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DESCRIPTION top

Thegetrandom() system call fills the buffer pointed to bybuf with up tosize random bytes. These bytes can be used to seed user-space random number generators or for cryptographic purposes. By default,getrandom() draws entropy from theurandom source (i.e., the same source as the/dev/urandom device). This behavior can be changed via theflags argument. If theurandom 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 errorEINTR. If theurandom source has not yet been initialized, thengetrandom() will block, unlessGRND_NONBLOCKis specified inflags. Theflags 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 therandom source (i.e., the same source as the/dev/random device) instead of theurandom source. Therandom source is limited based on the entropy that can be obtained from environmental noise. If the number of available bytes in therandom source is less than requested insize, the call returns just the available random bytes. If no random bytes are available, the behavior depends on the presence ofGRND_NONBLOCKin theflags argument.GRND_NONBLOCK By default, when reading from therandom source,getrandom() blocks if no random bytes are available, and when reading from theurandom source, it blocks if the entropy pool has not yet been initialized. If theGRND_NONBLOCKflag is set, thengetrandom() does not block in these cases, but instead immediately returns -1 witherrno set toEAGAIN.

RETURN VALUE top

On success,getrandom() returns the number of bytes that were copied to the bufferbuf. This may be less than the number of bytes requested viasize if eitherGRND_RANDOMwas specified inflags and insufficient entropy was present in therandom source or the system call was interrupted by a signal. On error, -1 is returned, anderrno is set to indicate the error.

ERRORS top

EAGAINThe requested entropy was not available, andgetrandom() would have blocked if theGRND_NONBLOCKflag was not set.EFAULTThe address referred to bybuf is outside the accessible address space.EINTRThe call was interrupted by a signal handler; see the description of how interruptedread(2) calls on "slow" devices are handled with and without theSA_RESTARTflag in thesignal(7) man page.EINVALAn invalid flag was specified inflags.ENOSYSThe glibc wrapper function forgetrandom() determined that the underlying kernel does not implement this system call.

NOTES top

For an overview and comparison of the various interfaces that can be used to obtain randomness, seerandom(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 wherechroot(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.Maximum number of bytes returned As of Linux 3.19 the following limits apply: • When reading from theurandom source, a maximum of 32Mi-1 bytes is returned by a single call togetrandom() on systems whereint has a size of 32 bits. • When reading from therandom source, a maximum of 512 bytes is returned.Interruption by a signal handler When reading from theurandom source (GRND_RANDOMis not set),getrandom() will block until the entropy pool has been initialized (unless theGRND_NONBLOCKflag 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 tobuf. When reading from therandom source (GRND_RANDOMis set),getrandom() will block until some random bytes become available (unless theGRND_NONBLOCKflag was specified). The behavior when a call togetrandom() that is blocked while reading from theurandom source is interrupted by a signal handler depends on the initialization state of the entropy buffer and on the request size,size. If the entropy is not yet initialized, then the call fails with theEINTRerror. If the entropy pool has been initialized and the request size is large (size > 256), the call either succeeds, returning a partially filled buffer, or fails with the errorEINTR. If the entropy pool has been initialized and the request size is small (size <= 256), thengetrandom() will not fail withEINTR. Instead, it will return all of the bytes that have been requested. When reading from therandom source, blocking requests of any size can be interrupted by a signal handler (the call fails with the errorEINTR). Usinggetrandom() to read small buffers (<= 256 bytes) from theurandom source is the preferred mode of usage. The special treatment of small values ofsize was designed for compatibility with OpenBSD'sgetentropy(3), which is nowadays supported by glibc. The user ofgetrandom()must always check the return value, to determine whether either an error occurred or fewer bytes than requested were returned. In the case whereGRND_RANDOMis not specified andsize 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!

COLOPHON top

This page is part of theman-pages (Linux kernel and C library user-space interface documentation) project. Information about the project can be found at ⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report for this manual page, see ⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩. This page was obtained from the tarball man-pages-6.16.tar.gz fetched from ⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on 2026-01-16. If you discover any rendering problems in this HTML version of the page, or you believe there is a better or more up- to-date source for the page, or you have corrections or improvements to the information in this COLOPHON (which isnot part of the original manual page), send a mail to man-pages@man7.orgLinux man-pages 6.16 2025-09-21getrandom(2)