The Open Group Base Specifications Issue 6
IEEE Std 1003.1, 2004 Edition
Copyright © 2001-2004 The IEEE and The Open Group, All Rights reserved.
A newer edition of this document exists here


rand, rand_r, srand - pseudo-random number generator


#include <stdlib.h>

int rand(void);

[TSF] [Option Start] int rand_r(unsigned *seed); [Option End]

void srand(unsigned


[CX] [Option Start] 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 IEEE Std 1003.1-2001 defers to the ISO C standard. [Option End]

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

[CX] [Option Start] The rand() function need not be reentrant. A function that is not required to be reentrant is not required to be thread-safe. [Option End]

[TSF] [Option Start] 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. [Option End]

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 IEEE Std 1003.1-2001 calls rand() or srand().


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

[TSF] [Option Start] The rand_r() function shall return a pseudo-random integer. [Option End]

The srand() function shall not return a value.


No errors are defined.

The following sections are informative.


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. */

/* 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; }


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.


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.




drand48(), the Base Definitions volume of IEEE Std 1003.1-2001, <stdlib.h>


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.

End of informative text.

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