NAME¶

getrandom - obtain a series of random bytes

LIBRARY¶

Standard C library (libc, -lc)

SYNOPSIS¶

#include <sys/random.h>

ssize_t getrandom(void buf[.buflen], size_t buflen, unsigned int flags);

DESCRIPTION¶

The getrandom() system call fills the buffer pointed to by buf with up to buflen random bytes. These bytes can be used to seed user-space random number generators or for cryptographic purposes.

By default, getrandom() draws entropy from the urandom source (i.e., the same source as the /dev/urandom device). This behavior can be changed via the flags argument.

If the urandom source has been initialized, reads of up to 256 bytes will always return as many bytes as requested and will not be interrupted by signals. No such guarantees apply for larger buffer sizes. For example, if the call is interrupted by a signal handler, it may return a partially filled buffer, or fail with the error EINTR.

If the urandom source has not yet been initialized, then getrandom() will block, unless GRND_NONBLOCK is specified in flags.

The flags argument is a bit mask that can contain zero or more of the following values ORed together:

GRND_RANDOM

If this bit is set, then random bytes are drawn from the random source (i.e., the same source as the /dev/random device) instead of the urandom source. The random source is limited based on the entropy that can be obtained from environmental noise. If the number of available bytes in the random source is less than requested in buflen, the call returns just the available random bytes. If no random bytes are available, the behavior depends on the presence of GRND_NONBLOCK in the flags argument.

GRND_NONBLOCK

By default, when reading from the random source, getrandom() blocks if no random bytes are available, and when reading from the urandom source, it blocks if the entropy pool has not yet been initialized. If the GRND_NONBLOCK flag is set, then getrandom() does not block in these cases, but instead immediately returns -1 with errno set to EAGAIN.

RETURN VALUE¶

On success, getrandom() returns the number of bytes that were copied to the buffer buf. This may be less than the number of bytes requested via buflen if either GRND_RANDOM was specified in flags and insufficient entropy was present in the random source or the system call was interrupted by a signal.

On error, -1 is returned, and errno is set to indicate the error.

ERRORS¶

EAGAIN

The requested entropy was not available, and getrandom() would have blocked if the GRND_NONBLOCK flag was not set.

EFAULT

The address referred to by buf is outside the accessible address space.

EINTR

The call was interrupted by a signal handler; see the description of how interrupted read(2) calls on "slow" devices are handled with and without the SA_RESTART flag in the signal(7) man page.

EINVAL

An invalid flag was specified in flags.

ENOSYS

The glibc wrapper function for getrandom() determined that the underlying kernel does not implement this system call.

STANDARDS¶

Linux.

HISTORY¶

Linux 3.17, glibc 2.25.

NOTES¶

For an overview and comparison of the various interfaces that can be used to obtain randomness, see random(7).

Unlike /dev/random and /dev/urandom, getrandom() does not involve the use of pathnames or file descriptors. Thus, getrandom() can be useful in cases where chroot(2) makes /dev pathnames invisible, and where an application (e.g., a daemon during start-up) closes a file descriptor for one of these files that was opened by a library.

BUGS¶

As of Linux 3.19, the following bug exists:

•

Depending on CPU load, getrandom() does not react to interrupts before reading all bytes requested.

SEE ALSO¶

getentropy(3), random(4), urandom(4), random(7), signal(7)