[MAN] rand

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RAND

Section: OpenSSL (1SSL)
Updated: 2022-07-04
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NAME

openssl-rand, rand - generate pseudo-random bytes  

SYNOPSIS

openssl rand [-help] [-out file] [-rand file...] [-writerand file] [-base64] [-hex] num  

DESCRIPTION

The rand command outputs num pseudo-random bytes after seeding the random number generator once. As in other openssl command line tools, PRNG seeding uses the file $HOME/.rnd or .rnd in addition to the files given in the -rand option. A new $HOME/.rnd or .rnd file will be written back if enough seeding was obtained from these sources.  

OPTIONS

-help

Print out a usage message.

-out file

Write to file instead of standard output.

-rand file...

A file or files containing random data used to seed the random number generator. Multiple files can be specified separated by an OS-dependent character. The separator is ; for MS-Windows, , for OpenVMS, and : for all others.

[-writerand file]

Writes random data to the specified file upon exit. This can be used with a subsequent -rand flag.

-base64

Perform base64 encoding on the output.

-hex

Show the output as a hex string.

 

SEE ALSO

RAND_bytes(3)  

COPYRIGHT

Copyright 2000-2016 The OpenSSL Project Authors. All Rights Reserved.

Licensed under the OpenSSL license (the ``License''). You may not use this file except in compliance with the License. You can obtain a copy in the file LICENSE in the source distribution or at <https://www.openssl.org/source/license.html>.

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Index

NAME

SYNOPSIS

DESCRIPTION

OPTIONS

SEE ALSO

COPYRIGHT

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RAND

Section: Linux Programmer's Manual (3)
Updated: 2017-07-13
Index Return to Main Contents

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NAME

rand, rand_r, srand - pseudo-random number generator  

SYNOPSIS

#include <stdlib.h>

int rand(void);

int rand_r(unsigned int *seedp);

void srand(unsigned int seed);

Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

rand_r():

Since glibc 2.24:
    _POSIX_C_SOURCE >= 199506L
Glibc 2.23 and earlier
    _POSIX_C_SOURCE

 

DESCRIPTION

The rand() function returns a pseudo-random integer in the range 0 to RAND_MAX inclusive (i.e., the mathematical range [0, RAND_MAX]).

The srand() function sets its argument as the seed for a new sequence of pseudo-random integers to be returned by rand(). These sequences are repeatable by calling srand() with the same seed value.

If no seed value is provided, the rand() function is automatically seeded with a value of 1.

The function rand() is not reentrant, since it uses hidden state that is modified on each call. This might just be the seed value to be used by the next call, or it might be something more elaborate. In order to get reproducible behavior in a threaded application, this state must be made explicit; this can be done using the reentrant function rand_r().

Like rand(), rand_r() returns a pseudo-random integer in the range [0, RAND_MAX]. The seedp argument is a pointer to an unsigned int that is used to store state between calls. If rand_r() is called with the same initial value for the integer pointed to by seedp, and that value is not modified between calls, then the same pseudo-random sequence will result.

The value pointed to by the seedp argument of rand_r() provides only a very small amount of state, so this function will be a weak pseudo-random generator. Try drand48_r(3) instead.  

RETURN VALUE

The rand() and rand_r() functions return a value between 0 and RAND_MAX (inclusive). The srand() function returns no value.  

ATTRIBUTES

For an explanation of the terms used in this section, see attributes(7).

Interface	Attribute	Value
rand(), rand_r(), srand()	Thread safety	MT-Safe

 

CONFORMING TO

The functions rand() and srand() conform to SVr4, 4.3BSD, C89, C99, POSIX.1-2001. The function rand_r() is from POSIX.1-2001. POSIX.1-2008 marks rand_r() as obsolete.  

NOTES

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. Do not use this function in applications intended to be portable when good randomness is needed. (Use random(3) instead.)  

EXAMPLE

POSIX.1-2001 gives the following example of an implementation of rand() and srand(), possibly useful when one needs the same sequence on two different machines.

static unsigned long next = 1;

/* RAND_MAX assumed to be 32767 */ int myrand(void) {
    next = next * 1103515245 + 12345;
    return((unsigned)(next/65536) % 32768); }

void mysrand(unsigned int seed) {
    next = seed; }

The following program can be used to display the pseudo-random sequence produced by rand() when given a particular seed.

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

int main(int argc, char *argv[]) {
    int j, r, nloops;
    unsigned int seed;

    if (argc != 3) {
        fprintf(stderr, "Usage: %s <seed> <nloops>\n", argv[0]);
        exit(EXIT_FAILURE);
    }

    seed = atoi(argv[1]);
    nloops = atoi(argv[2]);

    srand(seed);
    for (j = 0; j < nloops; j++) {
        r =  rand();
        printf("%d\n", r);
    }

    exit(EXIT_SUCCESS); }  

SEE ALSO

drand48(3), random(3)  

COLOPHON

This page is part of release 4.15 of the Linux man-pages project. A description of the project, information about reporting bugs, and the latest version of this page, can be found at https://www.kernel.org/doc/man-pages/.

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Index

NAME

SYNOPSIS

DESCRIPTION

RETURN VALUE

ATTRIBUTES

CONFORMING TO

NOTES

EXAMPLE

SEE ALSO

COLOPHON

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Time: 04:45:45 GMT, September 16, 2022