1getrandom(2) System Calls Manual getrandom(2)
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6 getrandom - obtain a series of random bytes
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9 Standard C library (libc, -lc)
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12 #include <sys/random.h>
13 ssize_t getrandom(void buf[.buflen], size_t buflen, unsigned int flags);
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17 The getrandom() system call fills the buffer pointed to by buf with up
18 to buflen random bytes. These bytes can be used to seed user-space
19 random number generators or for cryptographic purposes.
20 By default, getrandom() draws entropy from the urandom source (i.e.,
22 the same source as the /dev/urandom device). This behavior can be
23 changed via the flags argument.
24 If the urandom source has been initialized, reads of up to 256 bytes
26 will always return as many bytes as requested and will not be inter‐
27 rupted by signals. No such guarantees apply for larger buffer sizes.
28 For example, if the call is interrupted by a signal handler, it may re‐
29 turn a partially filled buffer, or fail with the error EINTR.
30 If the urandom source has not yet been initialized, then getrandom()
32 will block, unless GRND_NONBLOCK is specified in flags.
33 The flags argument is a bit mask that can contain zero or more of the
35 following values ORed together:
36 GRND_RANDOM
38 If this bit is set, then random bytes are drawn from the random
39 source (i.e., the same source as the /dev/random device) instead
40 of the urandom source. The random source is limited based on
41 the entropy that can be obtained from environmental noise. If
42 the number of available bytes in the random source is less than
43 requested in buflen, the call returns just the available random
44 bytes. If no random bytes are available, the behavior depends
45 on the presence of GRND_NONBLOCK in the flags argument.
46 GRND_NONBLOCK
48 By default, when reading from the random source, getrandom()
49 blocks if no random bytes are available, and when reading from
50 the urandom source, it blocks if the entropy pool has not yet
51 been initialized. If the GRND_NONBLOCK flag is set, then ge‐
52 trandom() does not block in these cases, but instead immediately
53 returns -1 with errno set to EAGAIN.
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56 On success, getrandom() returns the number of bytes that were copied to
57 the buffer buf. This may be less than the number of bytes requested
58 via buflen if either GRND_RANDOM was specified in flags and insuffi‐
59 cient entropy was present in the random source or the system call was
60 interrupted by a signal.
61 On error, -1 is returned, and errno is set to indicate the error.
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65 EAGAIN The requested entropy was not available, and getrandom() would
66 have blocked if the GRND_NONBLOCK flag was not set.
67 EFAULT The address referred to by buf is outside the accessible address
69 space.
70 EINTR The call was interrupted by a signal handler; see the descrip‐
72 tion of how interrupted read(2) calls on "slow" devices are han‐
73 dled with and without the SA_RESTART flag in the signal(7) man
74 page.
75 EINVAL An invalid flag was specified in flags.
77 ENOSYS The glibc wrapper function for getrandom() determined that the
79 underlying kernel does not implement this system call.
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82 Linux.
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85 Linux 3.17, glibc 2.25.
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88 For an overview and comparison of the various interfaces that can be
89 used to obtain randomness, see random(7).
90 Unlike /dev/random and /dev/urandom, getrandom() does not involve the
92 use of pathnames or file descriptors. Thus, getrandom() can be useful
93 in cases where chroot(2) makes /dev pathnames invisible, and where an
94 application (e.g., a daemon during start-up) closes a file descriptor
95 for one of these files that was opened by a library.
96 Maximum number of bytes returned
98 As of Linux 3.19 the following limits apply:
99 • When reading from the urandom source, a maximum of 32Mi-1 bytes is
101 returned by a single call to getrandom() on systems where int has a
102 size of 32 bits.
103 • When reading from the random source, a maximum of 512 bytes is re‐
105 turned.
106 Interruption by a signal handler
108 When reading from the urandom source (GRND_RANDOM is not set), getran‐
109 dom() will block until the entropy pool has been initialized (unless
110 the GRND_NONBLOCK flag was specified). If a request is made to read a
111 large number of bytes (more than 256), getrandom() will block until
112 those bytes have been generated and transferred from kernel memory to
113 buf. When reading from the random source (GRND_RANDOM is set), getran‐
114 dom() will block until some random bytes become available (unless the
115 GRND_NONBLOCK flag was specified).
116 The behavior when a call to getrandom() that is blocked while reading
118 from the urandom source is interrupted by a signal handler depends on
119 the initialization state of the entropy buffer and on the request size,
120 buflen. If the entropy is not yet initialized, then the call fails
121 with the EINTR error. If the entropy pool has been initialized and the
122 request size is large (buflen > 256), the call either succeeds, return‐
123 ing a partially filled buffer, or fails with the error EINTR. If the
124 entropy pool has been initialized and the request size is small (bu‐
125 flen <= 256), then getrandom() will not fail with EINTR. Instead, it
126 will return all of the bytes that have been requested.
127 When reading from the random source, blocking requests of any size can
129 be interrupted by a signal handler (the call fails with the error
130 EINTR).
131 Using getrandom() to read small buffers (<= 256 bytes) from the urandom
133 source is the preferred mode of usage.
134 The special treatment of small values of buflen was designed for com‐
136 patibility with OpenBSD's getentropy(3), which is nowadays supported by
137 glibc.
138 The user of getrandom() must always check the return value, to deter‐
140 mine whether either an error occurred or fewer bytes than requested
141 were returned. In the case where GRND_RANDOM is not specified and bu‐
142 flen is less than or equal to 256, a return of fewer bytes than re‐
143 quested should never happen, but the careful programmer will check for
144 this anyway!
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147 As of Linux 3.19, the following bug exists:
148 • Depending on CPU load, getrandom() does not react to interrupts be‐
150 fore reading all bytes requested.
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153 getentropy(3), random(4), urandom(4), random(7), signal(7)
154Linux man-pages 6.04 2023-03-30 getrandom(2)