/dev/urandom vs. /dev/random

[Ron Peterson]

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: %s\n\n", dev, dat );
} else {
exit( 1 );
}

close( fd );
}

int
main( void ) {
read_random( "/dev/random" );
read_random( "/dev/urandom" );
return( 0 );
}

506$ ./test

/dev/random: 200050617063526 437602510346655 072474031767426 306605046222320 604001730615000 243427435070303 503367456120754 173510400040000 000050000000000 007737204577300 001773560405044 500220434040000 340417736104640 464040000000000 000000000000000 000000000000000 000000000000000 000000000000000 000000640423461 000000000000000 000000000000000 000000000000024 040000000041007 000000063461000 000000000000000 000000000000000 000000000000000 000000000000000 000000005604000 000000000340437 730400377305043 000000047730000 000000000000000 000000000300010 000000000000000 000000000000000 000000000000000 000000000000000 000000000450045 004000000002000 700413043304070 400001704340417 040504277307040 504003016045773 240400006704000 000000000000000 007004200064040 000340400006404 677300045004000 000005773200070 041304330407040 000170434046404 400047730704400 007700000777324 040000570400000 000000000000000 000000003404710 007047773220417 046704000000000 000777300000000 000000000770077 354040504160463 040704577337730 204700020004004 477357731104641 464143773650464 145773340400005 404177322046404 5704

/dev/urandom: 103137002760751 330471453227547 317447153305403 055445577241174 112554437473707 174466515247147 230165510321673 032612147044376 361336471103425 426601777564215 275375065067605 320467556775450 023436261315455 011306014006753 403112324146646 175070523075164 557244007157744 425536470274352 332415164131435 115125573101442 745565447177770 545201523517642 416662075710077 001223562356301 231171774154712 461726216545424 416106151774201 651063200446176 135471662402411 424412532245563 115451424762236 647021720005536 526122475115504 245147261210474 110355324013261 471071077552457 243201117615642 610621436346764 640170271720054 525337367306022 203674555216357 653675736157531 236151625103600 640302611364212 474263063442224 321677233017736 071340646247534 536645354770361 405340770217451 441530701353725 673225577676034 024065113165756 376675063056177 024303522225056 407015430715065 670415500366164 627720133246523 045641760343566 266442731667313 365016666076430 236732103100735 453566475641705 242516630662601 345037070127506 753760326567356 031232252607702 215722340421132 304741363321332 6054

--
Ron Peterson
Network & Systems Manager
Mount Holyoke College

[Villy Kruse]

On Thu, 13 Jan 2005 00:18:48 GMT,
Keith Thompson <ks***@mib.or g> wrote:

<OT>
A system call generally works by invoking code within the OS kernel
rather than in a user-mode library. On a Unix-like system, system
calls are documented by man pages in section 2, library functions in
section 3.
</OT>

Occasionally confused with the standard library function called "system".
Villy

[Keith Thompson]

Villy Kruse <ve*@station02. ohout.pharmapar tners.nl> writes:

On Thu, 13 Jan 2005 00:18:48 GMT,
Keith Thompson <ks***@mib.or g> wrote:

<OT>
A system call generally works by invoking code within the OS kernel
rather than in a user-mode library. On a Unix-like system, system
calls are documented by man pages in section 2, library functions in
section 3.
</OT>

Occasionally confused with the standard library function called "system".

Good point.

<OT>
system() is typically implemented as a library function, not as a
system call.
</OT>

--
Keith Thompson (The_Other_Keit h) ks***@mib.org <http://www.ghoti.net/~kst>
San Diego Supercomputer Center <*> <http://users.sdsc.edu/~kst>
We must do something. This is something. Therefore, we must do this.