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27 .TH GETRANDOM 2 2017-09-15 "Linux" "Linux Programmer's Manual"
29 getrandom \- obtain a series of random bytes
31 .B #include <sys/random.h>
33 .BI "ssize_t getrandom(void *"buf ", size_t " buflen ", unsigned int " flags );
37 system call fills the buffer pointed to by
42 These bytes can be used to seed user-space random number generators
43 or for cryptographic purposes.
47 draws entropy from the
49 source (i.e., the same source as the
52 This behavior can be changed via the
58 source has been initialized,
59 reads of up to 256 bytes will always return as many bytes as
60 requested and will not be interrupted by signals.
61 No such guarantees apply for larger buffer sizes.
62 For example, if the call is interrupted by a signal handler,
63 it may return a partially filled buffer, or fail with the error
68 source has not yet been initialized, then
77 argument is a bit mask that can contain zero or more of the following values
81 If this bit is set, then random bytes are drawn from the
84 (i.e., the same source as the
92 source is limited based on the entropy that can be obtained from environmental
94 If the number of available bytes in the
96 source is less than requested in
98 the call returns just the available random bytes.
99 If no random bytes are available, the behavior depends on the presence of
106 By default, when reading from the
110 blocks if no random bytes are available,
111 and when reading from the
113 source, it blocks if the entropy pool has not yet been initialized.
118 does not block in these cases, but instead immediately returns \-1 with
125 returns the number of bytes that were copied to the buffer
127 This may be less than the number of bytes requested via
133 and insufficient entropy was present in the
135 source or the system call was interrupted by a signal.
137 On error, \-1 is returned, and
139 is set appropriately.
143 The requested entropy was not available, and
145 would have blocked if the
150 The address referred to by
152 is outside the accessible address space.
155 The call was interrupted by a signal
156 handler; see the description of how interrupted
158 calls on "slow" devices are handled with and without the
165 An invalid flag was specified in
169 The glibc wrapper function for
171 determined that the underlying kernel does not implement this system call.
174 was introduced in version 3.17 of the Linux kernel.
175 Support was added to glibc in version 2.25.
177 This system call is Linux-specific.
179 For an overview and comparison of the various interfaces that
180 can be used to obtain randomness, see
188 does not involve the use of pathnames or file descriptors.
191 can be useful in cases where
196 and where an application (e.g., a daemon during start-up)
197 closes a file descriptor for one of these files
198 that was opened by a library.
200 .SS Maximum number of bytes returned
201 As of Linux 3.19 the following limits apply:
203 When reading from the
205 source, a maximum of 33554431 bytes is returned by a single call to
209 has a size of 32 bits.
211 When reading from the
213 source, a maximum of 512 bytes is returned.
214 .SS Interruption by a signal handler
215 When reading from the
221 will block until the entropy pool has been initialized
225 If a request is made to read a large number of bytes (more than 256),
227 will block until those bytes have been generated and transferred
228 from kernel memory to
230 When reading from the
236 will block until some random bytes become available
241 The behavior when a call to
243 that is blocked while reading from the
245 source is interrupted by a signal handler
246 depends on the initialization state of the entropy buffer
247 and on the request size,
249 If the entropy is not yet initialized, then the call fails with the
252 If the entropy pool has been initialized
253 and the request size is large
254 .RI ( buflen "\ >\ 256),"
255 the call either succeeds, returning a partially filled buffer,
256 or fails with the error
258 If the entropy pool has been initialized and the request size is small
259 .RI ( buflen "\ <=\ 256),"
264 Instead, it will return all of the bytes that have been requested.
266 When reading from the
268 source, blocking requests of any size can be interrupted by a signal handler
269 (the call fails with the error
274 to read small buffers (<=\ 256 bytes) from the
276 source is the preferred mode of usage.
278 The special treatment of small values of
280 was designed for compatibility with
283 which is nowadays supported by glibc.
288 always check the return value,
289 to determine whether either an error occurred
290 or fewer bytes than requested were returned.
295 is less than or equal to 256,
296 a return of fewer bytes than requested should never happen,
297 but the careful programmer will check for this anyway!
299 As of Linux 3.19, the following bug exists:
300 .\" FIXME patch proposed https://lkml.org/lkml/2014/11/29/16
302 Depending on CPU load,
304 does not react to interrupts before reading all bytes requested.
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