313 lines
7.1 KiB
HTML
313 lines
7.1 KiB
HTML
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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
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<HTML><HEAD><TITLE>Man page of RAND</TITLE>
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</HEAD><BODY>
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<H1>RAND</H1>
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Section: Linux Programmer's Manual (3)<BR>Updated: 2019-03-06<BR><A HREF="#index">Index</A>
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<A HREF="/cgi-bin/man/man2html">Return to Main Contents</A><HR>
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<A NAME="lbAB"> </A>
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<H2>NAME</H2>
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rand, rand_r, srand - pseudo-random number generator
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<A NAME="lbAC"> </A>
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<H2>SYNOPSIS</H2>
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<PRE>
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<B>#include <<A HREF="file:///usr/include/stdlib.h">stdlib.h</A>></B>
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<B>int rand(void);</B>
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<B>int rand_r(unsigned int *</B><I>seedp</I><B>);</B>
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<B>void srand(unsigned int </B><I>seed</I><B>);</B>
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</PRE>
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<P>
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Feature Test Macro Requirements for glibc (see
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<B><A HREF="/cgi-bin/man/man2html?7+feature_test_macros">feature_test_macros</A></B>(7)):
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<P>
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<B>rand_r</B>():
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<DL COMPACT><DT id="1"><DD>
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Since glibc 2.24:
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<BR> _POSIX_C_SOURCE >= 199506L
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<BR>
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Glibc 2.23 and earlier
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<BR> _POSIX_C_SOURCE
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</DL>
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<A NAME="lbAD"> </A>
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<H2>DESCRIPTION</H2>
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The
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<B>rand</B>()
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function returns a pseudo-random integer in the range 0 to
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<B>RAND_MAX</B>
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inclusive (i.e., the mathematical range [0, <B>RAND_MAX</B>]).
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<P>
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The
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<B>srand</B>()
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function sets its argument as the seed for a new
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sequence of pseudo-random integers to be returned by
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<B>rand</B>().
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These sequences are repeatable by calling
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<B>srand</B>()
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with the same seed value.
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<P>
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If no seed value is provided, the
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<B>rand</B>()
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function is automatically seeded with a value of 1.
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<P>
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The function
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<B>rand</B>()
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is not reentrant, since it
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uses hidden state that is modified on each call.
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This might just be the seed value to be used by the next call,
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or it might be something more elaborate.
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In order to get reproducible behavior in a threaded
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application, this state must be made explicit;
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this can be done using the reentrant function
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<B>rand_r</B>().
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<P>
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Like
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<B>rand</B>(),
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<B>rand_r</B>()
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returns a pseudo-random integer in the range [0, <B>RAND_MAX</B>].
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The
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<I>seedp</I>
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argument is a pointer to an
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<I>unsigned int</I>
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that is used to store state between calls.
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If
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<B>rand_r</B>()
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is called with the same initial value for the integer pointed to by
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<I>seedp</I>,
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and that value is not modified between calls,
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then the same pseudo-random sequence will result.
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<P>
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The value pointed to by the
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<I>seedp</I>
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argument of
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<B>rand_r</B>()
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provides only a very small amount of state,
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so this function will be a weak pseudo-random generator.
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Try
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<B><A HREF="/cgi-bin/man/man2html?3+drand48_r">drand48_r</A></B>(3)
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instead.
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<A NAME="lbAE"> </A>
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<H2>RETURN VALUE</H2>
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The
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<B>rand</B>()
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and
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<B>rand_r</B>()
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functions return a value between 0 and
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<B>RAND_MAX</B>
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(inclusive).
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The
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<B>srand</B>()
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function returns no value.
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<A NAME="lbAF"> </A>
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<H2>ATTRIBUTES</H2>
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For an explanation of the terms used in this section, see
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<B><A HREF="/cgi-bin/man/man2html?7+attributes">attributes</A></B>(7).
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<TABLE BORDER>
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<TR VALIGN=top><TD><B>Interface</B></TD><TD><B>Attribute</B></TD><TD><B>Value</B><BR></TD></TR>
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<TR VALIGN=top><TD>
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<B>rand</B>(),
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<B>rand_r</B>(),
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<B>srand</B>()
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</TD><TD>Thread safety</TD><TD>MT-Safe<BR></TD></TR>
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</TABLE>
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<A NAME="lbAG"> </A>
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<H2>CONFORMING TO</H2>
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The functions
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<B>rand</B>()
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and
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<B>srand</B>()
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conform to SVr4, 4.3BSD, C89, C99, POSIX.1-2001.
