The Open Group Base Specifications Issue 7
IEEE Std 1003.1, 2013 Edition
Copyright © 2001-2013 The IEEE and The Open Group

#### NAME

> rand, rand_r, srand - pseudo-random number generator

#### SYNOPSIS

> ``#include <stdlib.h>
> int rand(void);
> ``

> ``[OB CX] int rand_r(unsigned *`seed`);``
> `
> void srand(unsigned` seed`);
> `

#### DESCRIPTION

> For rand() and srand(): [CX] The functionality described on this reference page is aligned with the ISO C standard. Any conflict between the requirements described here and the ISO C standard is unintentional. This volume of POSIX.1-2008 defers to the ISO C standard.

> The rand() function shall compute a sequence of pseudo-random integers in the range [0, {RAND_MAX}] [XSI] with a period of at least 232.

> [CX] The rand() function need not be thread-safe.

> [OB CX] The rand_r() function shall compute a sequence of pseudo-random integers in the range [0, {RAND_MAX}]. (The value of the {RAND_MAX} macro shall be at least 32767.)

> If rand_r() is called with the same initial value for the object pointed to by seed and that object is not modified between successive returns and calls to rand_r(), the same sequence shall be generated.

> The srand() function uses the argument as a seed for a new sequence of pseudo-random numbers to be returned by subsequent calls to rand(). If srand() is then called with the same seed value, the sequence of pseudo-random numbers shall be repeated. If rand() is called before any calls to srand() are made, the same sequence shall be generated as when srand() is first called with a seed value of 1.

> The implementation shall behave as if no function defined in this volume of POSIX.1-2008 calls rand() or srand().

#### RETURN VALUE

> The rand() function shall return the next pseudo-random number in the sequence.

> [OB CX] The rand_r() function shall return a pseudo-random integer.

> The srand() function shall not return a value.

#### ERRORS

> No errors are defined.

The following sections are informative.

#### EXAMPLES

> ##### Generating a Pseudo-Random Number Sequence

> The following example demonstrates how to generate a sequence of pseudo-random numbers.

>     #include <stdio.h>
>     #include <stdlib.h>
>     ...
>         long count, i;
>         char *keystr;
>         int elementlen, len;
>         char c;
>     ...
>     /* Initial random number generator. */
>         srand(1);

>         /* Create keys using only lowercase characters */
>         len = 0;
>         for (i=0; i<count; i++) {
>             while (len < elementlen) {
>                 c = (char) (rand() % 128);
>                 if (islower(c))
>                     keystr[len++] = c;
>             }

>             keystr[len] = '\0';
>             printf("%s Element%0*ld\n", keystr, elementlen, i);
>             len = 0;
>         }

> ##### Generating the Same Sequence on Different Machines

> The following code defines a pair of functions that could be incorporated into applications wishing to ensure that the same sequence of numbers is generated across different machines.

>     static unsigned long next = 1;
>     int myrand(void)  /* RAND_MAX assumed to be 32767. */
>     {
>         next = next * 1103515245 + 12345;
>         return((unsigned)(next/65536) % 32768);
>     }

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

#### APPLICATION USAGE

> The drand48() function provides a much more elaborate random number generator.

> The limitations on the amount of state that can be carried between one function call and another mean the rand_r() function can never be implemented in a way which satisfies all of the requirements on a pseudo-random number generator. Therefore this function should be avoided whenever non-trivial requirements (including safety) have to be fulfilled.

#### RATIONALE

> The ISO C standard rand() and srand() functions allow per-process pseudo-random streams shared by all threads. Those two functions need not change, but there has to be mutual-exclusion that prevents interference between two threads concurrently accessing the random number generator.

> With regard to rand(), there are two different behaviors that may be wanted in a multi-threaded program:

> 1. A single per-process sequence of pseudo-random numbers that is shared by all threads that call rand()

> 2. A different sequence of pseudo-random numbers for each thread that calls rand()

> This is provided by the modified thread-safe function based on whether the seed value is global to the entire process or local to each thread.

> This does not address the known deficiencies of the rand() function implementations, which have been approached by maintaining more state. In effect, this specifies new thread-safe forms of a deficient function.

#### FUTURE DIRECTIONS

> The rand_r() function may be removed in a future version.

#### SEE ALSO

> drand48

> XBD <stdlib.h>

#### CHANGE HISTORY

> First released in Issue 1. Derived from Issue 1 of the SVID.

#### Issue 5

> The rand_r() function is included for alignment with the POSIX Threads Extension.

> A note indicating that the rand() function need not be reentrant is added to the DESCRIPTION.

#### Issue 6

> Extensions beyond the ISO C standard are marked.

> The rand_r() function is marked as part of the Thread-Safe Functions option.

#### Issue 7

> Austin Group Interpretation 1003.1-2001 #156 is applied.

> The rand_r() function is marked obsolescent.

End of informative text.

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