arc4random - Man Page

cryptographically-secure pseudorandom number generator


Standard C library (libc, -lc)


#include <stdlib.h>

uint32_t arc4random(void);
uint32_t arc4random_uniform(uint32_t upper_bound);
void arc4random_buf(void buf[.n], size_t n);


These functions give cryptographically-secure pseudorandom numbers.

arc4random() returns a uniformly-distributed value.

arc4random_uniform() returns a uniformly-distributed value less than upper_bound (see Bugs).

arc4random_buf() fills the memory pointed to by buf, with n bytes of pseudorandom data.

The rand(3) and drand48(3) families of functions should only be used where the quality of the pseudorandom numbers is not a concern and there's a need for repeatability of the results. Unless you meet both of those conditions, use the arc4random() functions.

Return Value

arc4random() returns a pseudorandom number.

arc4random_uniform() returns a pseudorandom number less than upper_bound for valid input, or 0 when upper_bound is invalid.


For an explanation of the terms used in this section, see attributes(7).

arc4random(), arc4random_uniform(), arc4random_buf()Thread safetyMT-Safe




OpenBSD 2.1, FreeBSD 3.0, NetBSD 1.6, DragonFly 1.0, libbsd, glibc 2.36.


An upper_bound of 0 doesn't make sense in a call to arc4random_uniform(). Such a call will fail, and return 0. Be careful, since that value is not less than upper_bound. In some cases, such as accessing an array, using that value could result in Undefined Behavior.

See Also

getrandom(3), rand(3), drand48(3), random(7)

Referenced By

mk-configure(7), oksh(1).

The man pages arc4random_buf(3) and arc4random_uniform(3) are aliases of arc4random(3).

2023-10-31 Linux man-pages 6.7