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:
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.
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.
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 appropriately.
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.
When reading from the urandom source (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 of bytes (more than 256), getrandom() will block until those bytes have been generated and transferred from kernel memory to buf. When reading from the random source (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 the urandom source 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 fails 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 the random source, blocking requests of any size can be interrupted by a signal handler (the call fails with the error EINTR).
Using getrandom() to read small buffers (<= 256 bytes) from the urandom source is the preferred mode of usage.
The special treatment of small values of buflen was designed for compatibility with OpenBSD's getentropy(3), which is nowadays supported by glibc.
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!