Defined in header <stdlib.h> | ||
---|---|---|
int rand(); |
Returns a pseudo-random integer value between 0
and RAND_MAX
(0 and RAND_MAX
included).
srand()
seeds the pseudo-random number generator used by rand()
. If rand()
is used before any calls to srand()
, rand()
behaves as if it was seeded with srand(1)
. Each time rand()
is seeded with srand()
, it must produce the same sequence of values.
rand()
is not guaranteed to be thread-safe.
(none).
Pseudo-random integer value between 0
and RAND_MAX
, inclusive.
There are no guarantees as to the quality of the random sequence produced. In the past, some implementations of rand()
have had serious shortcomings in the randomness, distribution and period of the sequence produced (in one well-known example, the low-order bit simply alternated between 1
and 0
between calls). rand()
is not recommended for serious random-number generation needs, like cryptography.
POSIX requires that the period of the pseudo-random number generator used by rand
is at least 232
.
POSIX offered a thread-safe version of rand called rand_r, which is obsolete in favor of the drand48 family of functions.
#include <stdio.h> #include <stdlib.h> #include <time.h> int main(void) { srand(time(NULL)); // use current time as seed for random generator int random_variable = rand(); printf("Random value on [0,%d]: %d\n", RAND_MAX, random_variable); // roll a 6-sided die 20 times for (int n=0; n != 20; ++n) { int x = 7; while(x > 6) x = 1 + rand()/((RAND_MAX + 1u)/6); // Note: 1+rand()%6 is biased printf("%d ", x); } }
Possible output:
Random value on [0,2147483647]: 448749574 3 1 3 1 4 2 2 1 3 6 4 4 3 1 6 2 3 2 6 1
seeds pseudo-random number generator (function) |
|
maximum possible value generated by rand() (macro constant) |
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