1GETRANDOM(2) Linux Programmer's Manual GETRANDOM(2)
2
6 getrandom - obtain a series of random bytes
7
9 #include <sys/random.h>
10 ssize_t getrandom(void *buf, size_t buflen, unsigned int flags);
12
14 The getrandom() system call fills the buffer pointed to by buf with up
15 to buflen random bytes. These bytes can be used to seed user-space
16 random number generators or for cryptographic purposes.
17 By default, getrandom() draws entropy from the urandom source (i.e.,
19 the same source as the /dev/urandom device). This behavior can be
20 changed via the flags argument.
21 If the urandom source has been initialized, reads of up to 256 bytes
23 will always return as many bytes as requested and will not be inter‐
24 rupted by signals. No such guarantees apply for larger buffer sizes.
25 For example, if the call is interrupted by a signal handler, it may
26 return a partially filled buffer, or fail with the error EINTR.
27 If the urandom source has not yet been initialized, then getrandom()
29 will block, unless GRND_NONBLOCK is specified in flags.
30 The flags argument is a bit mask that can contain zero or more of the
32 following values ORed together:
33 GRND_RANDOM
35 If this bit is set, then random bytes are drawn from the random
36 source (i.e., the same source as the /dev/random device) instead
37 of the urandom source. The random source is limited based on
38 the entropy that can be obtained from environmental noise. If
39 the number of available bytes in the random source is less than
40 requested in buflen, the call returns just the available random
41 bytes. If no random bytes are available, the behavior depends
42 on the presence of GRND_NONBLOCK in the flags argument.
43 GRND_NONBLOCK
45 By default, when reading from the random source, getrandom()
46 blocks if no random bytes are available, and when reading from
47 the urandom source, it blocks if the entropy pool has not yet
48 been initialized. If the GRND_NONBLOCK flag is set, then
49 getrandom() does not block in these cases, but instead immedi‐
50 ately returns -1 with errno set to EAGAIN.
51
53 On success, getrandom() returns the number of bytes that were copied to
54 the buffer buf. This may be less than the number of bytes requested
55 via buflen if either GRND_RANDOM was specified in flags and insuffi‐
56 cient entropy was present in the random source or the system call was
57 interrupted by a signal.
58 On error, -1 is returned, and errno is set appropriately.
60
62 EAGAIN The requested entropy was not available, and getrandom() would
63 have blocked if the GRND_NONBLOCK flag was not set.
64 EFAULT The address referred to by buf is outside the accessible address
66 space.
67 EINTR The call was interrupted by a signal handler; see the descrip‐
69 tion of how interrupted read(2) calls on "slow" devices are han‐
70 dled with and without the SA_RESTART flag in the signal(7) man
71 page.
72 EINVAL An invalid flag was specified in flags.
74 ENOSYS The glibc wrapper function for getrandom() determined that the
76 underlying kernel does not implement this system call.
77
79 getrandom() was introduced in version 3.17 of the Linux kernel. Sup‐
80 port was added to glibc in version 2.25.
81
83 This system call is Linux-specific.
84
86 For an overview and comparison of the various interfaces that can be
87 used to obtain randomness, see random(7).
88 Unlike /dev/random and /dev/urandom, getrandom() does not involve the
90 use of pathnames or file descriptors. Thus, getrandom() can be useful
91 in cases where chroot(2) makes /dev pathnames invisible, and where an
92 application (e.g., a daemon during start-up) closes a file descriptor
93 for one of these files that was opened by a library.
94 Maximum number of bytes returned
96 As of Linux 3.19 the following limits apply:
97 * When reading from the urandom source, a maximum of 33554431 bytes is
99 returned by a single call to getrandom() on systems where int has a
100 size of 32 bits.
101 * When reading from the random source, a maximum of 512 bytes is
103 returned.
104 Interruption by a signal handler
106 When reading from the urandom source (GRND_RANDOM is not set), getran‐
107 dom() will block until the entropy pool has been initialized (unless
108 the GRND_NONBLOCK flag was specified). If a request is made to read a
109 large number of bytes (more than 256), getrandom() will block until
110 those bytes have been generated and transferred from kernel memory to
111 buf. When reading from the random source (GRND_RANDOM is set), getran‐
112 dom() will block until some random bytes become available (unless the
113 GRND_NONBLOCK flag was specified).
114 The behavior when a call to getrandom() that is blocked while reading
116 from the urandom source is interrupted by a signal handler depends on
117 the initialization state of the entropy buffer and on the request size,
118 buflen. If the entropy is not yet initialized, then the call fails
119 with the EINTR error. If the entropy pool has been initialized and the
120 request size is large (buflen > 256), the call either succeeds, return‐
121 ing a partially filled buffer, or fails with the error EINTR. If the
122 entropy pool has been initialized and the request size is small
123 (buflen <= 256), then getrandom() will not fail with EINTR. Instead,
124 it will return all of the bytes that have been requested.
125 When reading from the random source, blocking requests of any size can
127 be interrupted by a signal handler (the call fails with the error
128 EINTR).
129 Using getrandom() to read small buffers (<= 256 bytes) from the urandom
131 source is the preferred mode of usage.
132 The special treatment of small values of buflen was designed for com‐
134 patibility with OpenBSD's getentropy(3), which is nowadays supported by
135 glibc.
136 The user of getrandom() must always check the return value, to deter‐
138 mine whether either an error occurred or fewer bytes than requested
139 were returned. In the case where GRND_RANDOM is not specified and
140 buflen is less than or equal to 256, a return of fewer bytes than
141 requested should never happen, but the careful programmer will check
142 for this anyway!
143
145 As of Linux 3.19, the following bug exists:
146 * Depending on CPU load, getrandom() does not react to interrupts
148 before reading all bytes requested.
149
151 getentropy(3), random(4), urandom(4), random(7), signal(7)
152
154 This page is part of release 4.15 of the Linux man-pages project. A
155 description of the project, information about reporting bugs, and the
156 latest version of this page, can be found at
157 https://www.kernel.org/doc/man-pages/.
158Linux 2017-09-15 GETRANDOM(2)