648484913019536713400018031434012380511368173835296047361782363590900324442979506880200425497075817428109200515871132725419763955449965643205610662338099441079965090239835058688262541015044370484320696146208300581622032445457656923187565655142229321450667889684581664034312175421203842609068583814496145348996192319004679779630870771992965230958496008255678095829679128378996667544726319901350598963786237496380026027212712803720071903325318612085809426691843188133798061723913913711424525415380091954821781271034190574047677909474085485853003394617268058261532865988710695226333046882761562985860251045159006884365497700918591314079336300094266872450185905716172910319364909910982282171113034188814194168741059041047545569219310938636141579837579416940455279389371093841823524834691909773492295125953071398328562544812179152705046595958654656954779020092178370816222399126665698730839796362496210376149233471327600693023992176735118910577741564091153375237377967391642998745016394658581996592998334666059301647826349860479862872111189486311700077299334678795331022087853029064847682202470877226978827225703101293175999754152186080437662400387353762689053755715400425540320217748368675996336131595004238899499641854577857071050256848118475143977630753819681970818093539794067232920368908922783200763693358406584171498415879485891096945905672899713798415851643549629222750438948618153020439345247463008276498877765886574308087875413413076591503572256732064081391402979634937427007828401032999292566908606022083315983752819999156718709016301529164719115947920455896541741086277247149539333302596387664521161931270883282630583794072824775649170677720215796137315566388581174578829571182034171407714529475535797108182669757987365913307838682857595034767823707313759310787385400846883789925536631162479425047512615104634570611952476082258991470889730484997143514677051205872227048811238940155630842423255665822345240232500641846295922034770232544316636788345242347863518408683775566104412353145447124693099240549723842158184312016756039050317199725245656578583775808577538752965876118431967190114283315961863712299043641019251045901748994857351847545389669361365964528208636002427760428785759972672367893509433104811637811370730616577019136151552105444134731885196163472343099052391233625740331701021209610386118438680635978534912772759311376155125835880861436096960784145435311604736555422959099787360857819765485869182663659301669728836183187527585681057756759660676312618449087454840993232472607816610018772424475784827573256346192806768258063000257029675179576879240770103278259500518336875928686092661235311958664064242828565182177
... Snip so that I can post this comment ....
0000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000