In which practical situations would one want to use /dev/random in preference to /dev/urandom, and why? Basically none on a typical desktop or server system. Assuming the PRNG produces cryptographically secure random numbers, there is no reason to use /dev/random instead. The versions of rand and srand in the Linux C Library use the same random number generator as random(3) and srandom(3), so the lower-order bits should be as random as the higher-order bits. However, on older rand implementations, and on current implementations on different systems, the lower-order bits are much less random than the higher-order bits. Random ramblings of a sysadmin. Menu and widgets. Everytime i set up a new linux machine i try to remember what i did exactly to have the shell as i.
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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
The C Standard Library
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Selected Reading
Description
The C library function int rand(void) returns a pseudo-random number in the range of 0 to RAND_MAX.
RAND_MAX is a constant whose default value may vary between implementations but it is granted to be at least 32767.
Declaration
Following is the declaration for rand() function.
Dev Random Linux In C Free
Parameters
NA
Return Value
Auto tune coive changer. This function returns an integer value between 0 and RAND_MAX.
Example
The following example shows the usage of rand() function.
Let us compile and run the above program that will produce the following result −