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The function
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<B>rand_r</B>()
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is from POSIX.1-2001.
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POSIX.1-2008 marks
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<B>rand_r</B>()
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as obsolete.
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<A NAME="lbAH"> </A>
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<H2>NOTES</H2>
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The versions of
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<B>rand</B>()
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and
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<B>srand</B>()
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in the Linux C Library use the same random number generator as
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<B><A HREF="/cgi-bin/man/man2html?3+random">random</A></B>(3)
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and
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<B><A HREF="/cgi-bin/man/man2html?3+srandom">srandom</A></B>(3),
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so the lower-order bits should be as random as the higher-order bits.
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However, on older
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<B>rand</B>()
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implementations, and on current implementations on different systems,
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the lower-order bits are much less random than the higher-order bits.
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Do not use this function in applications intended to be portable
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when good randomness is needed.
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(Use
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<B><A HREF="/cgi-bin/man/man2html?3+random">random</A></B>(3)
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instead.)
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<A NAME="lbAI"> </A>
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<H2>EXAMPLE</H2>
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POSIX.1-2001 gives the following example of an implementation of
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<B>rand</B>()
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and
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<B>srand</B>(),
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possibly useful when one needs the same sequence on two different machines.
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<P>
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static unsigned long next = 1;
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<P>
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/* RAND_MAX assumed to be 32767 */
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int myrand(void) {
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<BR> next = next * 1103515245 + 12345;
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<BR> return((unsigned)(next/65536) % 32768);
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}
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<P>
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void mysrand(unsigned int seed) {
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<BR> next = seed;
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}
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<P>
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The following program can be used to display the
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pseudo-random sequence produced by
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<B>rand</B>()
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when given a particular seed.
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<P>
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#include <<A HREF="file:///usr/include/stdlib.h">stdlib.h</A>>
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#include <<A HREF="file:///usr/include/stdio.h">stdio.h</A>>
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<P>
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int
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main(int argc, char *argv[])
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{
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<BR> int j, r, nloops;
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<BR> unsigned int seed;
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<P>
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<BR> if (argc != 3) {
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<BR> fprintf(stderr, "Usage: %s <seed> <nloops>\n", argv[0]);
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<BR> exit(EXIT_FAILURE);
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<BR> }
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<P>
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<BR> seed = atoi(argv[1]);
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<BR> nloops = atoi(argv[2]);
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<P>
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<BR> srand(seed);
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<BR> for (j = 0; j < nloops; j++) {
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<BR> r = rand();
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<BR> printf("%d\n", r);
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<BR> }
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<P>
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<BR> exit(EXIT_SUCCESS);
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}
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<A NAME="lbAJ"> </A>
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<H2>SEE ALSO</H2>
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<B><A HREF="/cgi-bin/man/man2html?3+drand48">drand48</A></B>(3),
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<B><A HREF="/cgi-bin/man/man2html?3+random">random</A></B>(3)
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<A NAME="lbAK"> </A>
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<H2>COLOPHON</H2>
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This page is part of release 5.05 of the Linux
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<I>man-pages</I>
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project.
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A description of the project,
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information about reporting bugs,
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and the latest version of this page,
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can be found at
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<A HREF="https://www.kernel.org/doc/man-pages/.">https://www.kernel.org/doc/man-pages/.</A>
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<P>
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<HR>
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<A NAME="index"> </A><H2>Index</H2>
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<DL>
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<DT id="2"><A HREF="#lbAB">NAME</A><DD>
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<DT id="3"><A HREF="#lbAC">SYNOPSIS</A><DD>
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<DT id="4"><A HREF="#lbAD">DESCRIPTION</A><DD>
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<DT id="5"><A HREF="#lbAE">RETURN VALUE</A><DD>
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<DT id="6"><A HREF="#lbAF">ATTRIBUTES</A><DD>
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<DT id="7"><A HREF="#lbAG">CONFORMING TO</A><DD>
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<DT id="8"><A HREF="#lbAH">NOTES</A><DD>
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<DT id="9"><A HREF="#lbAI">EXAMPLE</A><DD>
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<DT id="10"><A HREF="#lbAJ">SEE ALSO</A><DD>
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<DT id="11"><A HREF="#lbAK">COLOPHON</A><DD>
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</DL>
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<HR>
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This document was created by
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<A HREF="/cgi-bin/man/man2html">man2html</A>,
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using the manual pages.<BR>
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Time: 00:05:53 GMT, March 31, 2021
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</BODY>
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</HTML>
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