The Single UNIX ® Specification, Version 2
Copyright © 1997 The Open Group


rand, rand_r - pseudo-random number generator


#include <stdlib.h>

int rand (void);
void srand(unsigned int seed);
int rand_r(unsigned int *seed);


The rand() function computes a sequence of pseudo-random integers in the range 0 to {RAND_MAX}  with a period of at least 232.

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 will be repeated. If rand() is called before any calls to srand() are made, the same sequence will be generated as when srand() is first called with a seed value of 1.

The implementation will behave as if no function defined in this document calls rand() or srand.

The rand() interface need not be reentrant.

The rand_r() function computes a sequence of pseudo-random integers in the range 0 to {RAND_MAX}. (The value of the {RAND_MAX} macro will 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 rand() function returns the next pseudo-random number in the sequence. The srand() function returns no value.

The rand_r() function returns a pseudo-random integer.


No errors are defined.




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

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

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

void mysrand(unsigned int seed)
    next = seed;




drand48(), srand(), <stdlib.h>.


rand() derived from Issue 1 of the SVID.

rand_r() derived from the POSIX Threads Extension (1003.1c-1995).

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Copyright © 1997 The Open Group
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