P: n/a

In the following piece of code, which simply generates a sequence of
(random) octal codes, I'm surprised by the apparent non-randomness of
/dev/random. It's not noticeable unless RAND_LENGTH is largish. I was
under the assumption that /dev/random was 'more random' than
/dev/urandom, and that it would block if it ran out of entropy until it
got more. Why am I seeing so many zeroes in my output?
#include <stdio.h>
#include <fcntl.h>
#define RAND_LEN 1024
void
read_random( const char* dev ) {
int i, fd;
char dat[RAND_LEN];
fd = open( dev, O_RDONLY );
dat[RAND_LEN] = '0';
if( fd != -1 ) {
read( fd, dat, RAND_LEN );
for( i = 0; i < RAND_LEN; i++ ) {
dat[i] = (dat[i] >> 4 & 0x07) + 48;
}
printf( '%s: %snn', dev, dat );
} else {
exit( 1 );
}
close( fd );
}
int
main( void ) {
read_random( '/dev/random' );
read_random( '/dev/urandom' );
return( 0 );
}
506$ ./test
/dev/random: 20005061706352643760251034665507247403176742630660 50462223206040017306150002434274350703035033674561 20754173510400040000000050000000000007737204577300 00177356040504450022043404000034041773610464046404 00000000000000000000000000000000000000000000000000 00000000000000000000000000640423461000000000000000 00000000000000000000000000002404000000004100700000 00634610000000000000000000000000000000000000000000 00000000000000000000000000005604000000000000340437 73040037730504300000004773000000000000000000000000 00003000100000000000000000000000000000000000000000 00000000000000000000000000000450045004000000002000 70041304330407040000170434041704050427730704050400 30160457732404000067040000000000000000000070042000 64040000340400006404677300045004000000005773200070 04130433040704000017043404640440004773070440000770 00007773240400005704000000000000000000000000000034 04710007047773220417046704000000000000777300000000 00000000077007735404050416046304070457733773020470 00200040044773577311046414641437736504641457733404 000054041773220464045704
/dev/urandom: 10313700276075133047145322754731744715330540305544 55772411741125544374737071744665152471472301655103 21673032612147044376361336471103425426601777564215 27537506506760532046755677545002343626131545501130 60140067534031123241466461750705230751645572440071 57744425536470274352332415164131435115125573101442 74556544717777054520152351764241666207571007700122 35623563012311717741547124617262165454244161061517 74201651063200446176135471662402411424412532245563 11545142476223664702172000553652612247511550424514 72612104741103553240132614710710775524572432011176 15642610621436346764640170271720054525337367306022 20367455521635765367573615753123615162510360064030 26113642124742630634422243216772330177360713406462 47534536645354770361405340770217451441530701353725 67322557767603402406511316575637667506305617702430 35222250564070154307150656704155003661646277201332 46523045641760343566266442731667313365016666076430 23673210310073545356647564170524251663066260134503 70701275067537603265673560312322526077022157223404 211323047413633213326054
--
Ron Peterson
Network & Systems Manager
Mount Holyoke College