Section (3) random

Linux manual pages Section 3  


random, srandom, initstate, setstate — random number generator


#include <stdlib.h>
long int random( void);  
void srandom( unsigned int seed);
char *initstate( unsigned int seed,
  char *state,
  size_t n);
char *setstate( char *state);
[Note] Note
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
random(), srandom(), initstate(), setstate():
_XOPEN_SOURCE >= 500 || /* Glibc since 2.19:
*/ _DEFAULT_SOURCE || /* Glibc versions <= 2.19:


The random() function uses a nonlinear additive feedback random number generator employing a default table of size 31 long integers to return successive pseudo-random numbers in the range from 0 to RAND_MAX. The period of this random number generator is very large, approximately 16 * ((2^31) − 1).

The srandom() function sets its argument as the seed for a new sequence of pseudo-random integers to be returned by random(). These sequences are repeatable by calling srandom() with the same seed value. If no seed value is provided, the random() function is automatically seeded with a value of 1.

The initstate() function allows a state array state to be initialized for use by random(). The size of the state array n is used by initstate() to decide how sophisticated a random number generator it should use—the larger the state array, the better the random numbers will be. Current optimal values for the size of the state array n are 8, 32, 64, 128, and 256 bytes; other amounts will be rounded down to the nearest known amount. Using less than 8 bytes results in an error. seed is the seed for the initialization, which specifies a starting point for the random number sequence, and provides for restarting at the same point.

The setstate() function changes the state array used by the random() function. The state array state is used for random number generation until the next call to initstate() or setstate(). state must first have been initialized using initstate() or be the result of a previous call of setstate().


The random() function returns a value between 0 and RAND_MAX. The srandom() function returns no value.

The initstate() function returns a pointer to the previous state array. On error, errno is set to indicate the cause.

On success, setstate() returns a pointer to the previous state array. On error, it returns NULL, with errno set to indicate the cause of the error.



The state argument given to setstate() was NULL.


A state array of less than 8 bytes was specified to initstate().


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

Interface Attribute Value

random(), srandom(),

initstate(), setstate()

Thread safety MT-Safe


POSIX.1-2001, POSIX.1-2008, 4.3BSD.


The random() function should not be used in multithreaded programs where reproducible behavior is required. Use random_r(3) for that purpose.

Random-number generation is a complex topic. Numerical Recipes in C: The Art of Scientific Computing (William H. Press, Brian P. Flannery, Saul A. Teukolsky, William T. Vetterling; New York: Cambridge University Press, 2007, 3rd ed.) provides an excellent discussion of practical random-number generation issues in Chapter 7 (Random Numbers).

For a more theoretical discussion which also covers many practical issues in depth, see Chapter 3 (Random Numbers) in Donald E. Knuth_zsingle_quotesz_s The Art of Computer Programming, volume 2 (Seminumerical Algorithms), 2nd ed.; Reading, Massachusetts: Addison-Wesley Publishing Company, 1981.


According to POSIX, initstate() should return NULL on error. In the glibc implementation, errno is (as specified) set on error, but the function does not return NULL.


getrandom(2), drand48(3), rand(3), random_r(3), srand(3)


This page is part of release 5.04 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−pages/.

  Copyright 1993 David Metcalfe (

Permission is granted to make and distribute verbatim copies of this
manual provided the copyright notice and this permission notice are
preserved on all copies.

Permission is granted to copy and distribute modified versions of this
manual under the conditions for verbatim copying, provided that the
entire resulting derived work is distributed under the terms of a
permission notice identical to this one.

Since the Linux kernel and libraries are constantly changing, this
manual page may be incorrect or out-of-date.  The author(s) assume no
responsibility for errors or omissions, or for damages resulting from
the use of the information contained herein.  The author(s) may not
have taken the same level of care in the production of this manual,
which is licensed free of charge, as they might when working

Formatted or processed versions of this manual, if unaccompanied by
the source, must acknowledge the copyright and authors of this work.

