rand(3) Library Functions Manual rand(3)
NAME
rand, rand_r, srand - pseudo-random number generator
LIBRARY
Standard C library (libc, -lc)
SYNOPSIS
#include <stdlib.h>
int rand(void);
void srand(unsigned int seed);
[[deprecated]] int rand_r(unsigned int *seedp);
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
rand_r():
Since glibc 2.24:
_POSIX_C_SOURCE >= 199506L
glibc 2.23 and earlier
_POSIX_C_SOURCE
DESCRIPTION
The rand() function returns a pseudo-random integer in the range 0 to
RAND_MAX inclusive (i.e., the mathematical range [0, RAND_MAX]).
The srand() function sets its argument as the seed for a new sequence of
pseudo-random integers to be returned by rand(). These sequences are
repeatable by calling srand() with the same seed value.
If no seed value is provided, the rand() function is automatically
seeded with a value of 1.
The function rand() is not reentrant, since it uses hidden state that is
modified on each call. This might just be the seed value to be used by
the next call, or it might be something more elaborate. In order to get
reproducible behavior in a threaded application, this state must be made
explicit; this can be done using the reentrant function rand_r().
Like rand(), rand_r() returns a pseudo-random integer in the range
[0, RAND_MAX]. The seedp argument is a pointer to an unsigned int that
is used to store state between calls. If rand_r() is called with the
same initial value for the integer pointed to by seedp, and that value
is not modified between calls, then the same pseudo-random sequence will
result.
The value pointed to by the seedp argument of rand_r() provides only a
very small amount of state, so this function will be a weak pseudo-ran-
dom generator. Try drand48_r(3) instead.
RETURN VALUE
The rand() and rand_r() functions return a value between 0 and RAND_MAX
(inclusive). The srand() function returns no value.
ATTRIBUTES
For an explanation of the terms used in this section, see attributes(7).
┌────────────────────────────────────────────┬───────────────┬─────────┐
│ Interface │ Attribute │ Value │
├────────────────────────────────────────────┼───────────────┼─────────┤
│ rand(), rand_r(), srand() │ Thread safety │ MT-Safe │
└────────────────────────────────────────────┴───────────────┴─────────┘
VERSIONS
The versions of rand() and srand() in the Linux C Library use the same
random number generator as random(3) and srandom(3), so the lower-order
bits should be as random as the higher-order bits. However, on older
rand() implementations, and on current implementations on different sys-
tems, the lower-order bits are much less random than the higher-order
bits. Do not use this function in applications intended to be portable
when good randomness is needed. (Use random(3) instead.)
STANDARDS
rand()
srand()
C11, POSIX.1-2008.
rand_r()
POSIX.1-2008.
HISTORY
rand()
srand()
SVr4, 4.3BSD, C89, POSIX.1-2001.
rand_r()
POSIX.1-2001. Obsolete in POSIX.1-2008.
EXAMPLES
POSIX.1-2001 gives the following example of an implementation of rand()
and srand(), possibly useful when one needs the same sequence on two
different machines.
static unsigned long next = 1;
/* RAND_MAX assumed to be 32767 */
int myrand(void) {
next = next * 1103515245 + 12345;
return((unsigned)(next/65536) % 32768);
}
void mysrand(unsigned int seed) {
next = seed;
}
The following program can be used to display the pseudo-random sequence
produced by rand() when given a particular seed. When the seed is -1,
the program uses a random seed.
#include <stdio.h>
#include <stdlib.h>
int
main(int argc, char *argv[])
{
int r;
unsigned int seed, nloops;
if (argc != 3) {
fprintf(stderr, "Usage: %s <seed> <nloops>\n", argv[0]);
exit(EXIT_FAILURE);
}
seed = atoi(argv[1]);
nloops = atoi(argv[2]);
if (seed == -1) {
seed = arc4random();
printf("seed: %u\n", seed);
}
srand(seed);
for (unsigned int j = 0; j < nloops; j++) {
r = rand();
printf("%d\n", r);
}
exit(EXIT_SUCCESS);
}
SEE ALSO
drand48(3), random(3)
Linux man-pages 6.9.1 2024-06-15 rand(3)
Generated by dwww version 1.16 on Tue Dec 16 04:25:28 CET 2025.