Ubuntu Manpages

getrandom

obtain a series of random bytes

#include <linux/random.h>

int getrandom(void *buf, size_t buflen, unsigned int flags);

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.

getrandom() relies on entropy gathered from device drivers and other sources of environmental noise. Unnecessarily reading large quantities of data will have a negative impact on other users of the /dev/random and /dev/urandom devices. Therefore, getrandom() should not be used for Monte Carlo simulations or other programs/algorithms which are doing probabilistic sampling.

By default, getrandom() draws entropy from the /dev/urandom pool. This behavior can be changed via the flags argument. If the /dev/urandom pool 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 pool has not yet been initialized, then the call blocks, 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:

If this bit is set, then random bytes are drawn from the /dev/random pool instead of the /dev/urandom pool. The /dev/random pool is limited based on the entropy that can be obtained from environmental noise. If the number of available bytes in /dev/random 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.
By default, when reading from /dev/random, getrandom() blocks if no random bytes are available, and when reading from /dev/urandom, 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.

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 GRND_RANDOM was specified in flags and insufficient entropy was present in the /dev/random pool, or if the system call was interrupted by a signal.

On error, -1 is returned, and errno is set appropriately.

An invalid flag was specified in flags.
The address referred to by buf is outside the accessible address space.
The requested entropy was not available, and getrandom() would have blocked if the GRND_NONBLOCK flag was not set.
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.

getrandom() was introduced in version 3.17 of the Linux kernel.

This system call is Linux-specific.

As of Linux 3.19 the following limits apply:

  • When reading from /dev/urandom, a maximum of 33554431 bytes is returned by a single call to getrandom() on a system where int has a size of 32 bits.
  • When reading from /dev/random, a maximum of 512 bytes is returned.

The kernel collects bits of entropy from environment. When a sufficient number of random bits has been collected, the /dev/urandom entropy pool is considered to be initialized. This state is normally reached early in the system bootstrap phase.

When reading from /dev/urandom (GRND_RANDOM is not set), getrandom() will block until the entropy pool has been initialized (unless the GRND_NONBLOCK flag was specified). If a request is made to read a large number (more than 256) of bytes, getrandom() will block until those bytes have been generated and transferred from kernel memory to buf. When reading from /dev/random (GRND_RANDOM is set), getrandom() will block until some random bytes become available (unless the GRND_NONBLOCK flag was specified).

The behavior when a call to getrandom() that is blocked while reading from /dev/urandom is interrupted by a signal handler depends on the initialization state of the entropy buffer and on the request size, buflen. If the entropy is not yet initialized, then the call will fail with the EINTR error. If the entropy pool has been initialized and the request size is large (buflen > 256), the call either succeeds, returning a partially filled buffer, or fails with the error EINTR. If the entropy pool has been initialized and the request size is small (buflen <= 256), then getrandom() will not fail with EINTR. Instead, it will return all of the bytes that have been requested.

When reading from /dev/random, blocking requests of any size can be interrupted by a signal (the call fails with the error EINTR).

Calling getrandom() to read /dev/urandom for small values (<= 256) of buflen is the preferred mode of usage.

The special treatment of small values of buflen was designed for compatibility with OpenBSD's getentropy() system call.

The user of getrandom() must always check the return value, to determine whether either an error occurred or fewer bytes than requested were returned. In the case where GRND_RANDOM is not specified and buflen is less than or equal to 256, a return of fewer bytes than requested should never happen, but the careful programmer will check for this anyway!

Unless you are doing long-term key generation (and perhaps not even then), you probably shouldn't be using GRND_RANDOM. The cryptographic algorithms used for /dev/urandom are quite conservative, and so should be sufficient for all purposes. The disadvantage of GRND_RANDOM is that it can block. Furthermore, dealing with the partially fulfilled getrandom() requests that can occur when using GRND_RANDOM increases code complexity.

The getentropy() system call in OpenBSD can be emulated using the following function:


int
getentropy(void *buf, size_t buflen)
{
    int ret;
    if (buflen > 256)
        goto failure;
    ret = getrandom(buf, buflen, 0);
    if (ret < 0)
        return ret;
    if (ret == buflen)
        return 0;
failure:
    errno = EIO;
    return -1;
}

As of Linux 3.19, the following bug exists:

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

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

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