References consulted:
    Linux libc source code
    Lewine_zsingle_quotesz_s _POSIX Programmer_zsingle_quotesz_s Guide_ (O_zsingle_quotesz_Reilly & Associates, 1991)
    386BSD man pages
Modified Sun Mar 28 00:25:51 1993, David Metcalfe
Modified Sat Jul 24 18:13:39 1993 by Rik Faith (
Modified Sun Aug 20 21:47:07 2000, aeb

Section (4) random

Linux manual pages Section 4  


random, urandom — kernel random number source devices


#include <linux/random.h>
int ioctl( fd,


The character special files /dev/random and /dev/urandom (present since Linux 1.3.30) provide an interface to the kernel_zsingle_quotesz_s random number generator. The file /dev/random has major device number 1 and minor device number 8. The file /dev/urandom has major device number 1 and minor device number 9.

The random number generator gathers environmental noise from device drivers and other sources into an entropy pool. The generator also keeps an estimate of the number of bits of noise in the entropy pool. From this entropy pool, random numbers are created.

Linux 3.17 and later provides the simpler and safer getrandom(2) interface which requires no special files; see the getrandom(2) manual page for details.

When read, the /dev/urandom device returns random bytes using a pseudorandom number generator seeded from the entropy pool. Reads from this device do not block (i.e., the CPU is not yielded), but can incur an appreciable delay when requesting large amounts of data.

When read during early boot time, /dev/urandom may return data prior to the entropy pool being initialized. If this is of concern in your application, use getrandom(2) or /dev/random instead.

The /dev/random device is a legacy interface which dates back to a time where the cryptographic primitives used in the implementation of /dev/urandom were not widely trusted. It will return random bytes only within the estimated number of bits of fresh noise in the entropy pool, blocking if necessary. /dev/random is suitable for applications that need high quality randomness, and can afford indeterminate delays.

When the entropy pool is empty, reads from /dev/random will block until additional environmental noise is gathered. If open(2) is called for /dev/random with the O_NONBLOCK flag, a subsequent read(2) will not block if the requested number of bytes is not available. Instead, the available bytes are returned. If no byte is available, read(2) will return -1 and errno will be set to EAGAIN.

The O_NONBLOCK flag has no effect when opening /dev/urandom. When calling read(2) for the device /dev/urandom, reads of up to 256 bytes will return as many bytes as are requested and will not be interrupted by a signal handler. Reads with a buffer over this limit may return less than the requested number of bytes or fail with the error EINTR, if interrupted by a signal handler.

Since Linux 3.16, a read(2) from /dev/urandom will return at most 32 MB. A read(2) from /dev/random will return at most 512 bytes (340 bytes on Linux kernels before version 2.6.12).

Writing to /dev/random or /dev/urandom will update the entropy pool with the data written, but this will not result in a higher entropy count. This means that it will impact the contents read from both files, but it will not make reads from /dev/random faster.


The /dev/random interface is considered a legacy interface, and /dev/urandom is preferred and sufficient in all use cases, with the exception of applications which require randomness during early boot time; for these applications, getrandom(2) must be used instead, because it will block until the entropy pool is initialized.

If a seed file is saved across reboots as recommended below (all major Linux distributions have done this since 2000 at least), the output is cryptographically secure against attackers without local root access as soon as it is reloaded in the boot sequence, and perfectly adequate for network encryption session keys. Since reads from /dev/random may block, users will usually want to open it in nonblocking mode (or perform a read with timeout), and provide some sort of user notification if the desired entropy is not immediately available.


If your system does not have /dev/random and /dev/urandom created already, they can be created with the following commands:

mknod −m 666 /dev/random c 1 8
mknod −m 666 /dev/urandom c 1 9
chown root:root /dev/random /dev/urandom

When a Linux system starts up without much operator interaction, the entropy pool may be in a fairly predictable state. This reduces the actual amount of noise in the entropy pool below the estimate. In order to counteract this effect, it helps to carry entropy pool information across shut-downs and start-ups. To do this, add the lines to an appropriate script which is run during the Linux system start-up sequence:

echo Initializing random number generator...
# Carry a random seed from start-up to start-up
# Load and then save the whole entropy pool
if [ −f $random_seed ]; then
    cat $random_seed >/dev/urandom
    touch $random_seed
chmod 600 $random_seed
[ −r $poolfile ] && bits=$(cat $poolfile) || bits=4096
bytes=$(expr $bits / 8)
dd if=/dev/urandom of=$random_seed count=1 bs=$bytes

Also, add the following lines in an appropriate script which is run during the Linux system shutdown:

# Carry a random seed from shut-down to start-up
# Save the whole entropy pool
echo Saving random seed...
touch $random_seed
chmod 600 $random_seed
[ −r $poolfile ] && bits=$(cat $poolfile) || bits=4096
bytes=$(expr $bits / 8)
dd if=/dev/urandom of=$random_seed count=1 bs=$bytes

In the above examples, we assume Linux 2.6.0 or later, where /proc/sys/kernel/random/poolsize returns the size of the entropy pool in bits (see below).

/proc interfaces

The files in the directory /proc/sys/kernel/random (present since 2.3.16) provide additional information about the /dev/random device:


This read-only file gives the available entropy, in bits. This will be a number in the range 0 to 4096.


This file gives the size of the entropy pool. The semantics of this file vary across kernel versions:

Linux 2.4:

This file gives the size of the entropy pool in bytes. Normally, this file will have the value 512, but it is writable, and can be changed to any value for which an algorithm is available. The choices are 32, 64, 128, 256, 512, 1024, or 2048.

Linux 2.6 and later:

This file is read-only, and gives the size of the entropy pool in bits. It contains the value 4096.


This file contains the number of bits of entropy required for waking up processes that sleep waiting for entropy from /dev/random. The default is 64.


This file contains the number of bits of entropy below which we wake up processes that do a select(2) or poll(2) for write access to /dev/random. These values can be changed by writing to the files.

uuid and boot_id

These read-only files contain random strings like 6fd5a44b-35f4-4ad4-a9b9-6b9be13e1fe9. The former is generated afresh for each read, the latter was generated once.

ioctl(2) interface

The following ioctl(2) requests are defined on file descriptors connected to either /dev/random or /dev/urandom. All requests performed will interact with the input entropy pool impacting both /dev/random and /dev/urandom. The CAP_SYS_ADMIN capability is required for all requests except RNDGETENTCNT.


Retrieve the entropy count of the input pool, the contents will be the same as the entropy_avail file under proc. The result will be stored in the int pointed to by the argument.


Increment or decrement the entropy count of the input pool by the value pointed to by the argument.


Removed in Linux 2.6.9.


Add some additional entropy to the input pool, incrementing the entropy count. This differs from writing to /dev/random or /dev/urandom, which only adds some data but does not increment the entropy count. The following structure is used:

struct rand_pool_info {
  int   entropy_count;  
  int   buf_size;  
  __u32   buf[0];  

Here entropy_count is the value added to (or subtracted from) the entropy count, and buf is the buffer of size buf_size which gets added to the entropy pool.


Zero the entropy count of all pools and add some system data (such as wall clock) to the pools.





For an overview and comparison of the various interfaces that can be used to obtain randomness, see random(7).


During early boot time, reads from /dev/urandom may return data prior to the entropy pool being initialized.


mknod(1), getrandom(2), random(7)

RFC 1750, Randomness Recommendations for Security


This page is part of release 4.16 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−pages/.

  Copyright (c) 1997 John S. Kallal (

This is free documentation; you can redistribute it and/or
modify it under the terms of the GNU General Public License as
published by the Free Software Foundation; either version 2 of
the License, or (at your option) any later version.

Some changes by tytso and aeb.

2004-12-16, John V. Belmonte/mtk, Updated init and quit scripts
2004-04-08, AEB, Improved description of read from /dev/urandom
2008-06-20, George Spelvin <>,
            Matt Mackall <>

Section (7) random

Linux manual pages Section 7  


random — overview of interfaces for obtaining randomness


The kernel random-number generator relies on entropy gathered from device drivers and other sources of environmental noise to seed a cryptographically secure pseudorandom number generator (CSPRNG). It is designed for security, rather than speed.

The following interfaces provide access to output from the kernel CSPRNG:

  • The /dev/urandom and /dev/random devices, both described in random(4). These devices have been present on Linux since early times, and are also available on many other systems.

  • The Linux-specific getrandom(2) system call, available since Linux 3.17. This system call provides access either to the same source as /dev/urandom (called the urandom source in this page) or to the same source as /dev/random (called the random source in this page). The default is the urandom source; the random source is selected by specifying the GRND_RANDOM flag to the system call. (The getentropy(3) function provides a slightly more portable interface on top of getrandom(2).)

Initialization of the entropy pool

The kernel collects bits of entropy from the environment. When a sufficient number of random bits has been collected, the entropy pool is considered to be initialized.

Choice of random source

Unless you are doing long-term key generation (and most likely not even then), you probably shouldn_zsingle_quotesz_t be reading from the /dev/random device or employing getrandom(2) with the GRND_RANDOM flag. Instead, either read from the /dev/urandom device or employ getrandom(2) without the GRND_RANDOM flag. The cryptographic algorithms used for the urandom source are quite conservative, and so should be sufficient for all purposes.

The disadvantage of GRND_RANDOM and reads from /dev/random is that the operation can block for an indefinite period of time. Furthermore, dealing with the partially fulfilled requests that can occur when using GRND_RANDOM or when reading from /dev/random increases code complexity.

Monte Carlo and other probabilistic sampling applications

Using these interfaces to provide large quantities of data for Monte Carlo simulations or other programs/algorithms which are doing probabilistic sampling will be slow. Furthermore, it is unnecessary, because such applications do not need cryptographically secure random numbers. Instead, use the interfaces described in this page to obtain a small amount of data to seed a user-space pseudorandom number generator for use by such applications.

Comparison between getrandom, /dev/urandom, and /dev/random

The following table summarizes the behavior of the various interfaces that can be used to obtain randomness. GRND_NONBLOCK is a flag that can be used to control the blocking behavior of getrandom(2). The final column of the table considers the case that can occur in early boot time when the entropy pool is not yet initialized.

Interface Pool Blocking behavior Behavior when pool is not yet ready
.I /dev/random Blocking pool If entropy too low, blocks until there is enough entropy again Blocks until enough entropy gathered
.I /dev/urandom CSPRNG output Never blocks Returns output from uninitialized CSPRNG (may be low entropy and unsuitable for cryptography)
getrandom() Same as .I /dev/urandom Does not block once is pool ready Blocks until pool ready
getrandom() GRND_RANDOM Same as .I /dev/random If entropy too low, blocks until there is enough entropy again Blocks until pool ready
getrandom() GRND_NONBLOCK Same as .I /dev/urandom Does not block once is pool ready EAGAIN
getrandom() GRND_RANDOM + GRND_NONBLOCK Same as .I /dev/random EAGAIN if not enough entropy available EAGAIN

Generating cryptographic keys

The amount of seed material required to generate a cryptographic key equals the effective key size of the key. For example, a 3072-bit RSA or Diffie-Hellman private key has an effective key size of 128 bits (it requires about 2^128 operations to break) so a key generator needs only 128 bits (16 bytes) of seed material from /dev/random.

While some safety margin above that minimum is reasonable, as a guard against flaws in the CSPRNG algorithm, no cryptographic primitive available today can hope to promise more than 256 bits of security, so if any program reads more than 256 bits (32 bytes) from the kernel random pool per invocation, or per reasonable reseed interval (not less than one minute), that should be taken as a sign that its cryptography is not skillfully implemented.


getrandom(2), getauxval(3), getentropy(3), random(4), urandom(4), signal(7)


This page is part of release 4.16 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−pages/.

  Copyright (C) 2008, George Spelvin <>,
and Copyright (C) 2008, Matt Mackall <>
and Copyright (C) 2016, Laurent Georget <>
and Copyright (C) 2016, Nikos Mavrogiannopoulos <>

Permission is granted to make and distribute verbatim copies of this
manual provided the copyright notice and this permission notice are
preserved on all copies.

Permission is granted to copy and distribute modified versions of
this manual under the conditions for verbatim copying, provided that
the entire resulting derived work is distributed under the terms of
a permission notice identical to this one.

Since the Linux kernel and libraries are constantly changing, this
manual page may be incorrect or out-of-date.  The author(s) assume.
no responsibility for errors or omissions, or for damages resulting.
from the use of the information contained herein.  The author(s) may.
not have taken the same level of care in the production of this.
manual, which is licensed free of charge, as they might when working.

Formatted or processed versions of this manual, if unaccompanied by
the source, must acknowledge the copyright and authors of this work.

The following web page is quite informative: